Sometimes I watch some of your older videos again. They are entertaining informative and helpful. With safety in mind. I got shocked once and lucky to be alive past about 18 years old from one of my Ham radios uh never again +s my fingers. I learned from this video and successfully biased the 6V6 tube in my little champ. I promise I won't modify it any more or any one else's. But my champ is mine. It's done but I'll just maintain it as it is now. I hope you are still putting out a new video once in a while we all would like to see it out here on yt Land.
And this is when the light came on for me - no pun intended. You can read forums for hours, observing well intended people argue back and forth. It becomes a battle in my mind between trying to grasp the information itself and then sorting thru the emotion to see if there's a consensus of some kind buried in the arguments of the "experts." Save yourself weeks, maybe months, of forum scrubbing and please just watch this video. The Air Force paid good money to send me thru electronic principles and I still have a massive 3 ring binder full of my notes, but none of that helped me as much as this video. Bravo - well done my friend! Thank YOU AGAIN! *And my Champ thanks you too. 👍
Uncle Doug's videos are a masterclass collection, I love watching them and learning. And then there are quotes like these: "the last thing you want to do is die, while you're trying to bias your single ended amplifier"..
cmpmuller Greetings, CM, and thanks for the very kind words. Rusty and I are glad you enjoy the videos. I guess dying is indeed "the last thing" we'll ever do.....it would just be a shame to hasten the process ;)
I'm wondering if anyone has systematically assembled his videos to construct a primer on all things amplifier for the beginner ? I have begun such an endeavour, but TH-cam seems to revel in scattering stuff all over the map. Here's what I've got so far (unedited): UNCLE DOUG NOTES How Tube Amps work ‑ HOW THE AMP WORKS 1,2 th-cam.com/video/x5SSKX74DKg/w-d-xo.html th-cam.com/video/901iaPVVzY0/w-d-xo.html TONE STACKS 1,2, th-cam.com/video/BatwDYFJ9ug/w-d-xo.html th-cam.com/video/uZHM5BwPLRw/w-d-xo.html CATHODE BYPASS CAPACITORS th-cam.com/video/HrkYUPmEUhk/w-d-xo.html OSCILOSCOPE BASICS 1,2 th-cam.com/video/ueOup-XBexU/w-d-xo.html th-cam.com/video/UswkMHK27F8/w-d-xo.html OSCILOSCOPE AC/DV TRACING, RMS OUTPUT MEASURING th-cam.com/video/PmVq7NLv31s/w-d-xo.html TUBE REVERB 1,2,3,4 th-cam.com/video/EZFuT4liLAw/w-d-xo.html th-cam.com/video/3KEiQBaYZDc/w-d-xo.html th-cam.com/video/wTwTcjMYnAY/w-d-xo.html th-cam.com/video/pjTpiQG-ov4/w-d-xo.html IISOLATION TRANSFORMERS th-cam.com/video/5SjVm-rF39A/w-d-xo.html TRANSFORMER IMPEDANCE RATIOS th-cam.com/video/5jUitplchok/w-d-xo.html CAPACITORS IN AUDIO CIRCUITS th-cam.com/video/l-u8J-yh9ZA/w-d-xo.html th-cam.com/video/9UULx3-hHv4/w-d-xo.html th-cam.com/video/YXChioNC5NY/w-d-xo.html RESISTORS th-cam.com/video/LBtoarc6Nls/w-d-xo.html th-cam.com/video/8ruznIHuh7M/w-d-xo.html th-cam.com/video/QFHcmST7AZ0/w-d-xo.html th-cam.com/video/ORjsSfab278/w-d-xo.html th-cam.com/video/TvQWX0M6hJY/w-d-xo.html POWER TRANSFORMERS th-cam.com/video/MXgG-0MIBGA/w-d-xo.html PT BASIC DESIGN AND FUNCTION th-cam.com/video/hJsPQdAhTkQ/w-d-xo.html GUITAR AMP VACUUM TUBES th-cam.com/video/Xo4nVjj4NSE/w-d-xo.html th-cam.com/video/Zh_51RP4V6I/w-d-xo.html PHASE INVERTERS th-cam.com/video/U6By31V9fDo/w-d-xo.html BUILD A CURRENT LIMITER th-cam.com/video/wRFRwOnLsZI/w-d-xo.html HUM AND ITS POSSIBLE SOURCE th-cam.com/video/GrVtX0QGNls/w-d-xo.html BIASING DOUBLE-ENDED AMPLIFIER OUTPUT TUBES, PART 1: BASIC EXPLANATION & METHODS th-cam.com/video/w9B0Rhr_Y-E/w-d-xo.html
Thank you very much for your videos. You explain things so clearly that even people that don't have English as their first language can understand everything very well.
Uncle Doug, I completed this procedure successfully today without incident, thanks to your excellent teaching. When I measured 450 volts after switching on my Vibro Champ, I felt a good amount of respect, but I didn't quake in my boots because I had watched your video. I did the measurements for several 6V6 tubes and found the newer ones (JJ) were running much hotter than an old GE tube (which was at 12.5 W with the original biasing). I also liked the overall sound better with the GE, so left it in there. Once I have to switch to JJs, I will have to rebias the amp because they're running at 15.5 - 16 watts of plate dissipation. I know how to do that now. The measurements were interesting. I also took the opportunity to glue the tolex back down and polish the front, using methods I learned from your videos. The amp is shiny and sounds great. I'm using a step-down transformer with this US-made amp, since I'm in Europe. I ran into some interesting polarity issues with that. If one turns the (Schuko) plug the wrong way around, the step-down transformer outputs 234 volts instead of 117. Not very nice. I believe many people aren't aware of this. I got myself a model with a polarity check so this doesn't happen anymore (it didn't hit the amp, I measured first). What is the result of running an amp made for 117 volts on 234 volts instead? I wonder.
Greetings, KD. A PD of 12.5W is still excessive and 16W is crazy. I tend to keep the cathode-biased 6V6's at around 11W. If 115VAC input can produce 450 plate volts, then 234V would yield close to 900 PV......which would breach all the capacitors and destroy the tubes.
@@UncleDoug I guess I might change the resistor then. I suspect previous owners in Europe didn't pay any attention to the plug orientation, so I'm surprised the amp still works and nothing looks destroyed inside... I should probably measure all the caps, though.
@@kneedeepinthedoomed Plug orientation is a safety risk but does not threaten the circuit components, KD. Increasing the bias resistor value will result in better tone (in my opinion) and longer tube life.
I am going through these videos slowly (about one every 2-3 days) and they're helping me get through these days, while educating me at the same time. You're a great teacher, and I appreciate you sharing your knowledge. Thanks Uncle Doug! :)
@@UncleDoug well you're here now and digital communication allows us to be in contact. I quit working as a technician 30 years ago. Digital technology made myself a thousands of other guys redundant. I worked maintaining telephone exchanges which were mostly relay switch operated. I also worked on big dc power supplies and have melted a few tools - lol
Hi Doug: I was about to correct you on a couple of points but I read the description and see that you cover them in your notes. Excellent tutorial and WOW what a clean little Gretsch! Usually the ones I've seen have been shall I say "faithful companions". Cheers Mike.
Just wanted to thank you for your awesome videos! An electrical engineer friend of mine (guitarist, electronics tinkerer) recommended your channel as the best way to learn tube amp electronics--he was totally right. You're a fantastic educator.
excellent doug,,this is the best i have seen,,i worked in electronics production,but i forgot all this stuff,,now im ready to get in there and test my amp,,i want a warmer sound and mabey this amp is set different,,thankx again do more old school electronics,,no one knows much about this stuff in the computer age we live in.
@@UncleDoug I had A-Fib for six years. THAT is like an earthquake going off in your heart, and it is horrifically miserable. Got that fixed. Now I just have six blown discs in my lower back. It's been a good past 15 years ...
Hi Doug, this is the first comment I have ever posted. I wanted to thank you for this video and the follow-up video. I have been playing guitar for 30yrs and during the last few years learning about amps. I had found it very hard to find a thorough demonstration of the biasing procedure even though I understood the theory around it. Its really good that someone with your experience is posting this type of experience. Warm regards John C
I am not into guitar amps; the plan is to build some HiFi tube amps, but principles stay the same - please correct if am wrong. However, your videos do help me understand what is going on in those circuits, how the electronic parts work and why. At school was I never really interested in physics (… it's just stuff that happens to some other stuff and it'll still happen without me understanding it …), but now, I want to know about it, and your videos do help a lot, they're epiphanic to me. Thank you very much for your effort in spreading knowledge in such a comprehensive and kind way. Please give your four legged companions an extra treat from me!
Apparently my little Monoprice 15w is set cold. It's got a test point and adjustment pot inside, from what I understand. Headed to watch the double ended vids next in hopes of learning to check and adjust mine. The last post I found at TDPRI said this guy's was around -5.86v and he set it to -12v but I'm not just going to accept that as correct until I learn how this all works and whether it actually is correct for 2 EL84s Edit: so I found the schematic and it plainly says 'set to -12.5V' at the bias pot. So I guess that settles that?
One of the things I really appreciate about these videos, besides the learning opportunity, is the fact that you leave your mistakes to be seen. The disclaimers and corrections are added later over the top. It greatly increases my faith in your presentation and adds to the humanity of the whole process. Good stuff.
Thanks, Keith. Unfortunately, TH-cam has decided to eliminate any notes or corrections within videos. So I guess the mistakes will have to go uncorrected :(
This video brings back a lot of memories of back when I was a U.S. Navy electronics technician...it has been a long, long, time since I worked on any tube type equipment(!)
Very helpful. I am writing notes as I watch to keep this all straight in my head. Have been away from tube amps for a while, so it's good to refresh. I learn more each time I visit.
Thank you Uncle Doug for sharing your knowledge Uncle Doug without charge you've been very helpful to me your friend Dwight from AllenPark Mi. I'm 62 now but I wish I would have been a student in your class. Your worth listening to thank you for teaching the wright way. You R A presious. Joul. Thank You.
Thanks Uncle Doug, this is the clearest explanation of cathode biasing of single ended amps that I've found so far. I really like the way that you explain things. I know have a much better understanding of biasing and it's relationship with headroom.
