I have to cool down a customers amp that's running at 147% plate dissipation and have never had to alter the cathode bias resistor in any other amp I've worked on, until now. Uncle Doug, I now fully understand the theory, principles and practice to get the job done when it arrives. Thanks again for all of your videos, you really are my favorite Uncle.
Why is it when you say something, it makes complete sense and easy to understand. Really enjoying your videos. I thank you for all the time and effort put forth!
After watching this video and the previous one, and the one on adding a bias pot to the Fender Princeton, a light bulb went on for me. I finally understand the concept of negative bias voltage! After carefully studying the Princeton schematic and that of my favorite Fender amp, the AB763 Deluxe Reverb, it all becomes pretty clear (I still have a long way to go) , in particular, how the bias board works. I have now elevated myself to the status of "potentially a little bit more knowledgeable than a rank amateur". By far, you present the most clear, concise videos on the theory of this stuff that I have come across so far. Thanks again.
Doug thanks for so clearly explaining so much valuable info about tube amps and how they work. Everyone who tinkers with these babies should watch your channel!
Excellent videos you have Sir uploaded, I want to tell you how I enjoy your presentation, brilliant lectures, best I've ever seen in guitar signal amplification. Thank you so much for your time, effort and devotion to share with us. God Bless You
+Imre Bencze You're quite welcome, Imre, and thanks so much for your very kind assessment. I appreciate the time you have spent watching my videos and wish you the very best :)
Thank you Doug! This is THE best explanation of how to do bias for tube amps I've seen. I'm old and worked in electronics all my life, so you done good son. :) two thumbs up.
As usual, thank you for a great easily understood video. Reminds me of my great teachers at John A. O'Connell Vocational High School in San Francisco back in the 60's.
I need to thank you & Rusty for your very informative tube amp series of videos. In the last hour, I have learned so much by watching several of your videos. My 1965 Vibro Champ has been with me for 43 yrs, but now I know so much more about it. I own four other vintage guitar amps and I am looking forward to being able to replace the power tubes and biasing and maybe more. Be sure to tell Rusty I said hi. Great videos!
You're quite welcome, Brian. Thanks so much for your kind words. It's great to hear that the videos are helpful. By coincidence, I was just on my way to feed Rusty tonight and I'll be sure to convey your best wishes to him. Thanks for watching and best of luck :)
Uncle Doug - just discovered your amp series vids today - you have an incredible way of teaching this to I'm sure, many of us players who, well we learned over the years to "just play" but over time have been frustrated in not being as able to understand, maintain and for some, modify our own tube amps. Since one of my amps currently needs basic re-tubing, watching you biasing vid and intro to amps 1 & 2 is making the process so much easier for me now. Thanks for your teaching series and really appreciate the time you've taken to publish!!!
Rusty and I really appreciate your kind words, i6, and are glad to hear that our videos are helpful. Best of luck with your biasing project....and be sure to follow all the safety rules :)
Uncle Doug Great videos. I was wondering would you mind to do a video on how to DIY switchable bias on the two ended amp? Can't find this mod for Bugera V55, as this cahnges a sound a lot from twnagy country style to nice and round bluesy, what I'm looking for, If you would need I have a diagram
Once again, an invaluable explanation by Uncle Doug. Thanks Doug I know I'll be using this (and part 2) as reference when biasing my own amps. (Keep 'em coming)
Uncle Doug, Sir you are a veritable textbook of knowledge and have a remarkable ability to convey that information in concise and effective terms. Your videos are simply wonderful. I’m sure I am not alone in offering you thanks and praise. Always looking forward to your next... and past videos. Well done sir.😊👍
Thanks Uncle Doug, these are essential procedures for building a tube amp. Reading about how to do this, or having it explained and demonstrated are two very different learning experiences. Thank you! BTW I just built a 5f2a with an AA764 reverb, inspired by your vid about adding a Twin reverb circuit to a Supro amp. I never thought I would ever be able to do something like that.
Thank you so much from europe, Vienna ! You videos are very good and clear spoken. Anybody can learn from them. I did understand, restored and bias my SANSUI AU-111 with your help. You are one of the best teachers I ever seen on electronics. Please make more Videos if you like to do them. If you ever need something from EU, I will get it for you. It is very nice to see how professional many People in US work on the Ideas they have.
Hi Doug! I want to start building tube guitar amps and they have been watching your videos and learning heaps! I have a notebook and have been taking notes :) I want to thankyou for the wealth of information you are giving away and I really appreciate how well you cover everything! You make it so easy to understand and are motivating me to pursue this hobby. The comic relief in the videos is hilarious! Cheers, Luke
Great videos. I have a matched tube suggestion for single combined bias. I mark and install the tubes at random....then measure or calculate the individual bias currents. They are seldom exactly the same value. Take notes, calculate the difference between the two bias currents for this (first try) tube's positions. Example Inside socket=32 ma., outside socket =29 ma., the difference being 3 ma of unbalanced current. Then SWAP the tube's physical positions and do it all again, noting a second "unbalanced" current value. It is likely one of these values will be smaller and therefore, a."better match". Ultimately, install the tubes in the position that has the smaller difference in the bias current. Works for me.
+dennis walsh An excellent suggestion, Dennis. I too have noted the difference in PC when tubes are moved from socket to socket. Thanks for your helpful input.
I have a question Doug... I have a Gibson GA77, which is negative grid biased. There is a 56k resistor and a 100uf polarized cap to ground on the negative bias supply (with the positive end on the ground side). There is no potentiometer in this amp, as you see in fenders, for adjusting the bias, so am I correct in assuming adjusting the bias would involve changing that 56k resistor to another value? Which direction should I take that value (up or down) in order to lower the plate dissipation of the output tubes?
Brad Linzy Brad, I checked the schematic and you're right about the bias voltage being controlled by the 56K resistor to ground. The higher the resistance to ground, the more negative bias voltage to the grids, and the lower the dissipation.....and vice versa. You could replace the 56K resistor with a 50K resistor and a 25K pot in series, to make it variable.
Thanks Doug. This amp had a LOT of issues that are now resolved after largely rewiring the entire amp. A previous 'tech' had hacked it and now she purrs. Thanks for clarifying what I thought I already knew. :D
This is yet another enjoyable and articulate video, Uncle Doug. I build both instrument and hi-fi amps, and I enjoy your videos very much. I'm presently building a pair of push-pull 2A3 audio amps, and it's an exciting journey!
Thanks so much, Adam. Wow....2A3's. I recall them, and their "brothers" the 45, from my old jukebox and radio days. Best of luck with all your projects :)
How do you know the tank and tubes are good? I ask because these are the "usual suspects", as well as the impedance-matching transformer that drives the tank. Check to be sure there is continuity in the primary and the secondary windings.
Thanks for the help. Just a heads up, if you ever work on this amp, just know there are at least 3 different versions I know about. Mine has a 6eu7 in the phase inverter and the rest of the 9-pin tubes are 12ax7. The schematic I found online shows no 6eu7. Also, there is another design which uses a third 6973 tube to power the reverb.
Brad, I'm glad the suggestion was helpful and you found the source of the reverb problem. Good luck with the repair. Also, your observation about the variations in Valco circuitry is right on the money. It's actually rare to find a Valco chassis that DOES completely match the schematic that is (supposedly) for it. In some cases, I simply draw my own schematic......it's easier in the long run.
Another well-done video Doug! I showed this to one of my customers, in order to simplify the answer to his question, and he called me afterward and told me that he wants me to bias his Fender Pro Reverb :) he said that any voltage over 12 volts scares him (LOL) :)
Thanks, Frank. I think healthy respect for the shock hazard of an amplifier is a good thing, but outright fear would definitely interfere with the biasing process. He's way better off having you do it :))
Doug, Your skill as a teacher is astounding. Had I had access to these lessons in the past I am dead certain I'd have been happily producing amplifiers instead of generating 3 ex-wives all with the same first name of "Plaintiff" This morning I finally understood those resistors.
Now I am really excited to learn! I have 2 of the same amps. One I keep original and the other I experiment with. I just installed a high voltage regulator board on that amp so that I can keep my pre-amp tube voltages regulated and keeping the filter cans still hooked up so that the ripple is clean. Next I will try to modify this amp with a Wing C EF86 tube right before the tone stack. Also my tone stack is before the inverter circuit which is good. Now I just have to figure out what values to connect to the EF86 tube. I need to find some schematics of someone else’s build of connecting the EF86 before the tone stack. I’m near 60yrs old and I grew up with tubes. Now I’m finally learning about them.
Uncle Doug, I guess these are the reqired first steps to take before getting all connected with some groupies or whatever-just kidding. You have a great sense of pacing the information, and the sequenceof the same as well. Saturated🎵!
Thanks for the quick reply. I am hoping that I may pursue this a bit further with you but don't want to become a pest. When I bought the amp the person I bought it from told me that the amp has the AB165 circuit and indeed when it arrived the tube chart fastened to the inside of the cabinet indicated an AB165 circuit. BUT when I opened the chassis up to take a look I found two 150 ohm 7 watt resistors connected between ground and the cathodes, along with a 5 - 50 capacitor in parallel with them. So after some digging online I found the schematic for the AC568 circuit and realized that my amp has that circuit, not the AB165. I know you said in one of your videos that this often happened - that Fender would use up the old tube charts even though the circuit had changed because the tubes were the same. The AC568 schematic also shows a negative 55 volt charge being sent to the grids through a set of components that appears to me to allow you to adjust separately how much voltage is applied to each grid. So the two 150 ohm resistors connected to the cathodes would suggest a cathode bias wouldn't it? But the voltage applied to the grids indicates that would affect the plate current as well wouldn't it? Again, I don't want to be a pest and you may not have time to pursue this but if you take a look at a schematic for the Bassman AC568 circuit you will see what I mean. I know that the AA 864 and AB165 circuits were designed in the Blackface era and the AC568 in the Silverface era so it makes me wonder - did Fender need to adjust some problem with the AB165 circuit or were they just "playing with the circuit for fun". BTW the serial number on the chassis of my amp and the dates on the transformers all indicate that my amp was made in early 1969. Hope you can help quench my curiosity but if this is too much trouble I understand.
Thanks for your excellent note and for bringing the unusual, hybrid-bias AC568 circuit to my attention. To be honest, I have never worked on one of these circuits. In a Princeton Reverb video, several years ago, I experimented with hybrid bias to perfectly balance two out-of-balance 6V6's. Applying different cathode bias resistance to each (I used fixed resistors, but you could use a 250 Ohm pot) resulted in equal plate current, while the (reduced -DC) grid bias helped to maintain a proper PC at idle. It worked and sounded great......but I caught all sorts of flak over the hybridization, some viewers saying "I couldn't do this". I wish I had known of the AC568 circuit at that time, to justify my experimentation. Bottom line: I think that this was Fender's attempt to do what I tried later, but perhaps the hybridization was like Harley Davidson introducing a water-cooled engine, simply not acceptable to purists......or possibly the tone was "different". Either way, it is an interesting modification for grid-biased amps and one that deserves more investigation.
I watched this a few times now. You have cleared it up for me. Thanks for the video! I have obtained a tube amplifier with no identification or schematics. I have found that it is mono with two KT66 tubes in double ended. It's going to be a repair project. It needs new capacitors and I'm sure it will need a bias. I will have to look out for a resistor to ground on the cathode.
