I have watched this video several times and great stuff. Just found a Heathkit AA-71 unbuilt on eBay and will have it soon. It is a mono but I have my dad's Jensen hi-fi 50's vintage speaker system to play it on (pre-stereo). But like you the joy is in the building and testing... and listening. Means I have stuff stacking up everywhere. It is a learning experience that I have looked forward to in retirement. I appreciate your videos and learning everytime. Thanks!
I think the LS series of UTC transformers have a very nice sound to them. A bit more clear, to my ear, than the Acrosound transformers. On the other hand, most of these UTC transformers perform poorly below 30 Hz. They have the same primary inductance as the Acrosound transformers, 20H. But after building and listening at length to the 300B stereo amplifier, I can only come to the conclusion that THD below probably 3-4% really doesn't matter much and may even be an artifact that enhances the sound. I know that flies-in-the-face of what we know technically and theoretically, but in the real-world I think many people would agree with the "sound" of SE amplifiers and their sawtooth harmonic profile. What else can I say? A somewhat similar example is audio on amateur radio. HiFi sound has too many lows in it and is easily obscured by every type of noise that can be found on the HF bands. I don't think a ham radio operator wants to sound exactly like a space shuttle radio transmission but we have to admit, it does get the message thru effectively. I think I have pretty well satisfied my own personal goal of evaluating the vintage high-end transformers, in particular the Acrosound, UTC, Peerless and the more modern day James transformers. They are all good but some of the Acrosound transformers, I had ten at one time, can have variations in performance that is hard to understand. I don't know if it is age or QC but I got to use all ten of them in the same circuit so there is something to be said for that. I have immersed myself in AM transmission lately and are having a blast. My 300 watt output transmitter with power supplies and modulator weighs in at about 350 pounds and puts out a lot of heat but it is beautiful. Great for the winter in the basement but not so much so in the summer. Always good to hear from you and I hope you are doing well. Everything is good here but I have slowed down a bit now at 71 but still having fun. :-)
@@ElPasoTubeAmps That's a bit unexpected to hear about the LF performance of LS series transformers. I understood them to be -3dB at 10Hz and 40KHz and they certainly have enough iron to get down that low. But yes, 3% THD is too high at 20Hz. A hi-fi amplifier should only increase the amplitude of a signal, adding or subtracting nothing along the way. Unfortunately, this new trend toward "euphonics" is not hi fidelity. Amps that distort belong in the musical instrument end of the spectrum, IMHO. When you're MAKING music, shaping the tone is part of the sound. But reproduction should be agnostic and faithful to the original sound. I generally agree about communication audio, but there is a HAM near me, K1KBW, who runs a Class E rig on AM that sounds better than broadcast quality. His modulation positive peaks are around 180% and he really cuts through when other AM'ers are barely modulating the noise floor. He's the new modulation king, even well beyond WA1HLR (Timtron), whose transmissions impressed me in the early 1970s. YayM is still the best transmission mode for enjoyment of HAM radio. Slopbucket never appealed to me. I suppose it's because building a filter that doesn't kill the lows below 300Hz is so difficult that almost no rigs have a decent audio response on sideband. Nice to hear you're still doing okay, Dave!
Thanks David. I remember hearing the local AM stations through my tube-driven Heathkit phonograph - good reception. Your videos are always packed with information. I am self-taught in all things vacuum-tube and your experience is invaluable to me. Beautiful workmanship, as always.
Thanks, Gary. I am self taught in vacuum tubes also. I have looked back at my college books of electronics and there were some vacuum tube circuits but I think they were there only as principles of ideas rather than circuits of the day. Transistors were the rage in the late 1960's and even IC's were available. When I went to work as a contractor in January 1970, our mainframes were called third-generation as they had IC circuits in the main arithmetic section of the computer. There were Thousands and thousands of RT-176 (2N2222) transistors everywhere. We called them ICR for integrated circuit registers and had a cycle time of 125 nano seconds. OMG... could it possibly be any faster (that is 8 MHZ clock frequency). Can you imagine but it was a 36 bit (not 8 or 32) word that could be double precision at 72 bits. We were definitely in the big-time 50 years ago and had just gone to the moon six months earlier. It was a good time and lots of work. Wish I could do it all over again.