Well I have watched this a few times and I'm starting to get it. My 6V6 on the 9002 is perfect and removing the strange air gap resistor caused the thing to just start working. I am amazed, I actually fixed this thing... ;) Thank you Doug
Very good explanation on this Output Tube Biasing Uncle Doug. This makes it much more clear to me. I even built a Current Limiter for testing amps now. All such great information. I really enjoy all these videos. I recently repaired a Capacitor issue in a vintage Trend LR19952 - 801 5 watt Guitar amp made by Pine Electronics Products co. LTD Montreal. They were built in the late 50s up to the early 70s out of surplus electrical parts. There is no input transformer but uses a 35W4 Rectifier 12AX7 input & a 50C5 output tube. 8"speaker, 3 input jacks, Tone & Volume control & a 2 prong cord. The Chassis Ground actually operates with 39 DC volts. It actually sounds pretty good for a vintage small amp. I can send you a schematic if your interested & pictures. Thank you kindly.
What would happen if you earth grounded the chassis with a 3-wire power cord, Kevin? If the circuit still functioned, most do, this would greatly lessen any shock hazard.
I actually tried earth grounding the Chassis & then I received a loud hum over the speaker. So I measured the Chassis to have 37 Volts DC. I guess it is the way it is designed. Quite different from traditional amps I can see from your various Schematics you have illustrated. I am careful with the amp & only use it in the living room on the carpet. My uncle gave it to me when it had the Capacitor Hum issue. The Guitar store told Him to ground the Speaker Chassis to the ground on the cord but no improvement of course. They did not want to repair such a vintage tube amp & sold him a new one. lol I discovered it had a paper unicap containing a 50 mfd 30 mfd & a 20 mfd on a 150 Working Voltage. I had to wire 3 independent Capacitors in as they do not make the 3 in one capacitor any more that I could find. I removed the extra Ground Wire from the Speaker Housing as it was not designed to be there & then sounded better yet. Sounds good now with only a very minor hum that is likely normal to its design I believe. Thanks for your interest & reply. I tried cut & pasting the Schematic to this chat box but does not work. Thanks Uncle Doug & now going to watch your Part 2of Grid Biasing double Ended Tubes. Take Care.
Doug, all of your videos are great but this one is fantastic! I can't wait to use what you've shown here in my own builds. Oh yes, great advice: "Don't die while biasing your tubes. It's a real downer!"
Hope this is not getting redundant, but thank you so much for all these great videos. I/We really appreciate your love of teaching, and all the little gifts you give us!
I am new to vintage tube amps and found this lesson to be of great value. I always wondered how one would modify an amp to have more or less "head-room". I always wondered what caused an amp to distort with manipulation of the electrical components. Now I know, thanks to Uncle Doug. I really appreciate your lessons, videos and topics covered.
Holy moley UD, I went to the URL you suggested and boy do I have a lot more studying to do. But at least I'm seeing a little light the further down this rabbit hole I go. Or should I say rabbit tube? It's like an alternate universe down here. The more you learn the more you need to learn... magic... And a little trippy too.
Indeed, if you really want to go into detail on topics like this, the depth of information is almost unlimited. You need to ask yourself.....how much do I really want and need to know ?
@@UncleDoug I've said this before, and I'll say it again. Your advice is always spot on. At this point in time this is a perfect fit for me. I hobble around some, but I will soon get my ankles fused and after that I should be walking around normal. No worries :) until then I have some vintage test gear to work on and plan on repairing electronics for a living. Doing some of that already, mostly computers, but there is something about the sound of my guitar playing through that crazy Eico Signal Tracer... sounds strange good. But your right, this bunny hole is bottomless. So I've decided to focus only mostly on the Fender 5F1 & the Eico 147A schematics. So that is why I am very grateful for your first rate "tubetorials". Keep 'em comin! BTW I will be happily throwin down some well spent clams on your patreon as soon as I am able! Thanks again!
Thank you Uncle Doug A million times over! I've been ignoring this subject matter for 30 years for 2 reasons; at 13 years old I had better things to do, like learn my instrument (guitar). Now that I've kinda done that, I'm obsessed with finding that tone that gets to me down real deep and you are helping me understand how and why I'm on my quest and believe that I'm closing in on it.THANKS HOMEBOY! Now I'm one step closer to having a meaningful conversation with the other UNCLE, the one and only legendary "UNCLE ALBERT" here in my home town of INDIANAPOLIS. Some of you out there may have heard of him (heh heh heh) Thanks again Sir!
Will Do UD! Might you inform us (if you can)a tad bit about U. A.'s recently patented 'Del Ray' tube amp? He told me he made 25 of them, all 10W amps, 8 or 10"(not sure which) with 'plenty of headroom" for my cleans. I could not get the $ quite fast enough, but I MUST have one. I WILL have one! HEHEHEHEHE!
Since they are so hard to get, WT, it sounds like you have a better shot at getting one than we do.....so if you do get one, please let us know what you think of it.
Will do UD- I will call UA first thing after he opens and ask (cross your fingers!) if he will let one go to me IF HE Has one. Very least Ill get more specs. Might get the link to the schematic. Peace on Earth okay? Later!
Wow 👍 I picked one of those up at the goodwill store for 7 dollars awhile back soldered the speaker wire's back on and cleaned the controls and it sounds great
Awesome info , I have a bit of learning of electronics and this is much more straightforward than other biasing issues, also more understandable, thanks.
I use to dislike electronics, but I needed to know more and more. Lucky for me I found your channel. Now I find it interesting and intriguing and have already carried out some successful repairs. I would like to build my own amp someday, but I am a long way from that at the moment. Thanks for sharing all your knowledge.
Thanks, you explain topics really clearly, it's amazing how you improve circuits and your knowledge of electrics seems endless. I have learnt some really interesting topics here on your channel about tubes, love it. Having said that it remains a little abstract thinking about what electricity is and how it travels through copper wire. Hats off to Tesla!
Hello Doug, I have a Vibrolux reverb custom amp which I converted to an eyelet board and have wired it like the AA964 schematic. Two weeks ago I wanted to check the bias to the tubes and found it cold, so I lowered my resistor value at the bias pot so I could get a higher current value and when I powered the amp I blew a fuse. Well when inspecting the circuit and chassis I found a piece of heat shield by the heater and pilot light section. This came off from above, I guess I caught a piece when pulling the chassis in and out. I then troubleshoot and found the power tranny was shorted. I then installed a vibrolux tranny that was a Weber brand and had to remove the end bell where the wires come out of in order for the tranny to fit. The amp powered on and my plate voltage was now 384 as opposed to the 430 volts I had on previous transformer. I like the lower plate voltage. I am running about 42 ma and using the power dissipation formula comes out to 16 watts for 6L6GC tubes. With the plate voltage running at 384 can I use the 5881 tubes? Thank you for you time and videos
Both the plate current and plate voltage are rather conservative for 6L6's, so I would say, yes, 5881's would be safe in this circuit. Simply recheck the bias with the 5881's installed and verify that the PD is within acceptable limits. Good luck.
@15:33 > Tube plate dissipation is calculated as follows : (Vp - Vrk) x Ip. Uncle Doug measured the plate voltage to ground as Vp, but did not subtract the voltage drop across the cathode resistor , Vrk = 14.8 volts . So the Vp used was 14.8 volts too high. Using the corrected plate voltage Vp of (approx) 296 - 14.8 = 281 Vdc shows that the tube is biased at 281 Vdc x 0.0415 mA = 11.66 watts. This video was an excellent presentation, aside from this error.
Thanks, PC. This was a very basic presentation. In all subsequent videos the PV has been measured between Plate and Cathode......even though it only matters in cathode-biased amps ;)
Many ppl have harped on this point but U Doug DID actually explain this in a 'roundabout' way at @18:32 in this video where he mentions that he measured across the cathode resistor (meaning from plate and over the cathode resistor) and thus the calc with a higher voltage will err a little on the pessimistic side, showing a slightly higher pwr dissipation than it really is. This means you won't be tempted to run the tube at the max limit. 😁
I really enjoy your videos! Great slow speech and thorough explanations without being too slow and never boring. I've built a similar amp using a 6L6 driven by a 12AX7. I'm using solid state rectification and resistors for voltage regulation- no chokes, just simple. The output transformer is rated at slightly higher plate impedance than what this tube is rated at using 292 volts on the plate. It dissipates about 11 watts bias and sounds great! I can't seem to find many 6L6 single ended amps.
Thanks, Ryan. I too have found very few SE 6L6 circuits. A PD of 11W is rather low for a 6L6 in fixed bias and real low if cathode biased.....yet it sounds good. That's the most important criteria ;)
@@UncleDoug I only used the 6L6 because I bought several at a yard sale for 50 cents each! I'm curious how it would compare to a 6V6. I didn't have any until yesterday I bought a Challenger HF8A which uses a pair of 6V6's in P.P. Can't wait to get started on the 1950's gem!
I have the 6156 version of the amp in your video. After watching the video again today I thought, what the heck, lets measure the plate dissipation. I followed your directions except I measured plate voltage across Pin 3 and 8, and came up with very similar numbers as you did. Plate dissipation is slightly lower at 11.28W. Here's the funny part. I've watched this video before but sometimes I like to watch informative videos like yours more than once. I have a schematic in the bottom of the amp. Guess what I wrote on it (last winter?)? Plate dissipation calculation values, except I used the method you used in the video. I came up with 11.87W that way. :)
Measuring the plate voltage between the plate and cathode, as you did, is the more accurate method, Michael, and will give you a slightly lower PD than measuring from plate to ground as I did in the video.
Uncle Doug. This is just a general comment about all the videos you have posted. I was looking for a video on how to cover a speaker cabinet with vinyl and I hit 'Pay Dirt' as I want to build a small valve combo amp from scratch, but I have struggled understanding any electrical theory from books. The joinery side I am fine with Thank you very much for the most informative and easy to understand films I have watched in a long time. They are very inspiring. I have just watched your video on identifying the wiring on transformers so I am off to find a flea market that has old electrical items for sale. Rob
Thanks so much, Rob. Rusty and I really appreciate your very kind and favorable appraisal of our work. We're glad that the videos have been helpful and wish you the best of luck with your projects.
Hi Doug! I like the way you explained cathode bias! I need to show this video to some of my customers...I have a tendency to forget that almost everyone I explain bias to is NOT a technician...I always forget and think I'm talking to another tech (LOL)...If the amp you used in the video wasn't a collector's piece, I would say you could install a potentiometer to vary the cathode bias, which would make it a lot easier to tweak when replacing the output tube...I've done that to many newer single-ended amps with fixed bias...I've also used extra resistors to limit the pot's range in some cases...Keep up the good work and pet Rusty for me :)
Thanks, Frank. Rusty sends his regards. I agree with the benefits of installing a cathode bias potentiometer, in fact I posted a video about the "adjustable bias" concept a while back. In this case, as you said, I didn't want to alter what was otherwise a pristine original amp.