We're glad the video was helpful, SED. Good luck with your biasing project.....and be very careful. The voltages can be lethal if not handled properly.
Uncle Doug Thanks doug. I understand the dangers of high voltage. I am reasonably knowledgeable in that area. My amateur radio licence has taught me a decent amount in that area.
Thank you very much for your videos Uncle Doug! Fabulous tutorials here. You are the best at delivering these informative lessons that I have ever seen. You should have a class room where we could sign up and attend the Uncle Dougs Amp-a-holics Unanimous University ! Thank you always for your willingness to share you knowledge. Keep on Rocking' - Perhaps Rusty could play some Stairway to Heaven for Us ?
+Uncle Doug I do so enjoy your videos. On the GA-40 schematic you traced from pin 8 of the rectifier thru a filter choke to the center tap and on to the plates. So do we include the resistance of the choke in our calculations? I'm guessing yes.
Greetings, 998. Yes, you would measure the total resistance between the rectifier output pin and each tube plate (which would include the filter choke in this case).....as well as the total voltage drop across each of those resistances.
Doug , Sorry to have bothered you before searching it . Here's the explanation I found. The filament center tap if for "hum-balancing" so the AC filament supply doesn't inject hum into the amplifier. It typically goes to ground. This is usually good enough in most cases, but if you want better hum rejection, it is customary to "elevate" the heater above the cathode voltage of the preamp tubes. In some amps, this is done with a voltage divider to set a fixed voltage, usually around 30-40V above ground. In low-budget designs, you can actually use the fact that the cathode-biased power tube cathodes are at a voltage anywhere from 10-40V above ground, so you can just stick the center tap there.
That's exactly what I would have told you, Michael. It's an old trick to reduce or eliminate hum with no extra components or expense. It's good that you did your own research.
I've never looked at amp circuits before but always been interested in how they work, so it's great to learn and you explain it perfectly. It's like being back at college but with a bit more enthusiasm. Is the gain on the preamp adjusting the bias to reduce the headroom and create distortion? Thanks for all the uploads and taking the time to respond.
You're welcome, RW, and thanks for the nice comments. Actually, the bias is adjusted primarily on the output tubes, not the preamp. If you bias the output tubes to a high level of plate dissipation, distortion will occur more easily and at lower volume.
These videos are a wonderful resource.Expertly stated, paced well and very user friendly. Thank you so much for this resource Do you offer any kind of a collection on DVD or DVD-r or maybe even a download? I wouldn't mind paying for it. I would like to have a collection that I could access offline for future reference.
To minimize the current unbalance (even with brand new, "matched" tubes), plug in the tubes randomly and measure current or calculate the plate disipation (and note it). THEN I swap the two tubes and do it a second time. Then I install the tubes in the position that had the smaller difference, the "best match". One "pairing" will almost always be "better" (slightly to notable) than the other due to differences in the tube gain, the output transformer (half) winding resistance, etc. Can't hurt, takes an extra 10-15 minutes, mostly rewarming.
Good suggestion, Dennis. I often get a bunch of output tubes and try them in different pairings to find two that have very similar plate current. It takes some time, but it's worth it.
Uncle Doug, I have watched pretty well all of your videos explaining all of the principles of amp design and they have been very helpful. Thanks again for posting them! I recently purchased a used Fender Bassman from the late 60s with the AC568 circuit. When I look at the schematic for this amp it looks like the power tubes are cathode biased but it also looks like there is a potentiometer that somehow affects the grid voltages which makes me wonder what is going on?? I have watched all of your videos about how to check the bias for single and double ended amps. Could you explain the theory behind this design? By the way when I switched to a 3 wire grounded power cord I used your idea of turning the ground switch into an on/off switch for the negative feedback loop. Wow, what a difference. It is almost too "hairy" without the NFB.
Thanks for your kind words, Walter. The output tubes in a Fender Bassman are not cathode biased. The cathodes are grounded and a -DC bias voltage is applied to the tube grids to regulate the plate current at idle. The potentiometer allows you to alter the -DC bias voltage and thus adjust the bias of the tubes.
Once again, I have learned a lot (I think in only a few minutes). You mention the different pin assignments for EL84. Would it be a simple matter to adapt a socket so that it could be switchable between 6V6 and EL84, I happen to have a pair of JJ EL844s which are reputed to have less headroom--great for a relatively quiet and readily distorting amp.
I guess you could have two separate sockets, wired in parallel, one for 6V6 and one for EL84, and could switch between them. It would be easier, however, to simply insert the output tube you want to use and leave the other one out.
Excellent video. Please advise me, I have Cayin A55T push pull KT88 HIFI amplifier. In cathode circuit is connected resistor of 10 ohms to ground. Negative voltage on the grid adjusts the Bias. According to manual bias is set to 0.4 V across cathode resistor of 10 ohms (40 mA). I would like to try EL34 pentode, so is it safe to put EL34 instead of KT88 and adjust bias to 0.4 V? Pin 1 and 8 are connected together so I assume it will work.
Greetings, Munja. I am no expert on KT-88/EL-34 HiFi amps, but after doing some research, it seems like this substitution will work as long as the 1-8 pins are connected for the EL-34's and you adjust the bias for them. I also read that the KT-88's have a 100K bias resistor, while the EL-34's prefer a 220K resistor. If you can't achieve proper bias for the EL-34's with the 100K resistor perhaps you will need to change to 220K. Good luck !!! Here is a link that addresses this tube substitution: www.diyaudio.com/forums/tubes-valves/168479-cayin-55t-want-use-el34.html
Hi Doug, thanks so much for the tutorial. It's so well explained. I had a question about what would constitute a mismatched pair. I had a set of old Sylvania 6L6GC's that tested well on my tube tester so I though I would give them a try. I measured a Gretsch 6165 Variety Plus and got measurements of 69.8mA(outer) & 61.8mA (inner)of plate current. The respective PD results were 28.3W(outer) & 25W(inner)
Some people feel that a discrepancy of 10% or more between PD values indicates a mismatch, Jeff, but others seem to think that some degree of mismatch will actually foster the production of even-ordered harmonics. I tend to avoid any strict numerical cutoff point and just go with what sounds good. The purists might give your tube pair a thumbs down, but I have a feeling they probably sound just fine. Also, as you found out, tube testers are not a good way to match tubes.....they measure other criteria than PC and PD.
Got it! Thanks again Doug. As always very helpful. I think I'll try cooling them down a bit and hear how that sounds. By the way any recommendations on wattage and type or brand of cathode resistors.
Hi Doug, quick question if you please. I just finished a AB763 Deluxe Reverb kit build and when I turn the bias pot my plate voltage also changes quite a bit along with the cathode current. I can swing the PV as much as 50 volts by turning the bias pot from one stop to the other. Is this normal? If not, could you possibly make a recommendation as to where to begin looking for the problem. Love your channel and I've learned a lot. Thank you. Rob, ps: please put a link to your sponsorship program. I forget what video I saw it in and would like to help so the videos keep coming.
Greetings, RC. It is completely normal for the PV to go down when the PC goes up, and vice versa. When current flows, voltage decreases. Think about holding your thumb over the end of a water hose. What happens to the pressure (voltage) within the hose when you release your thumb and water (current) flows ? Your circuit is operating as it should. Now you must come up with a PV and PC that provide an appropriate Plate Dissipation value. Good luck. Thanks for your offer of sponsorship. Here is the link to my Patreon site: www.patreon.com/uncledougsvintageamps
will try to do this later with 6j1 small signal tubes. may be 2 of them in push pull config will provide some audible sounds. not sure if i have audio transformer for this. may be will experiment with some mains to 6-0-6 or 9-0-9 ones. thanks.
Excellent tutorial. I've been reading schematics for many years, but you make it a lot easier following your explanation. RE: Your comment at 18:50 "much better way to bias each tube individually" . Just wondering why you would want to add individual cathode bias resistors, since the traditional push-pull (class AB) output circuit only works on the 180 degree phase shift of the output transformer. Ie: When one side of the o/p transformer is positive 180 degrees, the other side (from the centre tap) will be negative, and the plate of the tube (at the moment) will not be conducting, ( it will be cut off) , leaving the other power output of the push pull pair to conduct, (usually in Class AB mode), the bias is set up in such a way that the o/p tube conducts long enough around the "zero crossing point of the AC amplified waveform" to avoid the crossover notch distortion. So my question is: why would we even need separate cathode bias resistors in this case, since only one tube at a time will be conducting and drawing current through the normal single cathode resistance? This is the way I have always understood it.. Have I missed something through the years?
+SuperCarver2011 The way I visualize the process, SC, it's like two people rowing a boat.....one on each side of the boat. They take turns rowing, and if one rows harder than the other, then the process becomes unbalanced and the progress non-linear. Since no two output tubes have precisely the same plate current (and thus bias) characteristics, biasing each individually will neutralize this inherent mismatch and result in a much more balanced output, when they each "take their turn at rowing the boat". The use of a single bias resistor simply "splits the difference" between the two tubes resulting in an average plate dissipation value that may well be ideal, but conceals the fact that one tube is actually biased "hotter" and the other "colder". Naive and simplistic, perhaps, but it does seem to work quite well. Also, I believe the traditional reliance on a single cathode bias resistor has always been a concession to economy rather than optimum electronic reality.
+Uncle Doug; But if you use matched outputs ( 6V6, 6L6GC) then a slight difference in plate current is (probably) tolerable for both tubes. Perhaps there are custom made boutique amps out there somewhere that have individual cathode bias resistors, but for the most part, the output pair share one cathode bias resistor. Leo Fender built his amps with one cathode bias resistor in his vintage amps, and these vintage amps command a high resale price these days, a lot of guitar players seek them out. However, that being said, carry on Uncle Doug, I'll be tuned into the next part to hear your discussion.
It's a relatively minor consideration in the great scheme of things, SC. Thanks so much for your very knowledgeable input. I always enjoy hearing from you and look forward to further video rendezvous :)
Thank you Uncle Doug for your very clear and understandable explanations. I have tried your method of measuring PD through the center tap of the output transformer. It is a Twin Reverb "type" amp, with 4-5881s. Solid state rectifier. I get the following PD values: V7--9.85W; V8--9.85W; V9--8.87W V10--8.87W. The target PD is 23W@100% or 16.1W@70%. You see that the tubes are running cold, and causing low volume distortion. There is a bias pot on the amp--but the range of -V values it sends will not lift the PD measurements any higher. Could this be due to the value of the bias capacitor? Mine is a 50mf at 200V. Thanks.
LionBronstein You're welcome, LB. Since I don't have a specific schematic for your circuit, I will use a Fender Twin Reverb schematic to address your problem. The pot is not for bias, but for balance between the output tube pairs. The bias is controlled by the 15K resistor off the pot wiper....it allows a certain amount of bias voltage to go to ground. The higher the resistor value, the higher the bias voltage to the tube grids. Now, assuming your measurements were accurate.....and they must be, or the output tubes will be in jeopardy.....the PD values are way too low, as you know. Check the value of the 15K resistor.....is it low? If so, replace it and retest the bias. If not, then are the 5881's old (their PC goes down with age)? If so, replace them and retest. Check the DC output from the diode.....is it around -37VDC? If not, replace the diode and the 50uf filter cap for this circuit. After all this, if the PD is still too low, then increase the 15K resistor to 20K and retest.....continue in this manner with different resistances until you have a proper PD, which would be 70% of 23W (fixed bias tubes) or 16W. Good luck.