I have enough mystery holes in some of my chassis to get good air flow ventilation... I do recycle chassis sometimes if the holes are not such that it prevents me from properly mounting the new project components. It is not too hard to lay out the major components from input RCA connectors to speaker connections and tube sockets and transformers - all the common components, but it is difficult to determine all the best places for the solder terminals for the under chassis resistors and capacitors. They usually have to be drilled and mounted after all the major components are installed. Also, laying out tube sockets for best visual appeal from the top side of the chassis is often not the best layout for component placement under the chassis. Everything is a trade-off between visual appeal and performance, right down to a compromise of THD vs square wave response vs stability and on and on. I recently read an article that decades ago when guys like us did not have any test equipment other than a voltmeter/ammeter, they called amplifier building a "black art". I kind of understood that. How would one know if the amplifier had a parasitic at 70 KHz with out a spectrum analyzer or THD was most anything below 5% and clipping such that it could be heard? A whole lot of guess work was part of the home building art back then. It still amazes me the level of lab quality equipment we hobbyist can own these days. They made some good stuff back in the '70's/80's that still performs properly and is affordable. Stay safe and sane in these trying times.
I remember listening to Radio Moscow and Voice of America as a ham radio operator back then. The last thing I remember from the Russians was the "Russian woodpecker" which I think was an over-the-horizon radar system. Glad that thing is gone. I guess I am a little too far away across the pond for Radio Moscow to get into my stereo but the little 1 KW local AM radio station, WRAB, in the town of Arab, Alabama where I grew up could sure get into my equipment. That was the first radio station I got to see inside and I still remember those four beautiful 833A tubes glowing and probably some blue MV rectifiers at the bottom of the transmitter.
Your Altec's would sound so good. 20W per channel might not impress many these days where advertisers claim hundreds of watts output of a plastic box weighing 5 pounds but you guys know better. I drive Dahlquist DQ-20 speakers with this amplifier that are terrible inefficient at 86 dB SPL. compared to Klipschorn at 105 dB. That is 19 db difference almost exactly 4 times louder on the Klipschorns and still these Dahlquist speakers will run you out of the room and sound Great on this little amplifier. Hard to believe but I love it.
@@ElPasoTubeAmps I think they're rated at 30 watts and I bet a 20w amp like yours would absolutely sound great. my understanding is you don't want to exceed the rated watts on these EVER! but they sound so good so why go there! plenty loud! I LOVE how your amp is built! great goin !
That is good to hear. Thank you for letting me know. My usual camera is a small Leica camera made in Japan and this camera is a Sony, also made in Japan but a lot more camera and as mentioned 42 MP. I can see a difference in the quality of the video. Video files are huge but they combine in Movie Maker just as fast and upload plenty fast enough. Thanks again and stay safe.
I think it just takes a passion for what we do and always thinking about technology bordering on obsession. It is what keeps me alive today at 70 and I have been doing it in earnest since 13.
The Williamson phase splitter actually unbalances at supersonic frequency because the cathode and anode behave differently. The cathode coupled phase splitter when balanced is better at supersonic frequency and gives improved stability. Less correction is needed across the feedback resistor and the rc network across the plate load in stage 1 can be eliminated.
Howdy. Great demo. As You say. The 10 kHz square wave response is superior. My experience is that using ultralinear mode greatly improves the lower bands THD. Regards.
Beautiful! I've been trying to get in touch with you for a year or so sir. What's the best way to do so? Have a pair of amplifiers I think you'll be interested in :) Thanks, Justin
I’ve had to add an RC circuit on my Ultra Linear KT88 amplifier between plate and screen. It was oscillating at 111khz and although it still sounded fantastic, It was visible on the scope at a reasonably high level. After trying all manner of solutions I eventually came across a recommendation documented in a GEC tube datasheet giving a UL KT88 circuit example and gave resistor/capacitor values that may be required for this RC circuit. It solved my issue immediately. Im not exactly sure what I’ve traded off by doing this but looking forward to taking some comparative readings on the 8903! Interesting about the Mhz filter network on the front end. I would not have guessed the reason behind that one!