I just found you the other day. Im loving learning about tube amps. I would never try this at home but its fun to know how the things work. I have a marshall DLC5 , fender blues jr and a boogie mk5 35. cheers
Outstanding video. Thank you so much. I just built my first champ from a kit. You're helping understand so much. I can see how addicting this can be. Bart
redfishbum You're welcome, Bart....glad the video was helpful. A Champ built from a kit is a great way to start what may soon become a very addictive hobby. Welcome aboard :))
Hello Uncle Doug. Thank you for this excellent tutorial. I gave you attribution for this in a video I did where I used this method to check the bias on a IF Amp tube in a '53 Chevy Radio (Part 6). Just wanted to let you know I let everyone know where I got the technique, and a link to this video. Thanks again. [edit: spelling]
I'm so glad I found your videos, clean cut and to the point with laymans terms. Im hoping to build a BFPR soon for my next build and I loved your two recent videos on your BFPR
Very educational video on biasing an amp. Thanks, I now feel comfortable in checking/adjusting tube bias on my BF and SF VibroChamp's, A good way to think of the bias resistor, for me, is to think of a dimmer switch for a light bulb. Increase resistance dimmer bulb= lower output, decrease resistance brighter bulb= higher output.
@@UncleDoug I have a question for ya. My Blackface VC is alot darker sounding than my early 70's Silverface VC. The Silverface does have a Pyle Driver speaker, model MH810, in it. Other than that and new Tung Sol tubes both have all original components. Is that normal between the BF and SF VC?
Well I finally gave the Blackface VibroChamp a good going over to find out why it is so dark sounding. The problem is the speaker cone is shot, kaput, finished. Guess it's time to order a Weber 8A125
This was great. A very helpful refresher. And a new technique for me regarding measurement of the voltage drop. Keep up the good work, sir. (Also just a fun note - I picked up some parts off our local classified ad website last week, and the guy with the ad had linked to one of your vids as an example of a possible build using the parts. Of course I immediately recognized the Uncle Doug video. So just FYI, up here in Winnipeg total strangers are watching your videos, being inspired, and chatting about amps amongst themselves. Figured you might appreciate knowing that.)
Wow....thanks, Dr. S. It's good to hear that Rusty and I are now the spokespersons for a Canadian classified ad website. Today, Canada.....tomorrow the world :)) It's also good to know that people are chatting about tube amps. It's a great hobby and the more people who become involved, the better. Thanks again for a really nice comment :)
My first amp (used)was a Gretsch 6152 which has tremolo and Reverb. Years ago I removed the tubes because it was distorted at very low volumes and not knowing anything I thought one of the tubes might be bad however the 6V6 had broken the alignment pin because of age and I put the tube in one pin off. I bought a new rectifier and power tube and it would start playing very clearly for the very first minutes and then suddenly gets very hot and get distorted. So I shut it off and have not looked at it since. After looking at your video I was checking my 250 Ohm resistor and it crumbled in my fingers! There is the root of my problem. So of course I have a couple resistors coming in and I can't wait to replace it and see how it operates. Thank you so much for all your help.
@@frankscutari9516 I hope the new resistor solves the problem, Frank, but I wonder why the old one burned up. You might take a close look at your filter caps.....one or more may be shorted.
With the way you explain things it does make alot of sense. Thanks. I'v seen this vid. Of yours before But as a matter of study and learning I watch it again. Who's afraid of alittle Elesstrigerty ha. Don't touch! Oooh ah Pow boom yyooowoo I learned that lesson the hard Way back when I was 18 yrs old on an old Radio that I was repairing it didnt stop My heart but it was really beeting hard fast For about 5 or 10 minutes the shock Was painful. Guess I got lucky. Don't Forget REO spit. wagon. Dwight.
I haven't read all comments. Maybe somebody noticed before me, but: If you measure current thrugh cathode resistor, you actually measure sum of all currents in the tube. Cathode emits electrones and they go somewhere -> as your tube is 6V6, it's tetrode so you have measured Ia+Ig2 (anode + grid #2 current). Not only plate current as you said. So you have to subtract: Ia=Ik - Ig2 As you don't have resistor in series with g2, disconnect it from supply and measure with amper-metre. Than subtract. g1 (control grid) doesn't pull any current as long as it's on lower potential than cathode. It will drain some current if we had cathode follower before it and driving grid with positive signal. (Not for guitar but hi-fi) Plate dissipation: Pa = Ua * Ia -------> Ua=anode (plate) voltage=voltage between pins 3 & 8, not 3 and ground, as cathode is raised +14.84V above ground; Ia = Ik - Ig2 That's it.
+Ivan Horvat Yes, Ivan. The cathode current does include screen current and to be completely accurate, the SC should be subtracted and multiplied by the PV from plate to cathode to determine the PD.......but recall that this was a "basic" video and I chose to simplify the process. By using the cathode current times plate-to-ground PV, the PD that results will be a bit high, providing a safety factor for beginners. Thanks for your input.
Wonderful demonstrations and information as always with your videos, Uncle Doug. You make all these tube amp mysteries very clear to me, after struggling with understanding this material for years! Thank you very much for these invaluable videos, as cmpmuller said, they are truly a master class collection.
Thanx for another great and instructive video/lesson... And, for me anyway the typos wind up forcing my brain to work a wee bit more, and odd though it may seem, the overall learning experience was enhanced... thanx again and cheers
yes this was a really informative vid for me. i have a hand made Purdy "tramp" and changed the power.tube in it. the manual says i dont need to get biased but does run hot aftef a few hrs playing. breaks up around 6 on the volume. i might just take it in to a pro and see where the voltage is really at. thanks Doug :)
You're welcome, Scott. If your amp's output tube is cathode biased (check the literature or ask the builder), then it probably does not require re-biasing......but, if it's fixed bias (the cathode is grounded, which is quite unusual in single-ended amps), then a new tube must be re-biased.
That is a great demonstration of biasing with this type of method. The only concern is that carbon resistors have a tendency to drift at +-10%. I normally find the resistance goes higher, even though your readings are well over the 330 ohms it still well within the 10% margin as ohms law demonstrated with the dissipation factor of 12 watts with that particular tube, nicely done.
+John Cunningham Thanks, John. I agree that carbon resistors have their limitations, but lots of purists stick with them in their quest for "vintage tone". I personally prefer metal film resistors, especially for screen grids and power supplies.....but in this application, the carbon resistors seem to work well.
I just recently acquired a B&K 747 Tube Tester; the first thing I had to do was remove a handful of carbon resistors that had a tolerance of +-10% and were way out of spec. I ended up using metal film resistors that were rated at +-1%. On top of that endless amount of work went into it before calibration procedures could be performed. Yes! I do understand the nostalgia when it comes to preserving the original components, but in order to fix these amps accuracy is important along with reliability.
Yes, indeed. I tend to believe, however, that much of the so-called "vintage tone" that so many musicians seek is due to the variance from specs within the circuits of old amps. The sum total of all these variances yields a somewhat unstable and "dirty" tone that many people truly appreciate and seek to emulate. In a "great" old amp, the variances all work together.....but in some, they don't. Tweed Deluxes from the 1950's are an excellent example. Unmolested, most are rough, raw, and wonderful.....but if you replace all the carbon resistors and waxy caps with metal films and modern high-grade caps....they become much more sterile and (to many) less interesting.
wish you had a tube amp builders class... oh wait, this is it. good video... coincidentally I had a scare with a small arc.. discharge your caps folks....
i concur, another awesome video! it's great to finally see one on cathode biasing, rather than the typical Fender trim-pot videos (which are a dime a dozen.) i started a new thread and reposted the video over on the TDPRI amp forum: i think it'll help out newcomers quite a bit, since SE cathode biased kits are so popular, as well as stuff like old Champs scored off Craigslist and that sort of thing. can't wait for the next one!
Thanks, Cat. It's always good to hear from you. I think the next installment will be cathode biasing with dual output tubes. It's much the same process, but with a couple of extra steps. I also hope your Melodier project is going well.
Uncle Doug my computer decided it needed a new hard drive, so i had to put off getting my transformers for a couple weeks. minor setback and no big deal though. who knows, maybe i'll get lucky and Triode will have some sale prices coming up (i can always hope ;) .
Transformers are always a major expense in any amp project, but if you get them from Triode, it's less painful than from other sources. Also, If you get them around Christmas, they'll be a nice gift (to yourself)......way better than a lousy pair of socks or underwear.
cheers, again Doug thanks for another great video, as always from you. very informative, toke me back to my collage days. it good to get a refresher on basic principals every now and again. after using transistor, and now IC chip you forget the basics. will deff help me in my projects.
Hey from Japan, Doug! Enjoyed this video explanation, subscribed, and watching your others ... thank you! Got bitten by the amp-building bug a few years ago, built a few Champs and a 5E3, and am now planning either a Deluxe Reverb or Princeton Reverb head project. I see you have vids of both, and am thrilled. Being able to see what they look like, and what they are supposed to do is a big help, especially as there are so few hobbyists here. Fortunately, I can find tubes parts for the more well-known circuits here, but the more obscure stuff has to come from the US ... with associated shipping costs. Bew well, and looking forward to Push-pull bias, tremolo tweaking and reverb troubleshooting vids! ;)
Wow....it's great to hear from you, Charis, and thanks for subscribing. Please keep us posted on your DR or PR project. If you ever need transformers, please give Triode Electronics a try......they have a great selection at very reasonable prices, with quick shipping. I'm glad you're enjoying the videos and do indeed intend to post a double-ended bias video in the near future. Re the tremolo tweaking, be sure to watch both the PR videos ('66 and '67) I've posted, as they contain a great way to boost your tremolo intensity, and (as I recall) the '67 video goes into reverb repair. Thanks again for your kind words, and best of luck !!!!
Uncle Doug Thanks for the reply, Doug. Still haven't made the big decision ... The PR is a bit simpler, a bit smaller with less power, which makes sense in Japan, and this is strictly for home use. But the DR has more depth and versatility, as I understand it. Parts prices are not much different, and I'm building a head to plug into a nice oversize 2X12" cab I'm making, so it's a hard decision. Opinion? For sure about Triode! I have three amps with Classic Tone iron, and very pleased with price and performance. I found a local supplier that stocks some Edcor trannies, which though expensive, come pretty even with Triode after shipping. (sigh, another decision) Have watched all those vids, and will again with notepad in hand! Looking forward to more, please keep up the good work! btw, found you through Cat ... at TDPRI.