Uncle Doug - I noticed as you reviewed the various schematics in this video that on some, one leg of the output transformer's secondary was grounded while others were not. Would you explain why or why not ground one side? I once was told that grounding one side would cause ground loop hum. Does it make a difference in single ended versus push-pull amps? Thanks for your easy to understand teaching! Carl Espy
One of the two connections to complete the circuit to the speaker voice coil is generally done by grounding the appropriate OPT secondary wire and also grounding one leg of the speaker output jack. It causes no hum.....indeed, it minimizes it.
Your videos are phenomenal, I’m learning so much! Quick question, I’ve heard that some manufacturers of cathode biased amps will bias the output tubes to 100% rated plate dissipation. Is this advisable? I thought 60 to 70 percent was the recommended target, at least for grid biased amps.
Thanks so much, Tony. Recall that cathode biasing is self-compensating and CAN be biased at or near max PD. The 70% figure is only for grid-biased output tubes.
Hello Doug, I just started working on a Valco Golden Holiday and I got a bit confused because I believe the center tap of the 6.3 filament secondary goes to the cathodes of the 6v6 power tubes. At first I thought that it was a bias tap but I don't think that's right. I believe they are cathode biased through the 200 ohm resistor to ground. Could you tell me why they would attach the filament CT to the cathodes? Thanks.
This is a method of 'elevating' the heater supply. Rather than the heater voltage being centered around 0V, it is centered around a higher voltage, and the voltage at the cathodes of the power tubes is a convenient source. Heater elevation can be helpful in reducing hum, and is sometimes necessary to protect tubes such as cathode followers which have a high voltage on their cathodes.
Hello Uncle Doug! I want to retube a 100w Peavey Valveking Amp Head, it has 4 6L6GC´s and it ´s a grid biased design. I couldn´t get a matched tube quartet, I got two slightly different pairs: Pair A has Ip=39 Gm=4100. Pair B has Ip=39 Gm=4300 What do you recommend in regards to tube placement in the circuit? Pair A (Gm=4100) on one side of xformer and Pair B (Gm=4100) on the other side? Or sholud I mix one of each pair on each side of xformer? Thank You!!
I think the amp will work just fine with any configuration of the tubes you described......but, if you combine one tube from each pair in parallel, then the average of the two parallel pairs will be identical.
How do you recommend biasing hi-fi valve amplifiers? I get the method would be the same/similar but I am almost done making my own valve hi-fi amplifier and just need to know what sort of figures I should look for when adjusting the bias. With guitar amplifiers its a little bit lower than the maximum plate dissipation for the valves, but what sort of attitude should you have towards hi-fi amplifiers?
I would say that you're probably right on track at "a little bit lower than max. PD", Liz. You want good, clean tone with lots of headroom, and this should provide it.
Doug, another good video. One question: where does the table at 3:25 come from? And the information that max PD in grid-bias mode is 70% of PD in cathode-bias mode? Thanks again EJP. PS No Rusty?
You're welcome, QM. All of the PD data comes straight from the RCA and Sylvania Tube Manuals. Biasing at 70% of max for fixed/grid bias has been standard procedure for years. It provides a safety buffer as the PC and PD increase when the amp is driven. Cathode bias is self-regulating and no such increase occurs, thus they can be safely biased at (or near) max PD.
+Uncle Doug Thanks. I ask because I'm working on an amp which has everything at 100% rating: Va, PD, Ia, ... And it is grid biased. No 70% there. I was hoping it came out of a tube datasheet. I can see that Vak would be higher in a grid-biased amplifier, which may have something to do with it? EJP
Hi Doug. I’m struggling with the HH Scott 99D amplifier on determining if it is Cathode biased or grid biased. If you look at the HH Scott 99D schematic the cathode is connected to a 3K resistor (R47) to ground but it is also tied to 2 resistors, a grid and 375V. Also if you can figure out the biasing resistor would that control both tubes? Also, what would I set the bias for? Please help!
This is why I only work on vintage tube (guitar and PA) amps, Mark. This circuit is a convoluted mess, in my book. I believe it is cathode biased, simply because the cathodes are not directly grounded. I guess you could alter the value of R37 and R38 to achieve proper plate dissipation, and try biasing it as if cathode biased, but conservative.....say 75% of max. Try it and see if it works. Either that or seek assistance from someone better versed in HiFi amp topology. Best of luck.
Howdy. I have been wrong. Fighting hum in my DIY 4 x KT88 amp proved that individual biasing is mandatory. First I trimmed the anode current to 45 mA. After that careful tweaking of the individual biases produced the hum to drop under normal listening levels. Checking the bias voltages revealed that the steepest tube needs about - 61 V and the most shallow about - 59 V. The individual differences seems to be large for KT88:s. Otherwise KT88 is a great tube. Together with good output transformers the sensation of presence is fantastic. Regards.
Hey there Uncle Doug! :) Thanks a lot for all the videos that you're doing! You provide a truck load of good and well explained information! I have a little question for you though .. as I'm studying an old point-to-point tube guitar amp witch I don't know the brand or model ( with tow 12AX7 preamp, 5AR4 rectifier and a pair push-pull 6L6GC or 6V6GT power amp **I don't know what are the original power tubes**), I noticed something strange in the power amp section.. There is a 1.2K 10W resistor between the plate and the output transformer on each of the push-pull tubes.. ?!? Since the output transformer isn't original ( the OT is a Hammond 125B witch is a 5W OT.. doesn't make sense!) My guess is that these 10W 1.2K resistors are there to dissipate some of the output power so that the "too small" output transformer wont burn... is this makes any sense to you? What do you think? Also, If I replace the output transformer for a bigger one (wattage), is it safe to remove theses resistors? Best regards! Alex
Greetings, Alllx, and thanks for the kind words. The position of the resistors and their 10W power rating indicates that either someone was trying to protect a wimpy OPT (as you suggest) or trying to lower the plate voltage to the output tubes.....or both. Regardless, in a double-ended amp, the wimpy 5W OPT will have to be replaced with one that can handle at least 20W if you want to use 6V6's or about 40W if you want to use 6L6's. The two resistors will definitely have to be removed when you install the new OPT. Good luck !!!
Uncle Doug thanks Doug! this gives me some light for sure! Next stpe is the OPT swap! I'll probably stick with the 6L6GC, they sounds better to my Hears :) Keep it up with the videos, this helps A LOT!!
+Uncle Doug I have an RCA 6-HF3 AM/FM/Phonograph that I am almost finished restoring. I had to replace the two 6V6 output tubes in the amplifier section. When I checked the bias it was at 114% of max. I replaced the bias resistor and the "grid stoppers" as they were both significantly out of tolerance. When I rechecked the bias it was still at 96% I checked the bias because with the volume control at 2-3 (out of 10) the amp is very loud. At 5 is seems to be distorting. I'm thinking about reducing the bias to 85% - 90% for better tube longevity and to give me some room on the volume control. I'm not sure how that will affect the phono input. Do you think re-biasing is a good idea?
That sounds like a very reasonable idea, Michael. There is no need to bias any output tube at absolute max plate dissipation. I think you will be pleased with the lower bias levels you mentioned.....headroom should increase a bit and tone should remain good. There should be no problem with the phono input. Good luck.
Thanks Doug. Your channel is invaluable on a lot of levels. I typically haven't checked the output tube bias when I restore old radios. I will from now on. The RCA that I am working on is a '56 vintage. Two years older than me. Once I align the AM section, fix the FM section (it's dead), and re-bias the output tubes I've got to work on the turntable. That is going to be a new "thing" for me.
It's a good idea, Michael. Output tubes are under greater stress than other tubes in a circuit, and should have correct plate dissipation levels to survive and to produce good tone.
Hey Doug, I'm trying to bias my 1968 Sunn 200s. However, when I look at the schematic, I think there is a choke between pin 8 of the rectifier tube and the output transformer. I'm assuming this will effect my readings for calculation of plate current? Could I install a resistor in series with the wires from pin 8 to ground and measure that way?
hi, i have studied electronics for quite long, but nobody study those things nowadays, so these videos are really helpful. i can't find the first part where you discuss single ended amps, could you link me it?
Hi Doug. You said cathode biased and not Grid biased..Ah..on my schematic for the HH Scott 99D it shows R37 as part of the feedback loop and R38 as part of the AC balance circuit. Also important how do I know the correct bias current that the tubes should running at? I am going to replacing the 6L6GB with Tung Sol 6L6GC tubes. I have no idea if this is class A, AB or B in my output circuit.
Bias is based on plate dissipation, not current, Mark. It's determined by the tube and circuit. All 6L6GC's have a max PD of 30W. Whether you bias at 50% to 70% (grid biased) or up to 100% (cathode biased) depends on the circuit. Your circuit is weird. Perhaps you should see where the factory biased the output tubes in your amp and go from there. My answer was based on the schematic I found online.
It says this is part one but you say you recommend watching the first video? I've watched your excellent introduction to tube amps where you explain the fender champ. Is that the first video you refer to, or is there another video specifically on biasing?
@@UncleDoug Thanks, Doug. I'll check that one out. Just so you know, your video on how Tube amps work helped me get my head around something I thought so complex, I'd never be able to understand. I've now started playing around with building small amps and I'm learning all the time. I'm loving the hobby. I'd wanted to get into it for years. I wish I'd had a teacher like you in school! All the best.
When you measure the center tap of the xmfr to plates of the tubes you are really measuring the imbalance of the resistance winding of the xmfr which will appear as a voltage difference. When you measure the voltage across the tube you are measuring the current through the tube. If the voltages are equal then the tubes are matched.
Some of what you say eludes me, Henry. Once you know the exact resistance and voltage drop across each of the OPT primary windings, you can use Ohm's Law to calculate the current through the tube. If the calculated plate current of both tubes is the same, then yes, the tubes are balanced.
When you measure the cathode current it also takes in the screen current. There for the current for the plate resistance, impedance, you can find in the tube manual. It will also tell you what the screen current is. The tube manual will also tell you what the bias should be for any given plate voltage is and the screen voltage for the best operating point the tube should be biased at.
quick question doug and I notice that in my amp that one filament Glows Brighter then the rest. I did not try changing the tubes around. Love your videos Doug.Thanks c];-)
JazzyFizzleDrummers Greetings, JFD. In the circuits I am familiar with, if the output tubes are in parallel, they are not in a push-pull relationship. It is biased like a single-ended amp, but with two output tubes instead of one. You measure the resistance and voltage drop across the cathode bias resistor (assuming there is only one for both tubes). Divide the voltage drop by the resistance to get total plate current for both tubes, then divide by two to get the average PC for each tube. Other than the division by two, the process is like a SE amp.
Hi Doug, love your vids! I have a Gibson Skylark GA-5 crestline. It has a 6X4 rectifier en 6AQ5 tubes (pin5 is the plate). Which pin on the 6X4 can use for biasing like this methode? Pin 3 (not sure)? So pin 3 of the 6x4 to pin 5 of the 6AQ5? Hope you can help me.