Isn't it great to actually catch these anomalies? Do the math and check the 3 dB point of the RC circuit and you will see that many, from days of old, are in the frequency range of AM broadcast stations (somewhere between 500KHz and 1.6 MHz) to help keep out your local AM radio station. Not much of that left nowadays but 50 years ago it was serious. You will find that it does affect the HF response up at 20KHz and beyond but it is usually a good idea to put one in usually on the output of the first stage as you will see in virtually all commercial made amplifiers. Your amplifier may be perfectly stable with out it (during any particular test) but you can change the tubes, components can age, your cat can walk in front of you, and some will not need it and some will. I add them in now.
Really nice amp. Pretty much what my next build is going to be. Not too sure on the output tubes though. Those EL34s are great tubes, but I may go with 6L6s. Definitely going with the 6sn7 Williamson design. Hopefully I can find some output transformers as good as those UTC. That UV glue is pretty amazing. I have some and use it every once in a while. My go to is usually Loctite instant epoxy or Goop. Depends wether I can let it sit all night or not. I definitely need to get an Audio Discovery. Take care!
I am definitely a 6L6 fan but it seems the EL34 has a little brighter high end and so does the UTC output transformers over the Acrosound transformers. I think they are both great OPT but the UTC transformers seem to hold a special place in the audio world. It seemed only appropriate to finally build a stereo amp with them. I truly do like the sound of this amplifier. Of course, I have to know it performs well on the test bench and then it also sounds great to my ears and music so that is a match made in heaven for me. I like the quick epoxy also. I think I mentioned I already had to replace one of the voltage doubler caps and by sticking it on with Goop, it was nice and easy to remove and clean off the residue and stick another one back in its place. I don't think the UV glue would be the best for sticking on the electrolytic capacitors. It is a little light-weight in my opinion. Great to hear from you. I hope all is well.
I have always had a liking of Williamson design power amps. Weren't they also known as ultralinear amps too? I know I have said it before, buy you do VERY nice work! Great sounding and Great to look at! I really enjoy seeing your work. Thank You. Stay Safe!
Thanks Bill, In the beginning of the Williamson design, I don't think UL existed yet but came along shortly afterwards. That is kind of the interesting about these great sound UTC transformers. They are simple CT primary with on fancy UL taps, etc. I am wondering if they wound the primary wires side by side for better coupling rather than in a bobbin style. They actually do live up to their reputation.
Most any preamp should work fine. I am driving it with a McIntosh MX-113 which has a tuner. It really sounds nice and clean with great high end. I attribute that to the UTC transformers and only about 15-16 dB NFB. Too much NFB seems to make them sound more compressed although it does bring down THD. There is always a trade-off in these amplifiers. I have found that over focus on making one parameter perfect often times causes an anomaly we are not aware we have created. In particular, too much NFB or making the NFB bypass capacitor too large may cause a parasitic around 100KHz that we will only see if we use a spectrum analyzer. I hope to make a video on that soon. Thanks for you comments. Stay safe.
Igor s. Popovich in audiophile vacuum tube amplifiers vol . 3 suggested a solid state diode on the center tap of the transformer with two vacuum tube diodes would reduce IMD . This would be a fine test bench to test that out. ( ss diode neg to ground making a sort of full wave bridge 3 leg thing . A good name for it exscapes me.
Hi there, Im wondering if you think using two different tubes/transformers for tweeter and midbass is a good idea. IE acrosound for tweeters and UPC for bass.
I have a UTC special series I'm trying to find it and I'm running a little bit more about this from the 1930s can you give me some more information on that if you could please and maybe if you have some pictures
Hi David, I’ve just finished my four year tube amplifier build and have been running some tests on it with my newly acquired HP8903! Really exciting to get to this point. The frequency response is super flat! However the THD+N reads higher than I imagined it would. As it is a Single Ended KT88 Ultra Linear build what would you expect as reasonable THD values at moderate power level at 1Khz? I think i was up at around 3% I have been playing with Hum Balance and a few other ideas but unable to reduce it much. I wonder if the distortion which is present is mainly even order harmonics and if this can be expected with SE or Ive got some issues somewhere. I suppose this means I need to buy a spectrum Analyzer now!