I'm glad that Cat sent you my way, Charis, and also glad that you knew about Triode. They are my favorite source for transformers. To me, the main differences between the PR (AA764) and DR (AB763) are: 1.) The DR has a "Normal" and "Vibrato" input, while the PR has "Instruments" (= Vibrato). To me the DR "Normal" channel is a waste. 2.) The transformers in the DR are more robust (expensive), which is meaningless unless you really need more power and two channels. 3.) Phase Inverters: Long-Tailed-Pair 12AT7 for DR and half of a 12AX7 for PR. I really doubt that you could hear the difference, especially at moderate volume. The DR is going to be more expensive to build (9 tubes vs 7, among other things) and offers virtually no additional benefit to you, considering your intended use. All things considered, including your needs, I would go the PR route.
Uncle Doug Bless you, Doug!! :) Seriously, that is the most concise comparison of the two circuits I've ever heard. If I ask such a question at a forum leaning in the F*nder direction, it inevitably becomes an argument about the "tone" of the original amp, speaker size and manufacturer, who played one where, and everything *except* the basics of the circuit played at non-gig levels. I have 5 Champs, 5C1, 5F1, 5F2A (arguably a Princeton), an AA764 VC, and a 600RI, and each played through my oversized open-back 12" cab, they all sound good. Yes, a bit different from one another, but certainly not enough to get all emotional about ;) Good on ya, man!
Thanks, CJ. You have a nice collection of Champs. Re the 5F2A, I built a clone of this circuit (and posted a video about it :). I do consider it to be a Princeton (and so did Fender, as I recall).....granted, a single-ended, Champ-with-a-tone-control (and a cathode bypass cap on the 12AX7 and one extra 16uf filter cap) Princeton, but a Princeton nonetheless. It turned out great and I really like it. Your enthusiasm for Champs makes me even more certain that a AA764 PR (with GZ34 rectifier) is the right circuit for your next project. It's a logical progression :))
I do the bias on several amps without the math you know by using Webers bias calculator for class A/B or class A by switching his chart to class A ! I use a plate voltage probe and a grid bias probe to set the Webers recommended value at slightly lower value because voltage in my location varies - 123VAC to 116VAC at home ! You put your tube in the probe and plug the socket of the probe into the amp , first the plate voltage probe then bias using the grid probe setting my DMM to the probe instructions which is critical !
I seen my observation has already been commented on (calculating the valve's dissipation including the V-drop of the cathode biasing resistor vs cathode-plate V-drop only), but I agree with you that it is a good way to give yourself a safety cushion/margin! Valves are way before my time and I never learned much about them in university. If I imagine this correct, using the current flowing through the plate connection and the voltage measured from cathode to plate, it would pretty much get you the majority of the power dissipation. Thinking like a road, the traffic all originates from the cathode, and most exits via the plate, but some diverts to the screen grid. At least, I'm thinking this in Kirchhoff's Laws. As an EET, it certainly is interesting getting myself acquainted to valves and how things used to be before SST came about and comparing the different ways things were achieved :)
Doug: Thank you for the great video......again. Bye the way, I checked out your facebook page and was most impressed with the rest of the generator and his proud family name. The Triumph wasn't too shabby either. Did it have Amal's?(sp) Tough duty........ I thought I would practice by double checking the bias on my Champ. It has the stock resistor and cap values. Stock rectifier tube 5Y. 480 ohm tested, 22.5 voltage drop across the resistor (is that high?), and a plate voltage of 405 (pin 3). (the B+ is 370v by schematic and 385v actual) I come out with 18 watts or so with a 6v6. I have repeated the readings many times with the same result. Even if I plug in the schematic value of B+ it is still way high. I am happy with the 6v6 sound and the tube is not glowing heavily. My ultimate goal is to put a switchable bias in so I can use an el-34 or other tube at times.Could the feedback mod be causing this? I have tried it with two different OPT's. Oddly the math with the el-34 and a more powerful rectifier is 26 watts. Any thoughts on why my math is fuzzy but my amp is not?
Greetings, Steve. Yes, all the British bikes I restored had either their original Amal carburetors, or new replacements. They are devils to repair due to excess slide wear and air leakage, but once fixed, they worked fine. Re your Fender Champ, if the cathode bias resistor measured 480 ohms and the voltage drop across it was 22.5V, the plate current is 46.9mA, which is way too high (it should be about 30 mA). With a plate voltage of 405V, this would yield a plate dissipation of 18.98W, which is over 50% too high (max is 12W). If you like the amp as-is, I guess you could continue using it, but if it were mine, I would increase the value of the bias resistor and bring down the plate current to 30mA and plate dissipation to 12W. It will probably sound better and the tube will last a whole lot longer. Check out my recent video on the 1965 Vibro Champ....it had a very similar problem with high PC and PD, and I show how it was repaired. The feedback loop mod is not causing the problem. Re the PD of 26W with an EL34, this is a tiny bit high, but acceptable. Max for a cathode biased EL34 is 25W.
Hi Doug Damn it, the way you explain electronics is crystal clear. Two questions: 1-Have you ever got a high voltage shock working with these guys? 2-How can a tube amplifier be designed to sound good and have a way lower plate voltage? Cheers Pedro
Thanks, Pedro. Yes, I have been shocked a few times over the past 40+ years. Tubes must operate at the plate voltages for which they were designed; therefore, you would have to use an output tube(s) that was designed for low PV. See any Tube Manual to find one.
Thanks, Necro (Rusty is a little nervous about your nickname :) and you're welcome. I'm glad the videos have been helpful and interesting. Please stay tuned for Part 2 on the topic of tube biasing.
Doug I'm really enjoying these videos - very clearly presented and understandable. A couple of questions on this one: 1) I had the understanding that old tube amps with no ground were inherently unsafe but you are electing not to ground this one. Is it a matter of ensuring the 'death cap' is properly spec'd & functioning correctly? 2) Can you elaborate on the need for the lightbulb device? Cheers and thanks again! edit: I just found your video so #2 is answered :)
Thanks, Duane. With my own amps, if they are pristine originals, I keep them that way. When I sold the amp, however, I installed a 3-wire power cord and removed the "death cap".
Dylan Cole This is pretty typical of SF Champs. They are notorious for roasting their 6V6's. Stock up on bias resistors, Dylan.....probably near 1K ohms.
Sometimes I watch some of your older videos again.
They are entertaining informative and helpful. With safety in mind. I got shocked once and lucky to be alive past about 18 years old from one of my Ham radios uh never again +s my fingers. I learned from this video and successfully biased the 6V6 tube in my little champ. I promise I won't modify it any more or any one else's. But my champ is mine. It's done but I'll just maintain it as it is now. I hope you are still putting out a new video once in a while we all would like to see it out here on yt Land.
Thanks for the nice comments, Dwight. Be careful !!
And this is when the light came on for me - no pun intended. You can read forums for hours, observing well intended people argue back and forth. It becomes a battle in my mind between trying to grasp the information itself and then sorting thru the emotion to see if there's a consensus of some kind buried in the arguments of the "experts." Save yourself weeks, maybe months, of forum scrubbing and please just watch this video. The Air Force paid good money to send me thru electronic principles and I still have a massive 3 ring binder full of my notes, but none of that helped me as much as this video. Bravo - well done my friend! Thank YOU AGAIN! *And my Champ thanks you too. 👍
Wow....thanks so much, Chad. We're really glad to hear that the video was helpful :)
I appreciate his ability to explain things in a way that’s interesting and easy to understand.
......and I appreciate your nice comment. :)
military training is best,,you know your shit ,you forgot more than i know
Uncle Doug's videos are a masterclass collection, I love watching them and learning. And then there are quotes like these: "the last thing you want to do is die, while you're trying to bias your single ended amplifier"..
cmpmuller Greetings, CM, and thanks for the very kind words. Rusty and I are glad you enjoy the videos. I guess dying is indeed "the last thing" we'll ever do.....it would just be a shame to hasten the process ;)
Uncle Doug toujours clair Net et précis! Thanks !!!!
c'est absolument la vérité
I'm wondering if anyone has systematically assembled his videos to construct a primer on all things amplifier for the beginner ? I have begun such an endeavour, but TH-cam seems to revel in scattering stuff all over the map. Here's what I've got so far (unedited):
UNCLE DOUG NOTES How Tube Amps work
‑
HOW THE AMP WORKS 1,2
th-cam.com/video/x5SSKX74DKg/w-d-xo.html
th-cam.com/video/901iaPVVzY0/w-d-xo.html
TONE STACKS 1,2,
th-cam.com/video/BatwDYFJ9ug/w-d-xo.html
th-cam.com/video/uZHM5BwPLRw/w-d-xo.html
CATHODE BYPASS CAPACITORS
th-cam.com/video/HrkYUPmEUhk/w-d-xo.html
OSCILOSCOPE BASICS 1,2
th-cam.com/video/ueOup-XBexU/w-d-xo.html
th-cam.com/video/UswkMHK27F8/w-d-xo.html
OSCILOSCOPE AC/DV TRACING, RMS OUTPUT MEASURING
th-cam.com/video/PmVq7NLv31s/w-d-xo.html
TUBE REVERB 1,2,3,4
th-cam.com/video/EZFuT4liLAw/w-d-xo.html
th-cam.com/video/3KEiQBaYZDc/w-d-xo.html
th-cam.com/video/wTwTcjMYnAY/w-d-xo.html
th-cam.com/video/pjTpiQG-ov4/w-d-xo.html
IISOLATION TRANSFORMERS
th-cam.com/video/5SjVm-rF39A/w-d-xo.html
TRANSFORMER IMPEDANCE RATIOS
th-cam.com/video/5jUitplchok/w-d-xo.html
CAPACITORS IN AUDIO CIRCUITS
th-cam.com/video/l-u8J-yh9ZA/w-d-xo.html
th-cam.com/video/9UULx3-hHv4/w-d-xo.html
th-cam.com/video/YXChioNC5NY/w-d-xo.html
RESISTORS
th-cam.com/video/LBtoarc6Nls/w-d-xo.html
th-cam.com/video/8ruznIHuh7M/w-d-xo.html
th-cam.com/video/QFHcmST7AZ0/w-d-xo.html
th-cam.com/video/ORjsSfab278/w-d-xo.html
th-cam.com/video/TvQWX0M6hJY/w-d-xo.html
POWER TRANSFORMERS
th-cam.com/video/MXgG-0MIBGA/w-d-xo.html
PT BASIC DESIGN AND FUNCTION
th-cam.com/video/hJsPQdAhTkQ/w-d-xo.html
GUITAR AMP VACUUM TUBES
th-cam.com/video/Xo4nVjj4NSE/w-d-xo.html
th-cam.com/video/Zh_51RP4V6I/w-d-xo.html
PHASE INVERTERS
th-cam.com/video/U6By31V9fDo/w-d-xo.html
BUILD A CURRENT LIMITER
th-cam.com/video/wRFRwOnLsZI/w-d-xo.html
HUM AND ITS POSSIBLE SOURCE
th-cam.com/video/GrVtX0QGNls/w-d-xo.html
BIASING DOUBLE-ENDED AMPLIFIER OUTPUT TUBES, PART 1: BASIC EXPLANATION & METHODS
th-cam.com/video/w9B0Rhr_Y-E/w-d-xo.html
...thats a GOOD one all right-!!!