Thanks, Ramon. You don't use any pin of the 6X4 for biasing. The 6AQ5's are both cathode biased by a single 270 Ohm resistor connected to their cathodes at pin 2 of each tube.
+Uncle Doug Thanks. I divided the voltage drop with the resistance of the 270ohm resistor * plate voltage and divided by two for the bias. But how can I check the bias on each tube separately in this case? The method in your video is not working for the 6x4 and 6aq5?
You are not following the procedure in the video, Ramon. You must measure the voltage drop across each half of the output transformer primary, not across the single bias resistor. Also, this procedure is only for the output tubes, not the other tubes or rectifier.
+Uncle Doug Yes, I am aware of that its only for the output tubes. At first I measured according your vid with single ended output tube amp and divided by two. But to be accurate I want to measure it like this procedure with the transformer. In your vid you use pin 8, but on my schematic it goes from the 6x4 to the center of the transformer and then to the two plates of the output tubes. So that's why I was confused what pin to use on the 6x4. I think I am going to watch your vids again. Maybe I was wrong sorry ☺️
I'm sorry if I misunderstood your question, Ramon. The high voltage to the OPT center tap should be provided by the cathode (Pin 7) of the 6X4, so you could measure the resistance and voltage drop from pin 7 to each of the 6AQ5 plates (pin 5). Be careful. Good luck.
Is the reason you primarily discussed cathode biasing because it's the more common type or is it about 50/50 with fixed bias? Could you also please answer a question about grid biasing. Do you still have to go thru the steps to determine plate disapation. If you use a matched set of power tubes, does each different model amp have it's own target negative grid voltage? In other words, would it be correct to say...to bias any fixed bias amp, all you have to do is measure & the grid for a specific voltage?
I discuss grid biasing quite a bit in my more recent videos on more powerful amps, Mr. G. Since all tubes operate differently, you cannot rely on the applied -DC grid voltage for biasing. The best criteria is PD.
Hi Doug, one question. In your experience, when you measure the resistance values of each side of the output transformer, are they generally pretty much the same, or is there sometimes a mismatch? I was just thinking, if they're a little mismatched and you install a set of output tubes which themselves are a little mismatched, then in one configuration the two mismatches would add together, whereas if you switched the tubes around the two mismatches would kind of cancel themselves out somewhat. I hope that makes sense. Thanks very much for all your videos, they're highly informative and I'm enjoying them greatly!
I have never seen an OPT with precisely the same resistance in both half-windings.....but the difference should not be too great or you begin to suspect damage within the OPT. Yes, you could try switching output tubes from one half-winding to the other to optimize balance
Uncle Doug, I'm about to build bias probes for EL34 / KT77 Marshall from octal bases and sockets I've recently ordered. There seems to be 2 ways to directly measure current when assembling. The majority of people seem to split pin 8 to connect leads exiting octal socket in series with meter (cathode). Based on some observations from your videos I'm thinking pin 3 (anode)would be a more accurate albeit maybe more hazardous option as you are trying to determine plate dissipation. Which method would you recommend.
All of the current that passes through the tube, exiting from the plate and screen, must pass through the cathode (pin 8)......and the cathode is generally at a low voltage potential, while the plate is quite high......therefore it is safer to use pin 8 with a mA meter in series. However, it is more accurate to use pin 3 (plate) because you will only read the actual plate current, not the plate + screen current as you will using the cathode. Safest, most accurate method would be to use pin 8 for total PC, then pin 4 for screen current......and subtract SC from total PC for net PC.
@@UncleDoug thank you much for the quick reply, I am x automotive tech now IT, thought that I was understanding the nuances, thanks for the confirmation. I have the ultimate in respect for high voltage and regardless of job description we have been required to study HV concepts as they relate safety in regards to Electronic Vehicles. I was reluctant to go through the trouble of building probes that still required calculations for accuracy and will ensure that 600V leads will be utilized. One benefit of splitting pin 3 will be to stack to another meter (fluke 87 and 88) for B+, eliminating the need to manually probe live octals. Again great series. Its amazing to see individuals such as yourself providing such content to anyone with the incentive to learn. My son is 17 and becoming rather proficient on guitar. I do not play but try to support his learning as a seasoned tech though not an "amp tech". We recently acquired DSL100H and already replaced preamp valves with Tunsol12AX7's, I'm not sure this was the best selection but it made a noticeable difference in sustain ( he was favoring amp sim on PC over tube amp ) from 5 year old ECC83's next , want to evaluate the EL34's so I'll make two probes for splitting pin 3 and one for splitting 8 and 5 in the same package. This should allow me to evaluate then balance tubes one side at a time 5,7 6,8 swapping for best match then balance both sides 5, 6 or 5,8 on a re-bias dynamically without need for tube tester which is out of circuit anyhow. Watching videos such as yours makes me anxious to add reasonably priced used oscilloscope to my bench. Thanks again.
@@UncleDoug I built the probes splitting pin 3, they worked as expected. Thanks. One caveat that occurred to me is what happens if pin 3 were to go open, bad meter fuse or something. It seems that B+ would still be present at the screen. The bias setup hopefully would limit current. I checked the schematic and screens are in series with 1K 5W resisters ahead of tube and data sheet says grid capable of dissipating 8 watts.
Yes, but the voltage drop and resulting plate current calculations will be doubled, necessitating division by 2 to determine the average PC for each tube. Using a single cathode resistor drop cannot provide accurate PC for each tube......only the average, which could mask a serious imbalance.
Uncle Doug I justify mine by declaring that I am building up and inheritance for my kids and grandkids! They can sell all this stuff which hopefully will have gained some value by the time I kick off!
hi doug great video thanks a lot .just one question ,do tubes loose there tonal qualities with age when compared with new ones? say hi to rusty for me ,
You're welcome, Martyn. No, the amount of plate current diminishes with time, due to reduction in the cathode's ability to shed electrons......but the actual tone doesn't really change as long as you keep the tube(s) biased properly.
Can we measure the voltage drop between the center tap of the output transformer to both output tubes's socket without the output tubes in place? Only the 5u4 is used at the power supply end? If from pin 8 of the rectifier, it goes through some Resisters before reaching at the center tap of the ouput transformer, so do we measure after the last resister? or still measure at pin 8? In case we use two 5u4 in series with 3A each (B+ output at pin 8 of the second tube), do we need a 6A capability winding from the secondary of the Power transformer? Thank you.
Since you use the voltage drop to calculate plate current, and no plate current is flowing when the output tubes are missing, the answer is "No". There will be no voltage drop at all without tubes in place.
To be honest, Sterling, virtually all amps with more than one pre-amp stage are cascading......the plate of one stage is connected (through a coupling cap) to the grid of the next tube. This is standard practice.
@@UncleDoug Oh, I didn't know that. I read Sweetwater Catalog, as an educational Tool. So, I've noted some text mentioning the use of Cascading Tubes. Thanks for informing me.
The best teacher in the world. Uncle Doug you are a legend.
Wow, thanks so much, Keith :)
I have to say that your videos are the absolute most thorough and detailed that I've ever seen; it's like getting a PhD in amps!
Wow.....we really appreciate your kind comments, FN. Thanks !!!
I have to cool down a customers amp that's running at 147% plate dissipation and have never had to alter the cathode bias resistor in any other amp I've worked on, until now. Uncle Doug, I now fully understand the theory, principles and practice to get the job done when it arrives. Thanks again for all of your videos, you really are my favorite Uncle.
We're glad to hear that the videos have been informative, VTA. :)
Uncle Doug, Thanks very much. I'm restoring an RCA 77V1. It's a 1947 console radio. Your video came at just the right time!
You're welcome, EW. Best of luck with your project :)
After a busy week, I can't wait for sit down in my chair with a glass of wine and watch your videos. Thank you for share all this knowledge.
You're welcome, Alvin. We hope you enjoy it :)
Why is it when you say something, it makes complete sense and easy to understand. Really enjoying your videos. I thank you for all the time and effort put forth!
You're welcome, B. We're glad to hear that the videos are clear and informative. Thanks !!!
After watching this video and the previous one, and the one on adding a bias pot to the Fender Princeton, a light bulb went on for me. I finally understand the concept of negative bias voltage! After carefully studying the Princeton schematic and that of my favorite Fender amp, the AB763 Deluxe Reverb, it all becomes pretty clear (I still have a long way to go) , in particular, how the bias board works. I have now elevated myself to the status of "potentially a little bit more knowledgeable than a rank amateur". By far, you present the most clear, concise videos on the theory of this stuff that I have come across so far. Thanks again.
You are quite welcome, Joey. Congratulations on your focus and increased understanding.
Doug thanks for so clearly explaining so much valuable info about tube amps and how they work. Everyone who tinkers with these babies should watch your channel!
Thanks so much, SS. Rusty and I appreciate your kind words and support :)
Thank you for the most intelligent and easy to understand tutorials EVER!
You're welcome, JJ. Thanks for your nice comments :)
Excellent videos you have Sir uploaded, I want to tell you how I enjoy your presentation, brilliant lectures, best I've ever seen in guitar signal amplification. Thank you so much for your time, effort and devotion to share with us. God Bless You
+Imre Bencze You're quite welcome, Imre, and thanks so much for your very kind assessment. I appreciate the time you have spent watching my videos and wish you the very best :)
@@UncleDoug your the smartest
@@stevemitchell9848 Thanks, Steve, but there are lots of smarter folks out there in TH-cam Land :)
Thank you Doug! This is THE best explanation of how to do bias for tube amps I've seen. I'm old and worked in electronics all my life, so you done good son. :) two thumbs up.
Thanks, CA. I'm glad the video was helpful :)
As usual, thank you for a great easily understood video. Reminds me of my great teachers at John A. O'Connell Vocational High School in San Francisco back in the 60's.
You're quite welcome, Larry :)
I need to thank you & Rusty for your very informative tube amp series of videos. In the last hour, I have learned so much by watching several of your videos. My 1965 Vibro Champ has been with me for 43 yrs, but now I know so much more about it. I own four other vintage guitar amps and I am looking forward to being able to replace the power tubes and biasing and maybe more. Be sure to tell Rusty I said hi. Great videos!
You're quite welcome, Brian. Thanks so much for your kind words. It's great to hear that the videos are helpful. By coincidence, I was just on my way to feed Rusty tonight and I'll be sure to convey your best wishes to him. Thanks for watching and best of luck :)
Uncle Doug - just discovered your amp series vids today - you have an incredible way of teaching this to I'm sure, many of us players who, well we learned over the years to "just play" but over time have been frustrated in not being as able to understand, maintain and for some, modify our own tube amps. Since one of my amps currently needs basic re-tubing, watching you biasing vid and intro to amps 1 & 2 is making the process so much easier for me now. Thanks for your teaching series and really appreciate the time you've taken to publish!!!
Rusty and I really appreciate your kind words, i6, and are glad to hear that our videos are helpful. Best of luck with your biasing project....and be sure to follow all the safety rules :)
An outstanding demonstration, thank you! Looking forward to part 2 and beyond.
Thanks, BT. I'm glad you enjoyed the video.