Absolutely... congratulations on your build and measurements. The HP 8903 is a beautiful machine. HP or whatever they are called now, a few years ago, offered to buy back 8903's for almost the price of a new model that did not have the built in oscillator - so they could destroy them - the 8903's are that good and last that long - not like the limited-life instruments of today. First off, I assume you are not using any GNFB? If that is the case, 3% is to be expected for the 300B at around 6 or 7 watts and probably not any better for a triode connected KT88. I installed a switch in my 300B stereo SET to have no NFB or 5 dB of NFB. A little bit of NFB will allow you another watt or so. Your most "offending" harmonic is going to be the second at 2 KHz because as you mentioned, it is a characteristic of a SE amplifier - the reason, long ago, engineers went to push pull. A 50 ohm resistor across the filament winding with the wiper to ground is a good idea for balancing out 60 Hz filament hum. Just by listening, it is often hard to distinguish between 60 and 120 Hz hum where the 120 Hz hum is coming from your full wave HV power supply. The best FFT audio spectrum analyzer I have and know of is the SpectraPlus software. It only requires an inexpensive USB sound card. You can download it for 30 days, all features, for free. For years... I had a "Ghost" copy on a CD of a Win 98 SE OS that I would load back on my PC after a quick and full format after a US government full-disk-wipe (the SpectraPlus looked for remaining data after a format so it required a disk-wipe) because the SpectraPlus is so good. www.spectraplus.com/Downloads.htm I am only going to this length because the old HP 3580A is likely a bad investment being over 40 years old - even though it is the bench-mark that all of the high-end vintage equipment was measured by. Just my opinion. I am betting your amplifier sounds great. I just did not believe in the "magic" of the SET amplifier for over 50 years of building until I actually did build one and listen to it. It is still unbelievable and THD at 1 or 2% is to be expected and we love it... Thanks for letting me know of your projects. Mine have focused on high power AM (amplitude modulated) transmitters lately as my love-affair with electronics flips between audio frequencies and radio frequencies. Please stay safe and keep in touch.
@@ElPasoTubeAmps Thanks for your Input on the Analyzer. I am totally with you on going for a software based FFT with an audio card. Ive been looking at compatibility between various USB audio interfaces and Arta. Will check out SpectraPlus too! Actually the Schematic I based the amp around uses GNFB 100K from the 8ohm OPT secondary to first stage Cathode. I havnt calculated how many DB’s this is, But I feel the THD may be a little high for the GNFB in place. The Amp sounds pretty good to me though! I went on to test another much smaller Chinese SE kit tube amp I Purchased and built years ago and have been listening to in the lounge at home and it read something insane like 10-15% THD Which makes me feel a little better about the KT88 amp. But on comparing to a Solid state Rotel I have lying around the workshop, it reads right down sub 0.1% at full output. Which assured me the 8903 isnt wildly out of calibration. I did notice while measuring the chinese kit amp that the 8903’s voltage reading across the 8ohm load was reading much lower that it was on my MultiMeter (multimeter reading 4Vrms, Whereas the 8903 was saying like 1.3v) I still can’t understand this, I checked on the Rotel and it was consistent at its 25v output. I wonder if the voltage reading from the chinese amp was too low for the High Level BNC input on the 8903. Perhaps I need to run the tests again connected to the low input/output. Would love to share some photos and measurements of the finished amplifier. Do you still have a website (Im sure you mentioned one in one of your videos, but couldnt track it down) I will motivate myself to make some videos someday too!
Very nice David! Just a question, how do you usually adjust the feedback network? Never thought it can be so critical that it might need a variable capacitor.
Hi Manuel, I first start off by disabling all NFB and drive the amplifier up to a power output level somewhere at least half of what I expect to get at full power out of the amplifier. In my 20W amplifiers I drive them to 15 watts. I know how much NFB I want in dB and start lowering the NFB resistor (usually starting around 10K) until I get the power output level for 16 db NFB (or whatever you want and calculate) which would be 0.38 watts for 16 dB attenuation from 15 watts. You know how to do all the math. The capacitor is determined (if needed) by placing a variable capacitor across the resistor and tweaking for best square wave leading edge. Hope this makes sense.