Thank you very much for your videos. You explain things so clearly that even people that don't have English as their first language can understand everything very well.
You're welcome, Ed. Thanks for your very nice comments.
You should be a pro narrator for a wide range of educational videos.
Thanks, Trebor. I would love the opportunity.
Uncle Doug, I completed this procedure successfully today without incident, thanks to your excellent teaching. When I measured 450 volts after switching on my Vibro Champ, I felt a good amount of respect, but I didn't quake in my boots because I had watched your video. I did the measurements for several 6V6 tubes and found the newer ones (JJ) were running much hotter than an old GE tube (which was at 12.5 W with the original biasing). I also liked the overall sound better with the GE, so left it in there. Once I have to switch to JJs, I will have to rebias the amp because they're running at 15.5 - 16 watts of plate dissipation. I know how to do that now. The measurements were interesting. I also took the opportunity to glue the tolex back down and polish the front, using methods I learned from your videos. The amp is shiny and sounds great. I'm using a step-down transformer with this US-made amp, since I'm in Europe. I ran into some interesting polarity issues with that. If one turns the (Schuko) plug the wrong way around, the step-down transformer outputs 234 volts instead of 117. Not very nice. I believe many people aren't aware of this. I got myself a model with a polarity check so this doesn't happen anymore (it didn't hit the amp, I measured first). What is the result of running an amp made for 117 volts on 234 volts instead? I wonder.
Greetings, KD. A PD of 12.5W is still excessive and 16W is crazy. I tend to keep the cathode-biased 6V6's at around 11W. If 115VAC input can produce 450 plate volts, then 234V would yield close to 900 PV......which would breach all the capacitors and destroy the tubes.
@@UncleDoug I guess I might change the resistor then. I suspect previous owners in Europe didn't pay any attention to the plug orientation, so I'm surprised the amp still works and nothing looks destroyed inside... I should probably measure all the caps, though.
@@kneedeepinthedoomed Plug orientation is a safety risk but does not threaten the circuit components, KD. Increasing the bias resistor value will result in better tone (in my opinion) and longer tube life.
I am going through these videos slowly (about one every 2-3 days) and they're helping me get through these days, while educating me at the same time. You're a great teacher, and I appreciate you sharing your knowledge. Thanks Uncle Doug! :)
You're welcome, Kenji. Thanks for watching :)
Years of confusion cleared up in 20 minutes.
That's good to hear, Jan :)
@@UncleDoug Amen to that.
Good information Doug.
Safety, safety safety.
Thanks. Yes sir, Amo :)
This is the best explanation on biasing I've ever found on YT.. and the whole Internet..
Thanks, Ivan. One change is that it's better to measure the plate voltage between the plate and cathode....not ground.
So the actual plate voltage is voltage at pin 3 minus the voltage drop value?
Yes, I guess you could look at it that way.
Uncle Doug, I wish you'd been one of the instructors when I through tech school!!
Thanks bloke!!
I wish I could have been there for you, Vinny :)
@@UncleDoug well you're here now and digital communication allows us to be in contact. I quit working as a technician 30 years ago. Digital technology made myself a thousands of other guys redundant. I worked maintaining telephone exchanges which were mostly relay switch operated. I also worked on big dc power supplies and have melted a few tools - lol
@@ix-Xafra We retired guys have to keep busy :)
Hi Doug: I was about to correct you on a couple of points but I read the description and see that you cover them in your notes. Excellent tutorial and WOW what a clean little Gretsch! Usually the ones I've seen have been shall I say "faithful companions". Cheers Mike.
Thanks, Mike :)
Just wanted to thank you for your awesome videos! An electrical engineer friend of mine (guitarist, electronics tinkerer) recommended your channel as the best way to learn tube amp electronics--he was totally right. You're a fantastic educator.
Andy Reeves You're welcome, Andy. Rusty and I are really glad to hear that our videos have been helpful and informative. Thanks for watching :)
Thanks to you, i've build 2 amps for my self. One SE and one PP. You've opened a whole new world of me.
excellent doug,,this is the best i have seen,,i worked in electronics production,but i forgot all this stuff,,now im ready to get in there and test my amp,,i want a warmer sound and mabey this amp is set different,,thankx again do more old school electronics,,no one knows much about this stuff in the computer age we live in.
You're welcome, Noah. Good luck.
"It's a real Downer when your heart stops!" Lol, I have to agree Uncle Doug!
Trust me, it's scary as heck, especially when you feel it struggling to regain its proper rhythm.
@@UncleDoug I had A-Fib for six years. THAT is like an earthquake going off in your heart, and it is horrifically miserable. Got that fixed. Now I just have six blown discs in my lower back. It's been a good past 15 years ...
your videos are gold for all of us. thank you very much.
Glad you like them, Ivan :)
Hi Doug, this is the first comment I have ever posted. I wanted to thank you for this video and the follow-up video. I have been playing guitar for 30yrs and during the last few years learning about amps. I had found it very hard to find a thorough demonstration of the biasing procedure even though I understood the theory around it. Its really good that someone with your experience is posting this type of experience. Warm regards John C
You're welcome, John. We're glad the videos were clear and helpful :)
One of your best ever Doug! U.D. reigns supreme
Thanks so much, Austin :)
I am not into guitar amps; the plan is to build some HiFi tube amps, but principles stay the same - please correct if am wrong. However, your videos do help me understand what is going on in those circuits, how the electronic parts work and why. At school was I never really interested in physics (… it's just stuff that happens to some other stuff and it'll still happen without me understanding it …), but now, I want to know about it, and your videos do help a lot, they're epiphanic to me. Thank you very much for your effort in spreading knowledge in such a comprehensive and kind way.
Please give your four legged companions an extra treat from me!
Thanks for the very nice note, EW. Yes, the fundamentals are all the same. We're glad the videos have been helpful. Best of luck with your projects.
I find myself referring back to your videos often. You’re still the best Doug! 🥇
Thanks so much, Tony :)
Another great video! Please continue.
Thanks, DN. I will start production on a sequel, probably detailing the biasing process for dual output tubes. Stay tuned.......
Thanks Doug, helped save my StewMac Champ project. She was running way too hot from the get go.
I'm glad the video was helpful, Tom.
Apparently my little Monoprice 15w is set cold. It's got a test point and adjustment pot inside, from what I understand. Headed to watch the double ended vids next in hopes of learning to check and adjust mine.
The last post I found at TDPRI said this guy's was around -5.86v and he set it to -12v but I'm not just going to accept that as correct until I learn how this all works and whether it actually is correct for 2 EL84s
Edit: so I found the schematic and it plainly says 'set to -12.5V' at the bias pot. So I guess that settles that?
One of the things I really appreciate about these videos, besides the learning opportunity, is the fact that you leave your mistakes to be seen. The disclaimers and corrections are added later over the top. It greatly increases my faith in your presentation and adds to the humanity of the whole process. Good stuff.
Thanks, Keith. Unfortunately, TH-cam has decided to eliminate any notes or corrections within videos. So I guess the mistakes will have to go uncorrected :(
Yikes! (I know you will continue to make things right, in spite of ewetoob...)
We'll try, Keith. I will never understand the compulsive need to constantly modify systems that are working just fine.
This video brings back a lot of memories of back when I was a U.S. Navy electronics technician...it has been a long, long, time since I worked on any tube type equipment(!)
We're glad to help you reminisce, Dale. Please watch more of our videos.
Very helpful. I am writing notes as I watch to keep this all straight in my head. Have been away from tube amps for a while, so it's good to refresh. I learn more each time I visit.
Thank you Uncle Doug for sharing your knowledge Uncle Doug without charge you've been very helpful to me your friend Dwight from AllenPark Mi. I'm 62 now but I wish I would have been a student in your class. Your worth listening to thank you for teaching the wright way. You R A presious. Joul. Thank You.
You're welcome, Dwight. Thanks for the very nice comments.
Very well presented Doug. Thanks a lot, Colin UK 🇬🇧
You're welcome, Colin :)
Thanks Uncle Doug, this is the clearest explanation of cathode biasing of single ended amps that I've found so far. I really like the way that you explain things. I know have a much better understanding of biasing and it's relationship with headroom.
Thanks so much, David. I'm glad to hear that the video was informative and helpful.
Well I have watched this a few times and I'm starting to get it. My 6V6 on the 9002 is perfect and removing the strange air gap resistor caused the thing to just start working. I am amazed, I actually fixed this thing... ;) Thank you Doug
Congratulations, Anthony. This should give you confidence to continue your new career in amp repair :)
Very good explanation on this Output Tube Biasing Uncle Doug. This makes it much more clear to me. I even built a Current Limiter for testing amps now. All such great information. I really enjoy all these videos. I recently repaired a Capacitor issue in a vintage Trend LR19952 - 801 5 watt Guitar amp made by Pine Electronics Products co. LTD Montreal. They were built in the late 50s up to the early 70s out of surplus electrical parts. There is no input transformer but uses a 35W4 Rectifier 12AX7 input & a 50C5 output tube. 8"speaker, 3 input jacks, Tone & Volume control & a 2 prong cord. The Chassis Ground actually operates with 39 DC volts. It actually sounds pretty good for a vintage small amp. I can send you a schematic if your interested & pictures. Thank you kindly.
What would happen if you earth grounded the chassis with a 3-wire power cord, Kevin? If the circuit still functioned, most do, this would greatly lessen any shock hazard.
I actually tried earth grounding the Chassis & then I received a loud hum over the speaker. So I measured the Chassis to have 37 Volts DC. I guess it is the way it is designed. Quite different from traditional amps I can see from your various Schematics you have illustrated. I am careful with the amp & only use it in the living room on the carpet. My uncle gave it to me when it had the Capacitor Hum issue. The Guitar store told Him to ground the Speaker Chassis to the ground on the cord but no improvement of course. They did not want to repair such a vintage tube amp & sold him a new one. lol I discovered it had a paper unicap containing a 50 mfd 30 mfd & a 20 mfd on a 150 Working Voltage. I had to wire 3 independent Capacitors in as they do not make the 3 in one capacitor any more that I could find. I removed the extra Ground Wire from the Speaker Housing as it was not designed to be there & then sounded better yet. Sounds good now with only a very minor hum that is likely normal to its design I believe. Thanks for your interest & reply. I tried cut & pasting the Schematic to this chat box but does not work. Thanks Uncle Doug & now going to watch your Part 2of Grid Biasing double Ended Tubes. Take Care.