Uncle Doug Great videos. I was wondering would you mind to do a video on how to DIY switchable bias on the two ended amp? Can't find this mod for Bugera V55, as this cahnges a sound a lot from twnagy country style to nice and round bluesy, what I'm looking for, If you would need I have a diagram
Once again, an invaluable explanation by Uncle Doug. Thanks Doug I know I'll be using this (and part 2) as reference when biasing my own amps. (Keep 'em coming)
You're welcome, Quinn. I'm glad you enjoyed the video......and remember, one hand in the pocket !! :))
Thanks Uncle Doug for another great video. Very clear and well explained. I look forward to part 2.
Thank you, David. I hope to have Part 2 finished and posted today, if possible.
Uncle Doug,
Sir you are a veritable textbook of knowledge and have a remarkable ability to convey that information in concise and effective terms.
Your videos are simply wonderful.
I’m sure I am not alone in offering you thanks and praise.
Always looking forward to your next... and past videos.
Well done sir.😊👍
Wow, thanks so much, Bill. We really appreciate your very nice comments.
Thanks Uncle Doug, these are essential procedures for building a tube amp. Reading about how to do this, or having it explained and demonstrated are two very different learning experiences. Thank you! BTW I just built a 5f2a with an AA764 reverb, inspired by your vid about adding a Twin reverb circuit to a Supro amp. I never thought I would ever be able to do something like that.
You're welcome, Brad. I'm really glad to hear of your successful build, including the rather challenging addition of reverb. Keep up the good work :)
Thank you so much for sharing your time and knowledge. You're a great teacher!
Thank you, Scooter, and thanks for visiting my Facebook page.
Doug, you are a genius my friend!!! Thank you so much for sharing your knowledge in these videos
You're quite welcome, Joe :)
Great Vid! SUPER informative. Thank You Uncle Doug! Looking forward to part 2
Thanks, Jorma. I'm glad you liked it. Part 2 should be finished and posted in a day or two.
Thank you so much from europe, Vienna !
You videos are very good and clear spoken.
Anybody can learn from them.
I did understand, restored and bias my SANSUI AU-111 with your help.
You are one of the best teachers I ever seen on electronics.
Please make more Videos if you like to do them.
If you ever need something from EU, I will get it for you.
It is very nice to see how professional many People
in US work on the Ideas they have.
You're welcome, ZX. I am glad to hear that the videos were helpful. Thanks for your very kind words and best of luck with your projects :)
Hi Doug! I want to start building tube guitar amps and they have been watching your videos and learning heaps! I have a notebook and have been taking notes :) I want to thankyou for the wealth of information you are giving away and I really appreciate how well you cover everything! You make it so easy to understand and are motivating me to pursue this hobby. The comic relief in the videos is hilarious!
Cheers, Luke
You're quite welcome, Luke. It's great to hear that the videos are helpful and appreciated. Best of luck with your new hobby :)
Very Helpful, uncle Doug! Thanks for all the videos!
You're welcome, Paul :)
Thank you for this. So clear and easy to follow.
You're welcome, Ezra.
Thanks! This is all starting to make sense for me.
You're welcome, DN. I'm glad the videos are helpful.
Merry Christmas Doug thanks for all you do may God bless you this coming year
Thanks so much, Robert. Our best to you and yours.
Another great video, Thank you uncle Doug!
You're welcome, KM :)
Very well explained uncle doug. I will soon check bias on some of my amps soon. Thank you.
Thanks, Kevin. I hope your findings are all good ones :)
Sir, I thank you for all that you have done.great info with safety first advice
You're quite welcome, Mike. Glad you like the videos :)
Thank you Uncle Doug!
That was, again, extremely interesting and useful
You're welcome, PM.
Great videos. I have a matched tube suggestion for single combined bias. I mark and install the tubes at random....then measure or calculate the individual bias currents. They are seldom exactly the same value. Take notes, calculate the difference between the two bias currents for this (first try) tube's positions. Example Inside socket=32 ma., outside socket =29 ma., the difference being 3 ma of unbalanced current.
Then SWAP the tube's physical positions and do it all again, noting a second "unbalanced" current value.
It is likely one of these values will be smaller and therefore, a."better match".
Ultimately, install the tubes in the position that has the smaller difference in the bias current.
Works for me.
+dennis walsh An excellent suggestion, Dennis. I too have noted the difference in PC when tubes are moved from socket to socket. Thanks for your helpful input.
I have a question Doug... I have a Gibson GA77, which is negative grid biased. There is a 56k resistor and a 100uf polarized cap to ground on the negative bias supply (with the positive end on the ground side). There is no potentiometer in this amp, as you see in fenders, for adjusting the bias, so am I correct in assuming adjusting the bias would involve changing that 56k resistor to another value? Which direction should I take that value (up or down) in order to lower the plate dissipation of the output tubes?
Brad Linzy Brad, I checked the schematic and you're right about the bias voltage being controlled by the 56K resistor to ground. The higher the resistance to ground, the more negative bias voltage to the grids, and the lower the dissipation.....and vice versa. You could replace the 56K resistor with a 50K resistor and a 25K pot in series, to make it variable.
Thanks Doug. This amp had a LOT of issues that are now resolved after largely rewiring the entire amp. A previous 'tech' had hacked it and now she purrs. Thanks for clarifying what I thought I already knew. :D
You're welcome, Brad, and congratulations on a successful "save". Continued good luck.....
This is yet another enjoyable and articulate video, Uncle Doug. I build both instrument and hi-fi amps, and I enjoy your videos very much. I'm presently building a pair of push-pull 2A3 audio amps, and it's an exciting journey!
Thanks so much, Adam. Wow....2A3's. I recall them, and their "brothers" the 45, from my old jukebox and radio days. Best of luck with all your projects :)
@@UncleDoug many early 2A3s were just two 45s paralleled in one envelope. Both tubes make very beautiful sounding audio amplifiers!
@@UncleDoug Thanks! I do enjoy it!
Great video! Many thanks!
+SiliconSet You're welcome, SS.
This was extremely helpful! can't thank you enough. I have gotten less knowledge from DVDs I paid money for.
You're quite welcome, Brad. It's good to hear that the video was informative.
I have a Gretsch 6162 with dead reverb. Tank is good. I checked all the tubes too. They're good. Any hunches?
How do you know the tank and tubes are good? I ask because these are the "usual suspects", as well as the impedance-matching transformer that drives the tank. Check to be sure there is continuity in the primary and the secondary windings.
Thanks for the help. Just a heads up, if you ever work on this amp, just know there are at least 3 different versions I know about. Mine has a 6eu7 in the phase inverter and the rest of the 9-pin tubes are 12ax7. The schematic I found online shows no 6eu7. Also, there is another design which uses a third 6973 tube to power the reverb.
Brad, I'm glad the suggestion was helpful and you found the source of the reverb problem. Good luck with the repair. Also, your observation about the variations in Valco circuitry is right on the money. It's actually rare to find a Valco chassis that DOES completely match the schematic that is (supposedly) for it. In some cases, I simply draw my own schematic......it's easier in the long run.
Another well-done video Doug! I showed this to one of my customers, in order to simplify the answer to his question, and he called me afterward and told me that he wants me to bias his Fender Pro Reverb :) he said that any voltage over 12 volts scares him (LOL) :)
Thanks, Frank. I think healthy respect for the shock hazard of an amplifier is a good thing, but outright fear would definitely interfere with the biasing process. He's way better off having you do it :))
By the way, I just watched your video on the leslie speaker...I wrote a comment...at the end you'll find out something interesting :)
I'll go check it out. The Leslie video is, by far, my most popular video. There much be lots of Leslie fans out there in TH-cam Land.
Doug, Your skill as a teacher is astounding. Had I had access to these lessons in the past I am dead certain I'd have been happily producing amplifiers instead of generating 3 ex-wives all with the same first name of "Plaintiff" This morning I finally understood those resistors.
I here you, Pete. Glad the video was helpful :)
Love the videos Doug!
Thanks, RA. I'm glad you enjoy them.
Now I am really excited to learn! I have 2 of the same amps. One I keep original and the other I experiment with. I just installed a high voltage regulator board on that amp so that I can keep my pre-amp tube voltages regulated and keeping the filter cans still hooked up so that the ripple is clean. Next I will try to modify this amp with a Wing C EF86 tube right before the tone stack. Also my tone stack is before the inverter circuit which is good. Now I just have to figure out what values to connect to the EF86 tube. I need to find some schematics of someone else’s build of connecting the EF86 before the tone stack. I’m near 60yrs old and I grew up with tubes. Now I’m finally learning about them.
That sounds like an interesting project, Mark. Good luck with it.
Thanks Unc.
I wish Doug was my Uncle
mrGoesto11 We are all his nephews.
I like to think of us as one big, happy international family ;)
Uncle Doug is the BEST!
Wow.....thanks, Mark.
Nice. Great video.
Thanks, Joe.
Brilliant Video! thanks
You're welcome, JR. Glad you liked it. Rusty sends his regards to the kitty :)
Uncle Doug, I guess these are the reqired first steps to take before getting all connected with some groupies or whatever-just kidding. You have a great sense of pacing the information, and the sequenceof the same as well. Saturated🎵!
Thanks, John. We try to present information to others in the same way we would like it presented to us. Sort of like the Golden Rule :)
You are one of the best
Thanks, Svart.
Fan- effing- tastic. Thanks Uncle Doug.
Glad you enjoyed it, Pat.
Great! Again regards from Germany :-)
Thanks, OZ :)
Thanks for the quick reply. I am hoping that I may pursue this a bit further with you but don't want to become a pest. When I bought the amp the person I bought it from told me that the amp has the AB165 circuit and indeed when it arrived the tube chart fastened to the inside of the cabinet indicated an AB165 circuit. BUT when I opened the chassis up to take a look I found two 150 ohm 7 watt resistors connected between ground and the cathodes, along with a 5 - 50 capacitor in parallel with them. So after some digging online I found the schematic for the AC568 circuit and realized that my amp has that circuit, not the AB165. I know you said in one of your videos that this often happened - that Fender would use up the old tube charts even though the circuit had changed because the tubes were the same. The AC568 schematic also shows a negative 55 volt charge being sent to the grids through a set of components that appears to me to allow you to adjust separately how much voltage is applied to each grid. So the two 150 ohm resistors connected to the cathodes would suggest a cathode bias wouldn't it? But the voltage applied to the grids indicates that would affect the plate current as well wouldn't it? Again, I don't want to be a pest and you may not have time to pursue this but if you take a look at a schematic for the Bassman AC568 circuit you will see what I mean. I know that the AA 864 and AB165 circuits were designed in the Blackface era and the AC568 in the Silverface era so it makes me wonder - did Fender need to adjust some problem with the AB165 circuit or were they just "playing with the circuit for fun". BTW the serial number on the chassis of my amp and the dates on the transformers all indicate that my amp was made in early 1969. Hope you can help quench my curiosity but if this is too much trouble I understand.