@@ElPasoTubeAmps yes it makes a ton of sense,never found a good explanation on how the capacitor is adjusted! So your feedback amount depends upon how much drive voltage you want to apply and the capacitor is for best frequency response right?
@@manuelminutello3027 I think that is the right idea. The resistor is going to determine how much FB voltage goes back to the cathode of the first stage out of phase. If it goes into a square wave oscillation when you apply NFB then you must reverse the phase somewhere along the line - reverse drive to the final tubes or reverse the primary leads of the OPT on the plates of the tubes. The resistor method and calculating FB dB output with no NFB and then with NFB, as far as I can see, if flawless but the capacitor thing is not that cut and dry from my experience. I have worked hours trying to get the capacitor value determination down to real science, pretty much always with the Williamson circuit but with different output transformers and I can not guarantee any result with the capacitor. Values can vary greatly and if they are to large will do very strange things to the LF response of the amplifier. I have read that using the square wave test and trying to clean up the leading edge is the thing to do. In this case it worked perfectly but in other cases it is very confusing and nothing seems to go the way we want it to. That is my experience and I am likely just to leave the capacitor off. If you look at some of the old Williamson designs, they put in a formula that the FB resistor should be 1200*sqrt(Z), Z being the impedance tap on the transformer. I really do not buy that but that might give you a place to start and they do not even show a capacitor. BTW - I do not use any of the series F(3dB) RC circuits unless I have to especially the first one that I believe is for the suppression of AM stations. It only seems to degrade HF performance in the amplifier. You may have to use the 70 KHz circuits if you can see a parasitic out around 70 KHz or so.
The only Stromberg Carlson schematic I remember is more like the Dynaco ST70 using a 7199 driver. There may be designs that others can identify but in the simplest sense, there are three basic designs of the vintage amplifiers - the Williamson design, the Dynaco design and the McIntosh design. There are also some differences in phase inversion and the driver stages but these are the classic designs that are recognized.
Nice performing amplifier and great video. I have a pair of UTC LS55's toutputs that I would love to build an amp with, and have been wanting to do a Williamson build. Would you be willing to share a schematic or at least let know what your feedback values are. Thanks for any help you can give.
Hi David - yes, I will be happy to share anything I know (or think I know...). As for NFB values and the resistors necessary to get the values we desire in feedback, I start out by disconnecting all NFB then run the amplifier up to some level of power that is still very good, meaning not distorted, monitor with an oscilloscope. If it is driven into clipping the measurements don't work. For example, run a 20 watt amplifier up to 10 watts or so with no NFB. I then decide how much NFB I want and put a pot, something like a 25K or so where the original feedback resistor was and turn down the power to the level that the amount of feedback would give. I am thinking of a classic Williamson design where we connect a resistor between the 16 ohm tap of the output transformer back to the cathode of the first stage of the 6SN7 or 12AU7, etc. For example if your amplifier is running at 10 watts with no NFB and you want 10 dB of feedback, you would adjust your resistor for 1 watt output. That would be 10 dB. Then disconnect and read the value of resistor with your ohm meter and install that value. 16 dB of NFB is common in Williamson era amplifiers and as much as 20 dB may start causing instabilities at the low end causing the scope trace to oscillate up and down at about 1 HZ. I have also found that the more NFB introduced into an amplifier, the more compressed it may sound although (within reason - up to around 16 dB) the more NFB the more power you will get with lower distortion. Typical values I have found for Williamson amplifiers are between 4.7K and 6.2K - just as a starting point... Hope this help.
@@ElPasoTubeAmps Thanks, that does help. The LS55 outputs are the same as the LS57s you used, excepting the line output terminals, so should perform similar to your amp if keeping to el34 triode mode, similar voltage, feedback, etc. You said you found a 150 pf cap across the feedback resistor worked for you. Realize that will be dependent on the feedback resistor, but do you know the frequency you found worked best of the tuned rc combo? Thanks!