@@kevinsvpbiscayne Thanks for your follow up, Kevin. Good luck with it and your other projects.
This is what people need to learn especially if they rebuilt the amp or if it's been used heavily... Great video
Thanks, Mr. B :)
Doug, all of your videos are great but this one is fantastic! I can't wait to use what you've shown here in my own builds.
Oh yes, great advice: "Don't die while biasing your tubes. It's a real downer!"
Thanks, Jeff. Glad you enjoyed it. Be safe !! :)
"I've got one hand ion my pocket..." and the other one in an amp circuit! Ok, I'll see myself out now, lol. Thanks UD!!
Make sure it's YOUR pocket, Jack.
Hope this is not getting redundant, but thank you so much for all these great videos. I/We really appreciate your love of teaching, and all the little gifts you give us!
Appreciation can never be too often received, Ken :) You're quite welcome.
Excellent Video ! Lots of great information . Thank you Uncle Doug
Glad you enjoyed it, Joey.
I am new to vintage tube amps and found this lesson to be of great value. I always wondered how one would modify an amp to have more or less "head-room". I always wondered what caused an amp to distort with manipulation of the electrical components. Now I know, thanks to Uncle Doug. I really appreciate your lessons, videos and topics covered.
You're quite welcome, Daniel. It's great to hear that the videos are helpful :)
Holy moley UD, I went to the URL you suggested and boy do I have a lot more studying to do. But at least I'm seeing a little light the further down this rabbit hole I go. Or should I say rabbit tube? It's like an alternate universe down here. The more you learn the more you need to learn... magic... And a little trippy too.
Indeed, if you really want to go into detail on topics like this, the depth of information is almost unlimited. You need to ask yourself.....how much do I really want and need to know ?
@@UncleDoug I've said this before, and I'll say it again. Your advice is always spot on. At this point in time this is a perfect fit for me. I hobble around some, but I will soon get my ankles fused and after that I should be walking around normal. No worries :) until then I have some vintage test gear to work on and plan on repairing electronics for a living. Doing some of that already, mostly computers, but there is something about the sound of my guitar playing through that crazy Eico Signal Tracer... sounds strange good. But your right, this bunny hole is bottomless. So I've decided to focus only mostly on the Fender 5F1 & the Eico 147A schematics. So that is why I am very grateful for your first rate "tubetorials". Keep 'em comin! BTW I will be happily throwin down some well spent clams on your patreon as soon as I am able! Thanks again!
Thanks so much, FWR. Best wishes for a speedy recovery and good luck with all your projects.
Thank you Uncle Doug A million times over! I've been ignoring this subject matter for 30 years for 2 reasons; at 13 years old I had better things to do, like learn my instrument (guitar). Now that I've kinda done that, I'm obsessed with finding that tone that gets to me down real deep and you are helping me understand how and why I'm on my quest and believe that I'm closing in on it.THANKS HOMEBOY! Now I'm one step closer to having a meaningful conversation with the other UNCLE, the one and only legendary "UNCLE ALBERT" here in my home town of INDIANAPOLIS. Some of you out there may have heard of him (heh heh heh) Thanks again Sir!
You're welcome, WT. Give UA our regards :)
Will Do UD! Might you inform us (if you can)a tad bit about U. A.'s recently patented 'Del Ray' tube amp? He told me he made 25 of them, all 10W amps, 8 or 10"(not sure which) with 'plenty of headroom" for my cleans. I could not get the $ quite fast enough, but I MUST have one. I WILL have one! HEHEHEHEHE!
Since they are so hard to get, WT, it sounds like you have a better shot at getting one than we do.....so if you do get one, please let us know what you think of it.
Will do UD- I will call UA first thing after he opens and ask (cross your fingers!) if he will let one go to me IF HE Has one. Very least Ill get more specs. Might get the link to the schematic. Peace on Earth okay? Later!
Wow 👍 I picked one of those up at the goodwill store for 7 dollars awhile back soldered the speaker wire's back on and cleaned the controls and it sounds great
That has the be the best deal I've heard of in several years, DC. Congratulations !!
@@UncleDoug and it still has the price tag on it 👍 just couldn't bring myself to take it off
@@doctorcircuit5338 I don't blame you, DC. Why lose evidence of your major coup ??
Awesome info , I have a bit of learning of electronics and this is much more straightforward than other biasing issues, also more understandable, thanks.
I use to dislike electronics, but I needed to know more and more. Lucky for me I found your channel. Now I find it interesting and intriguing and have already carried out some successful repairs. I would like to build my own amp someday, but I am a long way from that at the moment. Thanks for sharing all your knowledge.
You're welcome, GW. Best of luck with your continued study of electronics.
Thanks, you explain topics really clearly, it's amazing how you improve circuits and your knowledge of electrics seems endless. I have learnt some really interesting topics here on your channel about tubes, love it. Having said that it remains a little abstract thinking about what electricity is and how it travels through copper wire. Hats off to Tesla!
The best educational videovI have ever seen. Thanks Doug. I have built 3 amp learning from you.
That's great news, Petar. Keep up the good work.
Fantastic class. And yes, I don't want to die:). Thanks Uncle Doug. And boy, do I hate math, but this was easy to follow.
You're welcome, Shaun.
Hello Doug, I have a Vibrolux reverb custom amp which I converted to an eyelet board and have wired it like the AA964 schematic. Two weeks ago I wanted to check the bias to the tubes and found it cold, so I lowered my resistor value at the bias pot so I could get a higher current value and when I powered the amp I blew a fuse. Well when inspecting the circuit and chassis I found a piece of heat shield by the heater and pilot light section. This came off from above, I guess I caught a piece when pulling the chassis in and out. I then troubleshoot and found the power tranny was shorted. I then installed a vibrolux tranny that was a Weber brand and had to remove the end bell where the wires come out of in order for the tranny to fit. The amp powered on and my plate voltage was now 384 as opposed to the 430 volts I had on previous transformer. I like the lower plate voltage. I am running about 42 ma and using the power dissipation formula comes out to 16 watts for 6L6GC tubes. With the plate voltage running at 384 can I use the 5881 tubes?
Thank you for you time and videos
Both the plate current and plate voltage are rather conservative for 6L6's, so I would say, yes, 5881's would be safe in this circuit. Simply recheck the bias with the 5881's installed and verify that the PD is within acceptable limits. Good luck.
I have to say this was fantastic information delivered in the most professional and entertaining fashion. I'm hooked !
+thebuzztone Thanks so much, TB. We have posted over 135 videos so far, so there are lots for you to watch :)
@15:33 > Tube plate dissipation is calculated as follows : (Vp - Vrk) x Ip. Uncle Doug measured the plate voltage to ground as Vp, but did not subtract the voltage drop across the cathode resistor , Vrk = 14.8 volts . So the Vp used was 14.8 volts too high. Using the corrected plate voltage Vp of (approx) 296 - 14.8 = 281 Vdc shows that the tube is biased at 281 Vdc x 0.0415 mA = 11.66 watts. This video was an excellent presentation, aside from this error.
Thanks, PC. This was a very basic presentation. In all subsequent videos the PV has been measured between Plate and Cathode......even though it only matters in cathode-biased amps ;)
Many ppl have harped on this point but U Doug DID actually explain this in a 'roundabout' way at @18:32 in this video where he mentions that he measured across the cathode resistor (meaning from plate and over the cathode resistor) and thus the calc with a higher voltage will err a little on the pessimistic side, showing a slightly higher pwr dissipation than it really is. This means you won't be tempted to run the tube at the max limit. 😁
I really enjoy your videos! Great slow speech and thorough explanations without being too slow and never boring. I've built a similar amp using a 6L6 driven by a 12AX7. I'm using solid state rectification and resistors for voltage regulation- no chokes, just simple. The output transformer is rated at slightly higher plate impedance than what this tube is rated at using 292 volts on the plate. It dissipates about 11 watts bias and sounds great! I can't seem to find many 6L6 single ended amps.
Thanks, Ryan. I too have found very few SE 6L6 circuits. A PD of 11W is rather low for a 6L6 in fixed bias and real low if cathode biased.....yet it sounds good. That's the most important criteria ;)
@@UncleDoug I only used the 6L6 because I bought several at a yard sale for 50 cents each! I'm curious how it would compare to a 6V6. I didn't have any until yesterday I bought a Challenger HF8A which uses a pair of 6V6's in P.P. Can't wait to get started on the 1950's gem!
I have the 6156 version of the amp in your video. After watching the video again today I thought, what the heck, lets measure the plate dissipation. I followed your directions except I measured plate voltage across Pin 3 and 8, and came up with very similar numbers as you did. Plate dissipation is slightly lower at 11.28W. Here's the funny part. I've watched this video before but sometimes I like to watch informative videos like yours more than once. I have a schematic in the bottom of the amp. Guess what I wrote on it (last winter?)? Plate dissipation calculation values, except I used the method you used in the video. I came up with 11.87W that way. :)
Measuring the plate voltage between the plate and cathode, as you did, is the more accurate method, Michael, and will give you a slightly lower PD than measuring from plate to ground as I did in the video.
Uncle Doug. This is just a general comment about all the videos you have posted. I was looking for a video on how to cover a speaker cabinet with vinyl and I hit 'Pay Dirt' as I want to build a small valve combo amp from scratch, but I have struggled understanding any electrical theory from books. The joinery side I am fine with
Thank you very much for the most informative and easy to understand films I have watched in a long time. They are very inspiring.
I have just watched your video on identifying the wiring on transformers so I am off to find a flea market that has old electrical items for sale.
Rob
Thanks so much, Rob. Rusty and I really appreciate your very kind and favorable appraisal of our work. We're glad that the videos have been helpful and wish you the best of luck with your projects.
Hi Doug! I like the way you explained cathode bias! I need to show this video to some of my customers...I have a tendency to forget that almost everyone I explain bias to is NOT a technician...I always forget and think I'm talking to another tech (LOL)...If the amp you used in the video wasn't a collector's piece, I would say you could install a potentiometer to vary the cathode bias, which would make it a lot easier to tweak when replacing the output tube...I've done that to many newer single-ended amps with fixed bias...I've also used extra resistors to limit the pot's range in some cases...Keep up the good work and pet Rusty for me :)
Thanks, Frank. Rusty sends his regards. I agree with the benefits of installing a cathode bias potentiometer, in fact I posted a video about the "adjustable bias" concept a while back. In this case, as you said, I didn't want to alter what was otherwise a pristine original amp.