Thanks for your excellent note and for bringing the unusual, hybrid-bias AC568 circuit to my attention. To be honest, I have never worked on one of these circuits. In a Princeton Reverb video, several years ago, I experimented with hybrid bias to perfectly balance two out-of-balance 6V6's. Applying different cathode bias resistance to each (I used fixed resistors, but you could use a 250 Ohm pot) resulted in equal plate current, while the (reduced -DC) grid bias helped to maintain a proper PC at idle. It worked and sounded great......but I caught all sorts of flak over the hybridization, some viewers saying "I couldn't do this". I wish I had known of the AC568 circuit at that time, to justify my experimentation. Bottom line: I think that this was Fender's attempt to do what I tried later, but perhaps the hybridization was like Harley Davidson introducing a water-cooled engine, simply not acceptable to purists......or possibly the tone was "different". Either way, it is an interesting modification for grid-biased amps and one that deserves more investigation.
Excellent, thanks you very much for your share and afforded.
You're welcome, PT :)
I watched this a few times now. You have cleared it up for me. Thanks for the video! I have obtained a tube amplifier with no identification or schematics. I have found that it is mono with two KT66 tubes in double ended. It's going to be a repair project. It needs new capacitors and I'm sure it will need a bias. I will have to look out for a resistor to ground on the cathode.
We're glad the video was helpful, SED. Good luck with your biasing project.....and be very careful. The voltages can be lethal if not handled properly.
Uncle Doug Thanks doug. I understand the dangers of high voltage. I am reasonably knowledgeable in that area. My amateur radio licence has taught me a decent amount in that area.
That's good, SED, because Rusty and I only want the best for our viewers. Best of luck with your project.
Uncle Doug thanks doug. I made a current limiter today also. Works just fine
That's good to hear, SED. You will find it to be a vital piece of equipment. Best of luck :)
Thank you very much for your videos Uncle Doug! Fabulous tutorials here. You are the best at delivering these informative lessons that I have ever seen. You should have a class room where we could sign up and attend the Uncle Dougs Amp-a-holics Unanimous University ! Thank you always for your willingness to share you knowledge. Keep on Rocking' - Perhaps Rusty could play some Stairway to Heaven for Us ?
Thanks so much, R55. Fortunately, my classroom days ended about 10 years ago when I retired :)
Yeah but you’re still sharing all of it with us! Retirement is the best slice of life for most of us!
@@Rodedog55 I agree, R55. Life begins at 65.
Thank you very much!!!
You're welcome, Ox :)
+Uncle Doug I do so enjoy your videos. On the GA-40 schematic you traced from pin 8 of the rectifier thru a filter choke to the center tap and on to the plates. So do we include the resistance of the choke in our calculations? I'm guessing yes.
Greetings, 998. Yes, you would measure the total resistance between the rectifier output pin and each tube plate (which would include the filter choke in this case).....as well as the total voltage drop across each of those resistances.
Doug , Sorry to have bothered you before searching it . Here's the explanation I found. The filament center tap if for "hum-balancing" so the AC filament supply doesn't inject hum into the amplifier. It typically goes to ground.
This is usually good enough in most cases, but if you want better hum rejection, it is customary to "elevate" the heater above the cathode voltage of the preamp tubes. In some amps, this is done with a voltage divider to set a fixed voltage, usually around 30-40V above ground. In low-budget designs, you can actually use the fact that the cathode-biased power tube cathodes are at a voltage anywhere from 10-40V above ground, so you can just stick the center tap there.
That's exactly what I would have told you, Michael. It's an old trick to reduce or eliminate hum with no extra components or expense. It's good that you did your own research.
I've never looked at amp circuits before but always been interested in how they work, so it's great to learn and you explain it perfectly. It's like being back at college but with a bit more enthusiasm. Is the gain on the preamp adjusting the bias to reduce the headroom and create distortion? Thanks for all the uploads and taking the time to respond.
You're welcome, RW, and thanks for the nice comments. Actually, the bias is adjusted primarily on the output tubes, not the preamp. If you bias the output tubes to a high level of plate dissipation, distortion will occur more easily and at lower volume.
These videos are a wonderful resource.Expertly stated, paced well and very user friendly. Thank you so much for this resource Do you offer any kind of a collection on DVD or DVD-r or maybe even a download? I wouldn't mind paying for it. I would like to have a collection that I could access offline for future reference.
Thanks, Greg. No, I have never produced a compilation disc, but all the videos are readily available if someone wanted to do it.
To minimize the current unbalance (even with brand new, "matched" tubes), plug in the tubes randomly and measure current or calculate the plate disipation (and note it).
THEN I swap the two tubes and do it a second time.
Then I install the tubes in the position that had the smaller difference, the "best match".
One "pairing" will almost always be "better" (slightly to notable) than the other due to differences in the tube gain, the output transformer (half) winding resistance, etc.
Can't hurt, takes an extra 10-15 minutes, mostly rewarming.
Good suggestion, Dennis. I often get a bunch of output tubes and try them in different pairings to find two that have very similar plate current. It takes some time, but it's worth it.
Uncle Doug, I have watched pretty well all of your videos explaining all of the principles of amp design and they have been very helpful. Thanks again for posting them! I recently purchased a used Fender Bassman from the late 60s with the AC568 circuit. When I look at the schematic for this amp it looks like the power tubes are cathode biased but it also looks like there is a potentiometer that somehow affects the grid voltages which makes me wonder what is going on?? I have watched all of your videos about how to check the bias for single and double ended amps. Could you explain the theory behind this design? By the way when I switched to a 3 wire grounded power cord I used your idea of turning the ground switch into an on/off switch for the negative feedback loop. Wow, what a difference. It is almost too "hairy" without the NFB.
Thanks for your kind words, Walter. The output tubes in a Fender Bassman are not cathode biased. The cathodes are grounded and a -DC bias voltage is applied to the tube grids to regulate the plate current at idle. The potentiometer allows you to alter the -DC bias voltage and thus adjust the bias of the tubes.
Once again, I have learned a lot (I think in only a few minutes). You mention the different pin assignments for EL84. Would it be a simple matter to adapt a socket so that it could be switchable between 6V6 and EL84, I happen to have a pair of JJ EL844s which are reputed to have less headroom--great for a relatively quiet and readily distorting amp.
I guess you could have two separate sockets, wired in parallel, one for 6V6 and one for EL84, and could switch between them. It would be easier, however, to simply insert the output tube you want to use and leave the other one out.
Excellent video. Please advise me, I have Cayin A55T push pull KT88 HIFI amplifier. In cathode circuit is connected resistor of 10 ohms to ground. Negative voltage on the grid adjusts the Bias. According to manual bias is set to 0.4 V across cathode resistor of 10 ohms (40 mA). I would like to try EL34 pentode, so is it safe to put EL34 instead of KT88 and adjust bias to 0.4 V? Pin 1 and 8 are connected together so I assume it will work.
Greetings, Munja. I am no expert on KT-88/EL-34 HiFi amps, but after doing some research, it seems like this substitution will work as long as the 1-8 pins are connected for the EL-34's and you adjust the bias for them. I also read that the KT-88's have a 100K bias resistor, while the EL-34's prefer a 220K resistor. If you can't achieve proper bias for the EL-34's with the 100K resistor perhaps you will need to change to 220K. Good luck !!!
Here is a link that addresses this tube substitution: www.diyaudio.com/forums/tubes-valves/168479-cayin-55t-want-use-el34.html
Thank you.
You're welcome, Joe.
Hi Doug, thanks so much for the tutorial. It's so well explained. I had a question about what would constitute a mismatched pair. I had a set of old Sylvania 6L6GC's that tested well on my tube tester so I though I would give them a try. I measured a Gretsch 6165 Variety Plus and got measurements of 69.8mA(outer) & 61.8mA (inner)of plate current. The respective PD results were 28.3W(outer) & 25W(inner)
Some people feel that a discrepancy of 10% or more between PD values indicates a mismatch, Jeff, but others seem to think that some degree of mismatch will actually foster the production of even-ordered harmonics. I tend to avoid any strict numerical cutoff point and just go with what sounds good. The purists might give your tube pair a thumbs down, but I have a feeling they probably sound just fine. Also, as you found out, tube testers are not a good way to match tubes.....they measure other criteria than PC and PD.
Got it! Thanks again Doug. As always very helpful. I think I'll try cooling them down a bit and hear how that sounds. By the way any recommendations on wattage and type or brand of cathode resistors.
With cathode-biased GC's you can go as high as 30W. If they are grid-biased (grounded cathodes) then 70% of 30W or 21W is the maximum.
As far as the cathode resistors, 2W is probably adequate. Metal film resistors are ultra precise and last forever.....virtually any brand will do.
Hi Doug, quick question if you please. I just finished a AB763 Deluxe Reverb kit build and when I turn the bias pot my plate voltage also changes quite a bit along with the cathode current. I can swing the PV as much as 50 volts by turning the bias pot from one stop to the other. Is this normal? If not, could you possibly make a recommendation as to where to begin looking for the problem. Love your channel and I've learned a lot. Thank you. Rob, ps: please put a link to your sponsorship program. I forget what video I saw it in and would like to help so the videos keep coming.
Greetings, RC. It is completely normal for the PV to go down when the PC goes up, and vice versa. When current flows, voltage decreases. Think about holding your thumb over the end of a water hose. What happens to the pressure (voltage) within the hose when you release your thumb and water (current) flows ? Your circuit is operating as it should. Now you must come up with a PV and PC that provide an appropriate Plate Dissipation value. Good luck.
Thanks for your offer of sponsorship. Here is the link to my Patreon site: www.patreon.com/uncledougsvintageamps
Thank mestre
You're quite welcome, Jorge.
will try to do this later with 6j1 small signal tubes. may be 2 of them in push pull config will provide some audible sounds.
not sure if i have audio transformer for this. may be will experiment with some mains to 6-0-6 or 9-0-9 ones.
thanks.
Good luck with your project, IB.
Excellent tutorial. I've been reading schematics for many years, but you make it a lot easier following your explanation.
RE: Your comment at 18:50 "much better way to bias each tube individually" .
Just wondering why you would want to add individual cathode bias resistors, since the traditional push-pull (class AB) output
circuit only works on the 180 degree phase shift of the output transformer.
Ie: When one side of the o/p transformer is positive 180 degrees, the other side (from the centre tap) will be negative, and the plate of the tube (at the moment) will not be conducting, ( it will be cut off) , leaving the other power output of the push pull pair to conduct, (usually in Class AB mode), the bias is set up in such a way that the o/p tube conducts long enough around the "zero crossing point of the AC amplified waveform" to avoid the crossover notch distortion.
So my question is: why would we even need separate cathode bias resistors in this case, since only one tube at a time will be conducting and drawing current through the normal single cathode resistance?
This is the way I have always understood it.. Have I missed something through the years?
+SuperCarver2011 The way I visualize the process, SC, it's like two people rowing a boat.....one on each side of the boat. They take turns rowing, and if one rows harder than the other, then the process becomes unbalanced and the progress non-linear. Since no two output tubes have precisely the same plate current (and thus bias) characteristics, biasing each individually will neutralize this inherent mismatch and result in a much more balanced output, when they each "take their turn at rowing the boat". The use of a single bias resistor simply "splits the difference" between the two tubes resulting in an average plate dissipation value that may well be ideal, but conceals the fact that one tube is actually biased "hotter" and the other "colder". Naive and simplistic, perhaps, but it does seem to work quite well. Also, I believe the traditional reliance on a single cathode bias resistor has always been a concession to economy rather than optimum electronic reality.