@@davidmcgown9425 I know I did the capacitor tuning on a square wave and if I remember correctly, it would have been at 1 KHz and probably checked also at 10KHz square wave. It was amazingly sensitive as I went above about 180 pF, if I remember correctly, it got a big hump in the square wave and THD went up quickly. Just realized you may be talking about the RC snubber. I think I used 220 pf in series with 4.7K (I think...) that would be about 154 KHz. It seems almost anything in the upper KHz range should be good. It definitely stopped and smoothed out all the parasitics. If you raise the capacitor to 470 pF it will have a pole at 72 KHz or you can leave the capacitor at 220 pF and change the resistor to 10K for 72 KHz pole if you prefer. I chose to keep the capacitor small as it does have some effect of raising THD at the 20KHz level that I can measure.
This is going to be "off the chart" so to speak, but I am considering using the UTC LS-61's that I have in the same manner I used the James transformers - that is, used them in a para-feed SET as the matching transformer between the choke which is the plate load for the 300B and the speaker. I know that sounds strange but it is exactly the same thing I did with the James transformers and the results are truly amazing. There is no current flowing thru the primary of the matching output transformer. You would have to hear it to believe it.
@@ElPasoTubeAmps Thanks again. Your video was so clear, I was able to read the resistor code off your feedback resistor off the output winding (6.5K) and your discussion on square wave response stated you found 150 pF was the optimum value in your build. I assume you took the feedback off the 7.5 ohm tap because of the way the LS transformer windings are connected. Again the video and your discussion of the schematics really tell it all after reviewing it again. Thanks again for sharing your experience with us!
lol, same here. I have 6 williamson amps and an st70, some will at some point be used for parts. Not the best ones though. i hope. Nice vid. learnt some new things.
We like to think that some of our best works will not just be parted out after we are gone. On the other hand, how many home brew amplifiers have you picked up over the years and how many have you restored rather than just using parts like the OPT or power transformer, tube sockets, etc? I can't think of a single one.
@@ElPasoTubeAmps none indeed. although i picked up one. Only have the tubes left. the rest was chinese stuff that whas just not up to par. Have you noticed by the way that if you look at the signal when switching from say 30 to 40 Herz, the signal bumps up and down a bit before stabilizing?
@@kuda_71 I have noticed the scope trace bounce up and down on lots of amplifiers. I think that indicates that a maximum level of safe NFB has been reached. I find instabilities caused by excessive NFB show up on a 20 Hz signal often times. NFB level and performance is, in every case I have dealt with, dependent on the output transformer. Acrosound and UTC transformers do not perform the same. Same for the Chicago Standard BO-13 and the Triad and the Dynaco amplifiers. That interface we call the output transformer is where things get passed on accurately from the Williamson voltage amplifier section (the 6SN7's) or the signal gets lots of artifacts added in by a less-than-perfect output transformer. With that said, it is my conviction that the NFB resistor and shunt capacitor should be very carefully selected when building these amplifiers. If an amplifier is built exact to a schematic with the exact output transformer, I would assume it is safe to use the values assigned in the schematic but often times we can have everything the same, i.e. capacitors, resistors, etc but the output transformer is likely to not be the one prescribed in the schematic - so, we have to do some serious investigation and testing to get a stable amplifier.
It does and it delivers quite a bit more power and punch. I still love triodes but it is hard to beat the beam power tubes and the pentodes. I think I would need a quad of 300B or 6B4G for each channel to out-do this amplifier. But then that would be a bit too much power for these LS-57 and I would need bigger transformers. Actually I have two larger UTC transformers I hope to start on with Svetlana 572-10 triodes soon. That should do the trick.
I think I did test it at 10W but I don't remember the results. I will have to try it again and let you know. I agree that the AM station reduction is on the front part of the amplifier but it seems the 70KHz region of parasitics are around the output tubes.
I use the PC based software that can be gotten off the Internet. The HP 3580A is a beautiful lab grade instrument but it is so slow at 60 Hz it is almost unusable, unless you have lots of time to watch it plot... over and over again and again... I do not recommend spending the type of money most people want for a 40+ year old 3580A. If it breaks, and you can't fix it, you know the answer... I use an inexpensive Tektronix scope with FFT function if I need to look for harmonics or parasitics above 20 HKz which is not uncommon in the Williamson design, often times with a parasitic in the 100 KHz range. There are a number of PC based audio spectrum analyzer software packages available on the Internet easily found with a search.