I just found you the other day. Im loving learning about tube amps. I would never try this at home but its fun to know how the things work. I have a marshall DLC5 , fender blues jr and a boogie mk5 35. cheers
Welcome aboard, Steve.
Thank you for taking the time to explain all this.
You're welcome, MP :)
Outstanding video. Thank you so much. I just built my first champ from a kit. You're helping understand so much. I can see how addicting this can be.
Bart
redfishbum You're welcome, Bart....glad the video was helpful. A Champ built from a kit is a great way to start what may soon become a very addictive hobby. Welcome aboard :))
Hello Uncle Doug. Thank you for this excellent tutorial. I gave you attribution for this in a video I did where I used this method to check the bias on a IF Amp tube in a '53 Chevy Radio (Part 6). Just wanted to let you know I let everyone know where I got the technique, and a link to this video. Thanks again. [edit: spelling]
We're glad the hint worked for you, PF :)
Thank you!!! You are the best Amp Teacher
We're glad the videos are helpful, Michele.
thank you for taking us through...
You're welcome, Gaurav.
This is great my understanding expand 's every time I watch one of Uncle Doug's video 's
That's our goal, Dain. Thanks !!!
these are the nicest videos. pen and paper first :)
Thanks !! Glad you like them :)
I'm so glad I found your videos, clean cut and to the point with laymans terms. Im hoping to build a BFPR soon for my next build and I loved your two recent videos on your BFPR
Thanks, Lego. I'm glad the videos were helpful. Good luck with your project.
Very educational video on biasing an amp. Thanks, I now feel comfortable in checking/adjusting tube bias on my BF and SF VibroChamp's,
A good way to think of the bias resistor, for me, is to think of a dimmer switch for a light bulb. Increase resistance dimmer bulb= lower output, decrease resistance brighter bulb= higher output.
You're welcome, OAG. You're right, the Plate Current is indirectly related to the bias resistance.
@@UncleDoug
I have a question for ya. My Blackface VC is alot darker sounding than my early 70's Silverface VC. The Silverface does have a Pyle Driver speaker, model MH810, in it.
Other than that and new Tung Sol tubes both have all original components.
Is that normal between the BF and SF VC?
Well I finally gave the Blackface VibroChamp a good going over to find out why it is so dark sounding. The problem is the speaker cone is shot, kaput, finished. Guess it's time to order a Weber 8A125
This was great. A very helpful refresher. And a new technique for me regarding measurement of the voltage drop.
Keep up the good work, sir.
(Also just a fun note - I picked up some parts off our local classified ad website last week, and the guy with the ad had linked to one of your vids as an example of a possible build using the parts. Of course I immediately recognized the Uncle Doug video. So just FYI, up here in Winnipeg total strangers are watching your videos, being inspired, and chatting about amps amongst themselves. Figured you might appreciate knowing that.)
Wow....thanks, Dr. S. It's good to hear that Rusty and I are now the spokespersons for a Canadian classified ad website. Today, Canada.....tomorrow the world :)) It's also good to know that people are chatting about tube amps. It's a great hobby and the more people who become involved, the better. Thanks again for a really nice comment :)
What a clear concise excellent job. Thank you so much
Thanks, Frank :)
My first amp (used)was a Gretsch 6152 which has tremolo and Reverb. Years ago I removed the tubes because it was distorted at very low volumes and not knowing anything I thought one of the tubes might be bad however the 6V6 had broken the alignment pin because of age and I put the tube in one pin off. I bought a new rectifier and power tube and it would start playing very clearly for the very first minutes and then suddenly gets very hot and get distorted. So I shut it off and have not looked at it since. After looking at your video I was checking my 250 Ohm resistor and it crumbled in my fingers! There is the root of my problem. So of course I have a couple resistors coming in and I can't wait to replace it and see how it operates. Thank you so much for all your help.
@@frankscutari9516 I hope the new resistor solves the problem, Frank, but I wonder why the old one burned up. You might take a close look at your filter caps.....one or more may be shorted.
Thank you Doug, I certainly will. have a good day
@@frankscutari9516 I hope so, Frank.
With the way you explain things it does make alot of sense.
Thanks.
I'v seen this vid.
Of yours before
But as a matter of study and learning
I watch it again.
Who's afraid of alittle
Elesstrigerty ha.
Don't touch! Oooh ah
Pow boom yyooowoo
I learned that lesson the hard
Way back when I was 18 yrs old on an old
Radio that I was repairing it didnt stop
My heart but it was really beeting hard fast
For about 5 or 10 minutes the shock
Was painful. Guess I got lucky. Don't
Forget REO spit. wagon.
Dwight.
Thanks for sharing, Dwight :)
Another vy good tutorial. Here's my vote for a dual pentode (PP) bias video..tnx!
Thanks, Chuck. The video you requested was posted a couple years ago. Please check my video list for Biasing Double-Ended Amplifiers.
I haven't read all comments. Maybe somebody noticed before me, but:
If you measure current thrugh cathode resistor, you actually measure sum of all currents in the tube. Cathode emits electrones and they go somewhere -> as your tube is 6V6, it's tetrode so you have measured Ia+Ig2 (anode + grid #2 current). Not only plate current as you said.
So you have to subtract: Ia=Ik - Ig2
As you don't have resistor in series with g2, disconnect it from supply and measure with amper-metre. Than subtract.
g1 (control grid) doesn't pull any current as long as it's on lower potential than cathode. It will drain some current if we had cathode follower before it and driving grid with positive signal. (Not for guitar but hi-fi)
Plate dissipation:
Pa = Ua * Ia -------> Ua=anode (plate) voltage=voltage between pins 3 & 8, not 3 and ground, as cathode is raised +14.84V above ground; Ia = Ik - Ig2
That's it.
+Ivan Horvat Yes, Ivan. The cathode current does include screen current and to be completely accurate, the SC should be subtracted and multiplied by the PV from plate to cathode to determine the PD.......but recall that this was a "basic" video and I chose to simplify the process. By using the cathode current times plate-to-ground PV, the PD that results will be a bit high, providing a safety factor for beginners. Thanks for your input.
Wonderful demonstrations and information as always with your videos, Uncle Doug. You make all these tube amp mysteries very clear to me, after struggling with understanding this material for years! Thank you very much for these invaluable videos, as cmpmuller said, they are truly a master class collection.
You're welcome, Jason. Thanks so much for your very nice comments :)
I really appreciate your videos. They are incredibly helpful in helping me to understand these concepts
You're quite welcome. It's great to hear that the videos are helpful :)
Great video.Anyone can understand everything.Thank you Uncle Doug.
You're welcome. Glad you liked it :)
Excellent! Thanks Uncle Doug!
I understood everything. Well taught!
You're welcome, Wally :)
Thanx for another great and instructive video/lesson...
And, for me anyway the typos wind up forcing my brain to work a wee bit more, and odd though it may seem, the overall learning experience was enhanced...
thanx again and cheers
You're welcome, Jonny
yes this was a really informative vid for me. i have a hand made Purdy "tramp" and changed the power.tube in it. the manual says i dont need to get biased but does run hot aftef a few hrs playing. breaks up around 6 on the volume. i might just take it in to a pro and see where the voltage is really at. thanks Doug :)
You're welcome, Scott. If your amp's output tube is cathode biased (check the literature or ask the builder), then it probably does not require re-biasing......but, if it's fixed bias (the cathode is grounded, which is quite unusual in single-ended amps), then a new tube must be re-biased.
That is a great demonstration of biasing with this type of method. The only concern is that carbon resistors have a tendency to drift at +-10%. I normally find the resistance goes higher, even though your readings are well over the 330 ohms it still well within the 10% margin as ohms law demonstrated with the dissipation factor of 12 watts with that particular tube, nicely done.
+John Cunningham Thanks, John. I agree that carbon resistors have their limitations, but lots of purists stick with them in their quest for "vintage tone". I personally prefer metal film resistors, especially for screen grids and power supplies.....but in this application, the carbon resistors seem to work well.
I just recently acquired a B&K 747 Tube Tester; the first thing I had to do was remove a handful of carbon resistors that had a tolerance of +-10% and were way out of spec. I ended up using metal film resistors that were rated at +-1%. On top of that endless amount of work went into it before calibration procedures could be performed. Yes! I do understand the nostalgia when it comes to preserving the original components, but in order to fix these amps accuracy is important along with reliability.
Yes, indeed. I tend to believe, however, that much of the so-called "vintage tone" that so many musicians seek is due to the variance from specs within the circuits of old amps. The sum total of all these variances yields a somewhat unstable and "dirty" tone that many people truly appreciate and seek to emulate. In a "great" old amp, the variances all work together.....but in some, they don't. Tweed Deluxes from the 1950's are an excellent example. Unmolested, most are rough, raw, and wonderful.....but if you replace all the carbon resistors and waxy caps with metal films and modern high-grade caps....they become much more sterile and (to many) less interesting.
wish you had a tube amp builders class... oh wait, this is it. good video... coincidentally I had a scare with a small arc.. discharge your caps folks....
Thanks, Jason.....and good advice :)
Excellent presentation, I will go home and try this soon!! Thanks, Uncle Doug!!
You're welcome, Eric. Be careful and best of luck.
i concur, another awesome video! it's great to finally see one on cathode biasing, rather than the typical Fender trim-pot videos (which are a dime a dozen.) i started a new thread and reposted the video over on the TDPRI amp forum: i think it'll help out newcomers quite a bit, since SE cathode biased kits are so popular, as well as stuff like old Champs scored off Craigslist and that sort of thing.
can't wait for the next one!
Thanks, Cat. It's always good to hear from you. I think the next installment will be cathode biasing with dual output tubes. It's much the same process, but with a couple of extra steps. I also hope your Melodier project is going well.
Uncle Doug my computer decided it needed a new hard drive, so i had to put off getting my transformers for a couple weeks. minor setback and no big deal though. who knows, maybe i'll get lucky and Triode will have some sale prices coming up (i can always hope ;) .
I never recall seeing any sort of sale prices from Triode. It's too bad that Musician's Friend doesn't sell transformers.....
yeah, probably just a bit of wishful thinking. the combined price of the ones i'm going to get is only around $75, so it's still a great price!
Transformers are always a major expense in any amp project, but if you get them from Triode, it's less painful than from other sources. Also, If you get them around Christmas, they'll be a nice gift (to yourself)......way better than a lousy pair of socks or underwear.
Nice look and its got that Gretsch sound too!