+Uncle Doug; But if you use matched outputs ( 6V6, 6L6GC) then a slight difference in plate current is (probably) tolerable for both tubes. Perhaps there are custom made boutique amps out there somewhere that have
individual cathode bias resistors, but for the most part, the output pair share one cathode bias resistor.
Leo Fender built his amps with one cathode bias resistor in his vintage amps, and these vintage amps command a high resale price these days, a lot of guitar players seek them out.
However, that being said, carry on Uncle Doug, I'll be tuned into the next part to hear your discussion.
It's a relatively minor consideration in the great scheme of things, SC. Thanks so much for your very knowledgeable input. I always enjoy hearing from you and look forward to further video rendezvous :)
Thank you Uncle Doug for your very clear and understandable explanations. I have tried your method of measuring PD through the center tap of the output transformer. It is a Twin Reverb "type" amp, with 4-5881s. Solid state rectifier. I get the following PD values: V7--9.85W; V8--9.85W; V9--8.87W V10--8.87W. The target PD is 23W@100% or 16.1W@70%. You see that the tubes are running cold, and causing low volume distortion. There is a bias pot on the amp--but the range of -V values it sends will not lift the PD measurements any higher. Could this be due to the value of the bias capacitor? Mine is a 50mf at 200V. Thanks.
LionBronstein You're welcome, LB. Since I don't have a specific schematic for your circuit, I will use a Fender Twin Reverb schematic to address your problem. The pot is not for bias, but for balance between the output tube pairs. The bias is controlled by the 15K resistor off the pot wiper....it allows a certain amount of bias voltage to go to ground. The higher the resistor value, the higher the bias voltage to the tube grids.
Now, assuming your measurements were accurate.....and they must be, or the output tubes will be in jeopardy.....the PD values are way too low, as you know. Check the value of the 15K resistor.....is it low? If so, replace it and retest the bias. If not, then are the 5881's old (their PC goes down with age)? If so, replace them and retest. Check the DC output from the diode.....is it around -37VDC? If not, replace the diode and the 50uf filter cap for this circuit.
After all this, if the PD is still too low, then increase the 15K resistor to 20K and retest.....continue in this manner with different resistances until you have a proper PD, which would be 70% of 23W (fixed bias tubes) or 16W. Good luck.
Uncle Doug Thank you very much for your quick, complete, and informative reply. I appreciate it.
You're welcome, LB. I hope the info helps.
Uncle Doug - I noticed as you reviewed the various schematics in this video that on some, one leg of the output transformer's secondary was grounded while others were not. Would you explain why or why not ground one side? I once was told that grounding one side would cause ground loop hum. Does it make a difference in single ended versus push-pull amps?
Thanks for your easy to understand teaching!
Carl Espy
One of the two connections to complete the circuit to the speaker voice coil is generally done by grounding the appropriate OPT secondary wire and also grounding one leg of the speaker output jack. It causes no hum.....indeed, it minimizes it.
Your videos are phenomenal, I’m learning so much! Quick question, I’ve heard that some manufacturers of cathode biased amps will bias the output tubes to 100% rated plate dissipation. Is this advisable? I thought 60 to 70 percent was the recommended target, at least for grid biased amps.
Thanks so much, Tony. Recall that cathode biasing is self-compensating and CAN be biased at or near max PD. The 70% figure is only for grid-biased output tubes.
Ampoholics anonymous, i don't understand why there are not more comments on this great phrase.
It was actually "Ampoholics Unanimous" at first, but we felt that anonymity should be preserved ;)
Hello Doug, I just started working on a Valco Golden Holiday and I got a bit confused because I believe the center tap of the 6.3 filament secondary goes to the cathodes of the 6v6 power tubes. At first I thought that it was a bias tap but I don't think that's right. I believe they are cathode biased through the 200 ohm resistor to ground. Could you tell me why they would attach the filament CT to the cathodes? Thanks.
This is a method of 'elevating' the heater supply. Rather than the heater voltage being centered around 0V, it is centered around a higher voltage, and the voltage at the cathodes of the power tubes is a convenient source. Heater elevation can be helpful in reducing hum, and is sometimes necessary to protect tubes such as cathode followers which have a high voltage on their cathodes.
Excellent answer, Silas. Thanks !!!
Hello Uncle Doug!
I want to retube a 100w Peavey Valveking Amp Head, it has 4 6L6GC´s and it ´s a grid biased design. I couldn´t get a matched tube quartet, I got two slightly different pairs: Pair A has Ip=39 Gm=4100. Pair B has Ip=39 Gm=4300
What do you recommend in regards to tube placement in the circuit? Pair A (Gm=4100) on one side of xformer and Pair B (Gm=4100) on the other side? Or sholud I mix one of each pair on each side of xformer? Thank You!!
I think the amp will work just fine with any configuration of the tubes you described......but, if you combine one tube from each pair in parallel, then the average of the two parallel pairs will be identical.
How do you recommend biasing hi-fi valve amplifiers? I get the method would be the same/similar but I am almost done making my own valve hi-fi amplifier and just need to know what sort of figures I should look for when adjusting the bias. With guitar amplifiers its a little bit lower than the maximum plate dissipation for the valves, but what sort of attitude should you have towards hi-fi amplifiers?
I would say that you're probably right on track at "a little bit lower than max. PD", Liz. You want good, clean tone with lots of headroom, and this should provide it.
Doug, another good video. One question: where does the table at 3:25 come from? And the information that max PD in grid-bias mode is 70% of PD in cathode-bias mode? Thanks again EJP. PS No Rusty?
You're welcome, QM. All of the PD data comes straight from the RCA and Sylvania Tube Manuals. Biasing at 70% of max for fixed/grid bias has been standard procedure for years. It provides a safety buffer as the PC and PD increase when the amp is driven. Cathode bias is self-regulating and no such increase occurs, thus they can be safely biased at (or near) max PD.
+Uncle Doug Thanks. I ask because I'm working on an amp which has everything at 100% rating: Va, PD, Ia, ... And it is grid biased. No 70% there. I was hoping it came out of a tube datasheet. I can see that Vak would be higher in a grid-biased amplifier, which may have something to do with it? EJP
You're welcome, QM.
Hi Doug. I’m struggling with the HH Scott 99D amplifier on determining if it is Cathode biased or grid biased. If you look at the HH Scott 99D schematic the cathode is connected to a 3K resistor (R47) to ground but it is also tied to 2 resistors, a grid and 375V. Also if you can figure out the biasing resistor would that control both tubes? Also, what would I set the bias for? Please help!
This is why I only work on vintage tube (guitar and PA) amps, Mark. This circuit is a convoluted mess, in my book. I believe it is cathode biased, simply because the cathodes are not directly grounded. I guess you could alter the value of R37 and R38 to achieve proper plate dissipation, and try biasing it as if cathode biased, but conservative.....say 75% of max. Try it and see if it works. Either that or seek assistance from someone better versed in HiFi amp topology. Best of luck.
The 6L6 was a popular tube for finals in guitar amps.
Yes, it was, Lyle.
Howdy. I have been wrong.
Fighting hum in my DIY 4 x KT88 amp proved that individual biasing is mandatory.
First I trimmed the anode current to 45 mA. After that careful tweaking of the individual biases produced the hum to drop under normal listening levels.
Checking the bias voltages revealed that the steepest tube needs about - 61 V and the most shallow about - 59 V. The individual differences seems to be large for KT88:s. Otherwise KT88 is a great tube. Together with good output transformers the sensation of presence is fantastic.
Regards.
Indeed, a serious output tube mismatch can indeed result in an audible hum.
Hey there Uncle Doug! :) Thanks a lot for all the videos that you're doing! You provide a truck load of good and well explained information!
I have a little question for you though .. as I'm studying an old point-to-point tube guitar amp witch I don't know the brand or model ( with tow 12AX7 preamp, 5AR4 rectifier and a pair push-pull 6L6GC or 6V6GT power amp **I don't know what are the original power tubes**), I noticed something strange in the power amp section.. There is a 1.2K 10W resistor between the plate and the output transformer on each of the push-pull tubes.. ?!? Since the output transformer isn't original ( the OT is a Hammond 125B witch is a 5W OT.. doesn't make sense!) My guess is that these 10W 1.2K resistors are there to dissipate some of the output power so that the "too small" output transformer wont burn... is this makes any sense to you? What do you think?
Also, If I replace the output transformer for a bigger one (wattage), is it safe to remove theses resistors?
Best regards!
Alex
Greetings, Alllx, and thanks for the kind words. The position of the resistors and their 10W power rating indicates that either someone was trying to protect a wimpy OPT (as you suggest) or trying to lower the plate voltage to the output tubes.....or both. Regardless, in a double-ended amp, the wimpy 5W OPT will have to be replaced with one that can handle at least 20W if you want to use 6V6's or about 40W if you want to use 6L6's. The two resistors will definitely have to be removed when you install the new OPT. Good luck !!!
Uncle Doug thanks Doug! this gives me some light for sure! Next stpe is the OPT swap! I'll probably stick with the 6L6GC, they sounds better to my Hears :)
Keep it up with the videos, this helps A LOT!!
You're welcome, Alllx. Good luck with your project. Be sure to let us know how it turns out :)
+Uncle Doug
I have an RCA 6-HF3 AM/FM/Phonograph that I am almost finished restoring. I had to replace the two 6V6 output tubes in the amplifier section. When I checked the bias it was at 114% of max. I replaced the bias resistor and the "grid stoppers" as they were both significantly out of tolerance. When I rechecked the bias it was still at 96%
I checked the bias because with the volume control at 2-3 (out of 10) the amp is very loud. At 5 is seems to be distorting. I'm thinking about reducing the bias to 85% - 90% for better tube longevity and to give me some room on the volume control. I'm not sure how that will affect the phono input. Do you think re-biasing is a good idea?
That sounds like a very reasonable idea, Michael. There is no need to bias any output tube at absolute max plate dissipation. I think you will be pleased with the lower bias levels you mentioned.....headroom should increase a bit and tone should remain good. There should be no problem with the phono input. Good luck.
Thanks Doug. Your channel is invaluable on a lot of levels. I typically haven't checked the output tube bias when I restore old radios. I will from now on. The RCA that I am working on is a '56 vintage. Two years older than me. Once I align the AM section, fix the FM section (it's dead), and re-bias the output tubes I've got to work on the turntable. That is going to be a new "thing" for me.
It's a good idea, Michael. Output tubes are under greater stress than other tubes in a circuit, and should have correct plate dissipation levels to survive and to produce good tone.
Hey Doug, I'm trying to bias my 1968 Sunn 200s. However, when I look at the schematic, I think there is a choke between pin 8 of the rectifier tube and the output transformer.
I'm assuming this will effect my readings for calculation of plate current?
Could I install a resistor in series with the wires from pin 8 to ground and measure that way?
Then simply take your readings after the choke, M&M. Best of luck.
hi, i have studied electronics for quite long, but nobody study those things nowadays, so these videos are really helpful.
i can't find the first part where you discuss single ended amps, could you link me it?
Glad you are finding the videos helpful, Matteo. Here is the SE video: th-cam.com/video/L3rRk3eSTnA/w-d-xo.html
Hi Doug. You said cathode biased and not Grid biased..Ah..on my schematic for the HH Scott 99D it shows R37 as part of the feedback loop and R38 as part of the AC balance circuit. Also important how do I know the correct bias current that the tubes should running at? I am going to replacing the 6L6GB with Tung Sol 6L6GC tubes. I have no idea if this is class A, AB or B in my output circuit.