The pot is 100 ohms and is across the 6.3 volt filament line with the wiper to ground. It is used to adjust out any possible 60 Hz hum that might be induced by the filament supply.
I agree but it is a lot of work to draw the schematics. However, I have gotten into using LTSpice to model them before the final build and I print and tape a schematic on the bottom cover for one channel. Usually I do not draw the full power supply. In many of my amplifiers, I use the Williamson Design which can easily be found with an Internet search.
I have never gotten into that level of detail for audio. I don't think a single channel preamp with tone controls would be that hard but if we wanted multiple inputs, the input selector switches, etc would be quite a challenge. I think I will stick with the store-bought preamps. :-)
Audio amplifiers of good quality, those having a frequency response that goes down to 30 or even 20 Hz, needs to have an output transformer with a high inductance (like 20 Henry's or more) in the primary so that the inductive reactance remains relatively high at these low frequencies and they are not attenuated. Most quality vacuum tube output transformers in the 20 watt range have a primary inductance around 20 to 40 Henry's of inductance.
RC circuit across the output transformer: According to the Audio Cyclopedia (page 615), quote "A limiting circuit consisting of a resistor and capacitor connected in series and in parallel with the output transformer, prevents damage to high-frequency speaker units caused by transient disturbances." It will not affect the frequency response. It is not a Zobel circuit. A Zobel circuit are seen in solid state output circuit to cancel out the reactive portion of a speaker (inductive load). As for the capacitor & resistor in parallel in the feedback circuit, it is there to help maintain the phase of the feedback at higher frequencies.
True . It acts like a rc damper to keep the circuit from hf ringing excited by transients. Looks like it improves the phase margin in the transient response.
@@johnwilliamson467 I have become a believer in the RC damper circuits. Maybe in a particular amp with a certain set of tubes, etc... measurements may be perfect and it may be decided that they are not necessary but the slightest bit of change and we may end up with a parasitic at 75KHz... and we will never know it unless we take the amplifier back to the bench and do lots of testing and look way out past the audio spectrum with a spectrum analyzer. I can see that they are a real necessity in a mass production environment calming down the variations in circuit components and especially ageing of component values. I have finally decided that the little RC snubbing circuits are our friends.
A lot would depend on how much a person had to pay for a pair of UTC transformers - probably $600 or so. Power transformer maybe less than $100. Chassis and capacitors and all small parts maybe less than $200 so a reasonable guess might be $900 for all the parts.
It also presents the OPT with a similar load with different speakers. Stops the speakers impedance at different frequencies having as much effect . I use one on all my valve amp builds, start with something like a 0.1u and a big 10r, you have to experiment to get the best result....DA
@@diabolicalartificer I knew you guys would know the best answer. I have used them but I usually don't just like the little RC circuits I mentioned in the video - but, if we feel they are needed it is good to know about all these remedies.
Here in the UK some manufacturers (Naim, Exposure, NVA) preferred not to fit one, along with the output inductor in their solid state designs. As such advised against the use of high capacitance Litz speaker cables which could (and did) burn-out the output stage if used.
@@stephenfurley1025 They're responsible for their own design decisions.. One of the main objectives of the zobel network is to provide load at high frequencies to prevent oscillation.
All that skill, thinking, work time and great vintage parts put altogether on probably great amplifiers but that we won't ear any of them. I's like a chief that shows great skill in preparing a great meal but instead of sharing the results with his audience, he put it into the garbage. That's really frustrating. Maybe having a partner doing it for him would be a great addition.