Thanks, Dave. Yes it does :)
Excellent. Love this stuff. Yes... I am just learning. Keep on Rock`en! From Sioux City,Iowa.
Thanks so much, Ronnie. It's great to hear that the video was helpful. Best regards from UD & Rusty :)
cheers, again Doug thanks for another great video, as always from you. very informative, toke me back to my collage days. it good to get a refresher on basic principals every now and again. after using transistor, and now IC chip you forget the basics. will deff help me in my projects.
Thanks, Ron. I'm very glad to hear that the video will help you and others with their electronic projects. Good luck !!
Hey from Japan, Doug! Enjoyed this video explanation, subscribed, and watching your others ... thank you! Got bitten by the amp-building bug a few years ago, built a few Champs and a 5E3, and am now planning either a Deluxe Reverb or Princeton Reverb head project. I see you have vids of both, and am thrilled. Being able to see what they look like, and what they are supposed to do is a big help, especially as there are so few hobbyists here. Fortunately, I can find tubes parts for the more well-known circuits here, but the more obscure stuff has to come from the US ... with associated shipping costs.
Bew well, and looking forward to Push-pull bias, tremolo tweaking and reverb troubleshooting vids! ;)
Wow....it's great to hear from you, Charis, and thanks for subscribing. Please keep us posted on your DR or PR project. If you ever need transformers, please give Triode Electronics a try......they have a great selection at very reasonable prices, with quick shipping.
I'm glad you're enjoying the videos and do indeed intend to post a double-ended bias video in the near future. Re the tremolo tweaking, be sure to watch both the PR videos ('66 and '67) I've posted, as they contain a great way to boost your tremolo intensity, and (as I recall) the '67 video goes into reverb repair.
Thanks again for your kind words, and best of luck !!!!
Uncle Doug Thanks for the reply, Doug. Still haven't made the big decision ... The PR is a bit simpler, a bit smaller with less power, which makes sense in Japan, and this is strictly for home use. But the DR has more depth and versatility, as I understand it. Parts prices are not much different, and I'm building a head to plug into a nice oversize 2X12" cab I'm making, so it's a hard decision. Opinion?
For sure about Triode! I have three amps with Classic Tone iron, and very pleased with price and performance. I found a local supplier that stocks some Edcor trannies, which though expensive, come pretty even with Triode after shipping. (sigh, another decision)
Have watched all those vids, and will again with notepad in hand! Looking forward to more, please keep up the good work!
btw, found you through Cat ... at TDPRI.
I'm glad that Cat sent you my way, Charis, and also glad that you knew about Triode. They are my favorite source for transformers.
To me, the main differences between the PR (AA764) and DR (AB763) are: 1.) The DR has a "Normal" and "Vibrato" input, while the PR has "Instruments" (= Vibrato). To me the DR "Normal" channel is a waste. 2.) The transformers in the DR are more robust (expensive), which is meaningless unless you really need more power and two channels. 3.) Phase Inverters: Long-Tailed-Pair 12AT7 for DR and half of a 12AX7 for PR. I really doubt that you could hear the difference, especially at moderate volume.
The DR is going to be more expensive to build (9 tubes vs 7, among other things) and offers virtually no additional benefit to you, considering your intended use. All things considered, including your needs, I would go the PR route.
Uncle Doug Bless you, Doug!! :)
Seriously, that is the most concise comparison of the two circuits I've ever heard. If I ask such a question at a forum leaning in the F*nder direction, it inevitably becomes an argument about the "tone" of the original amp, speaker size and manufacturer, who played one where, and everything *except* the basics of the circuit played at non-gig levels.
I have 5 Champs, 5C1, 5F1, 5F2A (arguably a Princeton), an AA764 VC, and a 600RI, and each played through my oversized open-back 12" cab, they all sound good. Yes, a bit different from one another, but certainly not enough to get all emotional about ;)
Good on ya, man!
Thanks, CJ. You have a nice collection of Champs. Re the 5F2A, I built a clone of this circuit (and posted a video about it :). I do consider it to be a Princeton (and so did Fender, as I recall).....granted, a single-ended, Champ-with-a-tone-control (and a cathode bypass cap on the 12AX7 and one extra 16uf filter cap) Princeton, but a Princeton nonetheless. It turned out great and I really like it.
Your enthusiasm for Champs makes me even more certain that a AA764 PR (with GZ34 rectifier) is the right circuit for your next project. It's a logical progression :))
Amazing tutorial! Please make more technical videos!
Thanks, AR. With such a favorable response, I can guarantee that sequels will be posted. :))
I do the bias on several amps without the math you know by using Webers bias calculator for class A/B or class A by switching his chart to class A !
I use a plate voltage probe and a grid bias probe to set the Webers recommended value at slightly lower value because voltage in my location varies - 123VAC to 116VAC at home !
You put your tube in the probe and plug the socket of the probe into the amp , first the plate voltage probe then bias using the grid probe setting my DMM to the probe instructions which is critical !
Whatever works for you, IJS. It sounds like you have a good procedure.
I seen my observation has already been commented on (calculating the valve's dissipation including the V-drop of the cathode biasing resistor vs cathode-plate V-drop only), but I agree with you that it is a good way to give yourself a safety cushion/margin!
Valves are way before my time and I never learned much about them in university. If I imagine this correct, using the current flowing through the plate connection and the voltage measured from cathode to plate, it would pretty much get you the majority of the power dissipation. Thinking like a road, the traffic all originates from the cathode, and most exits via the plate, but some diverts to the screen grid. At least, I'm thinking this in Kirchhoff's Laws.
As an EET, it certainly is interesting getting myself acquainted to valves and how things used to be before SST came about and comparing the different ways things were achieved :)
Your analysis is correct, Salcon. I now am careful to measure PV from plate to cathode in cathode-biased circuits.
Excellent.... I'm close to getting it.
Carl Rudd Thanks so much. Keep at it, Carl, and it will all become clear and comprehensible :)
Thank you, an excellent Video! Perfect timing too, as I am ready to start soldering a scratch built 5F1 with a solid state rectifier. Thanks again.
You're welcome, BT. I'm glad the video was of interest. Keep us posted on your project.....it sounds like an interesting one.
Great content Sir! And thanks for the safety tips. Be safe.
Thanks, you too!
Ótimo vídeo mestre
Thanks, Jorge. Glad you liked it :)
How many x ive watched, eye dunno but it makes a whee bit more progress towards my brain each time.
Glad to hear it, J ;)
Doug: Thank you for the great video......again. Bye the way, I checked out your facebook page and was most impressed with the rest of the generator and his proud family name. The Triumph wasn't too shabby either. Did it have Amal's?(sp) Tough duty........ I thought I would practice by double checking the bias on my Champ. It has the stock resistor and cap values. Stock rectifier tube 5Y. 480 ohm tested, 22.5 voltage drop across the resistor (is that high?), and a plate voltage of 405 (pin 3). (the B+ is 370v by schematic and 385v actual) I come out with 18 watts or so with a 6v6. I have repeated the readings many times with the same result. Even if I plug in the schematic value of B+ it is still way high. I am happy with the 6v6 sound and the tube is not glowing heavily. My ultimate goal is to put a switchable bias in so I can use an el-34 or other tube at times.Could the feedback mod be causing this? I have tried it with two different OPT's. Oddly the math with the el-34 and a more powerful rectifier is 26 watts. Any thoughts on why my math is fuzzy but my amp is not?
Greetings, Steve. Yes, all the British bikes I restored had either their original Amal carburetors, or new replacements. They are devils to repair due to excess slide wear and air leakage, but once fixed, they worked fine.
Re your Fender Champ, if the cathode bias resistor measured 480 ohms and the voltage drop across it was 22.5V, the plate current is 46.9mA, which is way too high (it should be about 30 mA). With a plate voltage of 405V, this would yield a plate dissipation of 18.98W, which is over 50% too high (max is 12W). If you like the amp as-is, I guess you could continue using it, but if it were mine, I would increase the value of the bias resistor and bring down the plate current to 30mA and plate dissipation to 12W. It will probably sound better and the tube will last a whole lot longer. Check out my recent video on the 1965 Vibro Champ....it had a very similar problem with high PC and PD, and I show how it was repaired. The feedback loop mod is not causing the problem.
Re the PD of 26W with an EL34, this is a tiny bit high, but acceptable. Max for a cathode biased EL34 is 25W.
Great video series for a very well done amp. I am really impressed!
Thanks, Bob.
Hi Doug
Damn it, the way you explain electronics is crystal clear.
Two questions:
1-Have you ever got a high voltage shock working with these guys?
2-How can a tube amplifier be designed to sound good and have a way lower plate voltage?
Cheers
Pedro
Thanks, Pedro. Yes, I have been shocked a few times over the past 40+ years. Tubes must operate at the plate voltages for which they were designed; therefore, you would have to use an output tube(s) that was designed for low PV. See any Tube Manual to find one.
The current you calculated is actually the sum of plate and screen grid.
The voltage you need is plate to cathode,not to ground.
+MrAletube You're right, Mr. A. I have since corrected this oversight. Thanks for your input.
thank you very much...great video
+thundervbc You're welcome, TV.....glad it was helpful :)
Great video, a masterclass indeed. Thanks
You're welcome, Andrew :)
Excellent. Been eagerly watching many of your amp tech videos and learning a lot. Thanks so much for sharing the knowledge :)
Thanks, Necro (Rusty is a little nervous about your nickname :) and you're welcome. I'm glad the videos have been helpful and interesting. Please stay tuned for Part 2 on the topic of tube biasing.
excellent video clear and informative, plus some wry comments ......
Thanks, WC :)
doug. brilliant video. exactly what i've been looking for. thanks a miilion!
Thanks so much, Greg.....glad you liked it :)
Doug I'm really enjoying these videos - very clearly presented and understandable. A couple of questions on this one: 1) I had the understanding that old tube amps with no ground were inherently unsafe but you are electing not to ground this one. Is it a matter of ensuring the 'death cap' is properly spec'd & functioning correctly? 2) Can you elaborate on the need for the lightbulb device?
Cheers and thanks again!
edit: I just found your video so #2 is answered :)
Thanks, Duane. With my own amps, if they are pristine originals, I keep them that way. When I sold the amp, however, I installed a 3-wire power cord and removed the "death cap".
Off to the store I go.. my silverface champ is really running hot. In the 20 range.
Dylan Cole This is pretty typical of SF Champs. They are notorious for roasting their 6V6's. Stock up on bias resistors, Dylan.....probably near 1K ohms.
Thanks for the vids Uncle Doug.
You're welcome, BN. I hope you are enjoying them :)