Bias is based on plate dissipation, not current, Mark. It's determined by the tube and circuit. All 6L6GC's have a max PD of 30W. Whether you bias at 50% to 70% (grid biased) or up to 100% (cathode biased) depends on the circuit. Your circuit is weird. Perhaps you should see where the factory biased the output tubes in your amp and go from there. My answer was based on the schematic I found online.
It says this is part one but you say you recommend watching the first video? I've watched your excellent introduction to tube amps where you explain the fender champ. Is that the first video you refer to, or is there another video specifically on biasing?
The "first video" was the one how to bias single-ended amps. JC.
@@UncleDoug Thanks, Doug. I'll check that one out. Just so you know, your video on how Tube amps work helped me get my head around something I thought so complex, I'd never be able to understand. I've now started playing around with building small amps and I'm learning all the time. I'm loving the hobby. I'd wanted to get into it for years. I wish I'd had a teacher like you in school! All the best.
@@jc5454 Thanks so much, JC. Best of luck with your new hobby :)
When you measure the center tap of the xmfr to plates of the tubes you are really measuring the imbalance of the resistance winding of the xmfr which will appear as a voltage difference. When you measure the voltage across the tube you are measuring the current through the
tube. If the voltages are equal then the tubes are matched.
Some of what you say eludes me, Henry. Once you know the exact resistance and voltage drop across each of the OPT primary windings, you can use Ohm's Law to calculate the current through the tube. If the calculated plate current of both tubes is the same, then yes, the tubes are balanced.
Just checking but on a single ended amp can you measure the plate current the same way as you show in the second method threw the output transformer?
Yes, but it's generally easier to measure the voltage drop across the cathode bias resistor and calculate the PC.
@@UncleDoug ok just looking for the more accurate numbers . Thanks for the info
@@OldIronShops Try it both ways and compare the measurements, OIS.
@@UncleDoug will do
Have you done any videos discussing biasing on EL84 amps?
All output tubes are biased using the same technique, PP, depending on if they're cathode or fixed bias.
When you measure the cathode current it also takes in the screen current. There for the current for the plate resistance, impedance, you can find in the tube manual. It will also tell you what the screen current is. The tube manual will also tell you what the bias should be for any given plate voltage is and the screen voltage for the best operating point the tube should be biased at.
Thank you for your input, Henry.
quick question doug and I notice that in my amp that one filament Glows Brighter then the rest. I did not try changing the tubes around. Love your videos Doug.Thanks c];-)
I think it has to do with different tube brands and designs, Al......not a fault of the circuit.
@@UncleDoug Thank you Doug. I appreciate your Quick Response to my question.c];-)
@@ALtheDoctorWho You're welcome, AL.
Hey Uncle Doug, how do I bias a parellel push pull output stage (I'm using 6c33c triodes)
JazzyFizzleDrummers Greetings, JFD. In the circuits I am familiar with, if the output tubes are in parallel, they are not in a push-pull relationship. It is biased like a single-ended amp, but with two output tubes instead of one. You measure the resistance and voltage drop across the cathode bias resistor (assuming there is only one for both tubes). Divide the voltage drop by the resistance to get total plate current for both tubes, then divide by two to get the average PC for each tube. Other than the division by two, the process is like a SE amp.
Hi Doug, love your vids! I have a Gibson Skylark GA-5 crestline. It has a 6X4 rectifier en 6AQ5 tubes (pin5 is the plate). Which pin on the 6X4 can use for biasing like this methode? Pin 3 (not sure)? So pin 3 of the 6x4 to pin 5 of the 6AQ5? Hope you can help me.
Thanks, Ramon. You don't use any pin of the 6X4 for biasing. The 6AQ5's are both cathode biased by a single 270 Ohm resistor connected to their cathodes at pin 2 of each tube.
+Uncle Doug Thanks. I divided the voltage drop with the resistance of the 270ohm resistor * plate voltage and divided by two for the bias. But how can I check the bias on each tube separately in this case? The method in your video is not working for the 6x4 and 6aq5?
You are not following the procedure in the video, Ramon. You must measure the voltage drop across each half of the output transformer primary, not across the single bias resistor. Also, this procedure is only for the output tubes, not the other tubes or rectifier.
+Uncle Doug Yes, I am aware of that its only for the output tubes. At first I measured according your vid with single ended output tube amp and divided by two. But to be accurate I want to measure it like this procedure with the transformer. In your vid you use pin 8, but on my schematic it goes from the 6x4 to the center of the transformer and then to the two plates of the output tubes. So that's why I was confused what pin to use on the 6x4. I think I am going to watch your vids again. Maybe I was wrong sorry ☺️
I'm sorry if I misunderstood your question, Ramon. The high voltage to the OPT center tap should be provided by the cathode (Pin 7) of the 6X4, so you could measure the resistance and voltage drop from pin 7 to each of the 6AQ5 plates (pin 5). Be careful. Good luck.
Is the reason you primarily discussed cathode biasing because it's the more common type or is it about 50/50 with fixed bias? Could you also please answer a question about grid biasing. Do you still have to go thru the steps to determine plate disapation. If you use a matched set of power tubes, does each different model amp have it's own target negative grid voltage? In other words, would it be correct to say...to bias any fixed bias amp, all you have to do is measure & the grid for a specific voltage?
I discuss grid biasing quite a bit in my more recent videos on more powerful amps, Mr. G. Since all tubes operate differently, you cannot rely on the applied -DC grid voltage for biasing. The best criteria is PD.
Hi Doug, one question. In your experience, when you measure the resistance values of each side of the output transformer, are they generally pretty much the same, or is there sometimes a mismatch? I was just thinking, if they're a little mismatched and you install a set of output tubes which themselves are a little mismatched, then in one configuration the two mismatches would add together, whereas if you switched the tubes around the two mismatches would kind of cancel themselves out somewhat. I hope that makes sense. Thanks very much for all your videos, they're highly informative and I'm enjoying them greatly!
I have never seen an OPT with precisely the same resistance in both half-windings.....but the difference should not be too great or you begin to suspect damage within the OPT. Yes, you could try switching output tubes from one half-winding to the other to optimize balance
@@UncleDoug Thank you.
Uncle Doug, I'm about to build bias probes for EL34 / KT77 Marshall from octal bases and sockets I've recently ordered. There seems to be 2 ways to directly measure current when assembling. The majority of people seem to split pin 8 to connect leads exiting octal socket in series with meter (cathode). Based on some observations from your videos I'm thinking pin 3 (anode)would be a more accurate albeit maybe more hazardous option as you are trying to determine plate dissipation. Which method would you recommend.
All of the current that passes through the tube, exiting from the plate and screen, must pass through the cathode (pin 8)......and the cathode is generally at a low voltage potential, while the plate is quite high......therefore it is safer to use pin 8 with a mA meter in series. However, it is more accurate to use pin 3 (plate) because you will only read the actual plate current, not the plate + screen current as you will using the cathode. Safest, most accurate method would be to use pin 8 for total PC, then pin 4 for screen current......and subtract SC from total PC for net PC.
@@UncleDoug thank you much for the quick reply, I am x automotive tech now IT, thought that I was understanding the nuances, thanks for the confirmation. I have the ultimate in respect for high voltage and regardless of job description we have been required to study HV concepts as they relate safety in regards to Electronic Vehicles. I was reluctant to go through the trouble of building probes that still required calculations for accuracy and will ensure that 600V leads will be utilized. One benefit of splitting pin 3 will be to stack to another meter (fluke 87 and 88) for B+, eliminating the need to manually probe live octals. Again great series. Its amazing to see individuals such as yourself providing such content to anyone with the incentive to learn. My son is 17 and becoming rather proficient on guitar. I do not play but try to support his learning as a seasoned tech though not an "amp tech". We recently acquired DSL100H and already replaced preamp valves with Tunsol12AX7's, I'm not sure this was the best selection but it made a noticeable difference in sustain ( he was favoring amp sim on PC over tube amp ) from 5 year old ECC83's next , want to evaluate the EL34's so I'll make two probes for splitting pin 3 and one for splitting 8 and 5 in the same package. This should allow me to evaluate then balance tubes one side at a time 5,7 6,8 swapping for best match then balance both sides 5, 6 or 5,8 on a re-bias dynamically without need for tube tester which is out of circuit anyhow. Watching videos such as yours makes me anxious to add reasonably priced used oscilloscope to my bench. Thanks again.
@@UncleDoug sorry mistake I'll make two probes for splitting pin 3 and one for splitting 8 and 4 in the same package.
Sounds good, SR. Best of luck :)
@@UncleDoug I built the probes splitting pin 3, they worked as expected. Thanks.
One caveat that occurred to me is what happens if pin 3 were to go open, bad meter fuse or something. It seems that B+ would still be present at the screen. The bias setup hopefully would limit current. I checked the schematic and screens are in series with 1K 5W resisters ahead of tube and data sheet says grid capable of dissipating 8 watts.
Does the cathode measurement method used here also apply to parallel single ended biasing?
Yes, but the voltage drop and resulting plate current calculations will be doubled, necessitating division by 2 to determine the average PC for each tube. Using a single cathode resistor drop cannot provide accurate PC for each tube......only the average, which could mask a serious imbalance.
Ampoholics Unamimous! Too funny, you're killing me here! LOL
The best way to cope with an addiction is to confront it publicly :)
Uncle Doug I justify mine by declaring that I am building up and inheritance for my kids and grandkids! They can sell all this stuff which hopefully will have gained some value by the time I kick off!
Let's hope, Bruce. Either that or sell it ourselves in a few years and blow the money on Depenz and Viagra :)
Uncle Doug Too funny!!! Well said. ;p
hi doug great video thanks a lot .just one question ,do tubes loose there tonal qualities with age when compared with new ones? say hi to rusty for me ,
You're welcome, Martyn. No, the amount of plate current diminishes with time, due to reduction in the cathode's ability to shed electrons......but the actual tone doesn't really change as long as you keep the tube(s) biased properly.
Can we measure the voltage drop between the center tap of the output transformer to both output tubes's socket without the output tubes in place? Only the 5u4 is used at the power supply end?
If from pin 8 of the rectifier, it goes through some Resisters before reaching at the center tap of the ouput transformer, so do we measure after the last resister? or still measure at pin 8?
In case we use two 5u4 in series with 3A each (B+ output at pin 8 of the second tube), do we need a 6A capability winding from the secondary of the Power transformer?
Thank you.
Since you use the voltage drop to calculate plate current, and no plate current is flowing when the output tubes are missing, the answer is "No". There will be no voltage drop at all without tubes in place.
Uncle Doug, Just Curious. Have you ever done and/or considered doing an Amp with Cascading Pre-Amp Tubes? I believe Orange does this.
To be honest, Sterling, virtually all amps with more than one pre-amp stage are cascading......the plate of one stage is connected (through a coupling cap) to the grid of the next tube. This is standard practice.
@@UncleDoug Oh, I didn't know that. I read Sweetwater Catalog, as an educational Tool. So, I've noted some text mentioning the use of Cascading Tubes. Thanks for informing me.