It is just that there is nothing but the music that TH-cam allows can be played without a copyright warning and I don't like any of it. I understand the copyright stuff but it is frustrating and I don't know how it seems some people get away with it. That is why I hardly every play music on my channel. I once got a copyright warning of a song that sounded like the fans running in my 813 amplifier. Seriously, their music sounded like the cooling fan in my amplifier. What is the world coming to... :-)
ElPaso TubeAmps could you make a video explaining how a linear Rf amplifier like the one in this link works and any improvements that could be made in this 200 watt grounded grid linear Rf amplifier if used to amplify an AM transmitter signal here is the link www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1955/06/page21/index.html
Parker, That is a very interesting amplifier. I read the entire article. Very nicely written and he touches on all the important points. The fact that the base of the tube from the 807 or 1625 must be removed to disconnect the beam forming plates is likely going to be a challenge and will make spare tubes a problem. You could not use the 807 and have a base pin to connect the beam forming plate to as all of the pins are used in the standard 5 pin socket. The 1625 is a 7 pin socket so you could use pin 2 or 5 for this re-connection of the beam forming plates so they could be grounded independently. I can see it working but this is truly a labor-of-love amplifier and complication beyond anything I have seen before. I find it interesting and I did not know the beam forming plate connection was external to the inside of the tube envelope. I don't know if that will every be used by me but it is interesting to know. In my opinion, having to remove the base from the tube is just too much to go thru. I hope you understand my thoughts on it. Not tuning the input circuit will cause a good bit more distortion in the RF output and make the tube(s) harder to drive. It is a good idea to always tune the cathode circuit in a GG amplifier. Lastly, using a "linear" to amplify an AM signal is very inefficient but it can be and is done in some cases. From my perspective, this is a novelty of an amplifier design and interesting to observe the uniqueness and ingenuity of the designer but I would look elsewhere for something that did not require modification of the actual vacuum tube to make it work. Just my thoughts on it. 73 WA4QGA
I have to admit that I am stuck with the 1960's military equipment look. I did paint my grandson't vacuum tube headphone amplifier blue. Maybe someday I will paint one black... :-)
I have watched this video several times and great stuff. Just found a Heathkit AA-71 unbuilt on eBay and will have it soon. It is a mono but I have my dad's Jensen hi-fi 50's vintage speaker system to play it on (pre-stereo). But like you the joy is in the building and testing... and listening. Means I have stuff stacking up everywhere. It is a learning experience that I have looked forward to in retirement. I appreciate your videos and learning everytime. Thanks!
About 50 years ago, I had an LS-58 output transformer that was part of an amplifier with four 5881 output tubes.
I think the LS series of UTC transformers have a very nice sound to them. A bit more clear, to my ear, than the Acrosound transformers. On the other hand, most of these UTC transformers perform poorly below 30 Hz. They have the same primary inductance as the Acrosound transformers, 20H. But after building and listening at length to the 300B stereo amplifier, I can only come to the conclusion that THD below probably 3-4% really doesn't matter much and may even be an artifact that enhances the sound. I know that flies-in-the-face of what we know technically and theoretically, but in the real-world I think many people would agree with the "sound" of SE amplifiers and their sawtooth harmonic profile. What else can I say?
A somewhat similar example is audio on amateur radio. HiFi sound has too many lows in it and is easily obscured by every type of noise that can be found on the HF bands. I don't think a ham radio operator wants to sound exactly like a space shuttle radio transmission but we have to admit, it does get the message thru effectively.
I think I have pretty well satisfied my own personal goal of evaluating the vintage high-end transformers, in particular the Acrosound, UTC, Peerless and the more modern day James transformers. They are all good but some of the Acrosound transformers, I had ten at one time, can have variations in performance that is hard to understand. I don't know if it is age or QC but I got to use all ten of them in the same circuit so there is something to be said for that.
I have immersed myself in AM transmission lately and are having a blast. My 300 watt output transmitter with power supplies and modulator weighs in at about 350 pounds and puts out a lot of heat but it is beautiful. Great for the winter in the basement but not so much so in the summer. Always good to hear from you and I hope you are doing well. Everything is good here but I have slowed down a bit now at 71 but still having fun. :-)
@@ElPasoTubeAmps That's a bit unexpected to hear about the LF performance of LS series transformers. I understood them to be -3dB at 10Hz and 40KHz and they certainly have enough iron to get down that low. But yes, 3% THD is too high at 20Hz.
A hi-fi amplifier should only increase the amplitude of a signal, adding or subtracting nothing along the way. Unfortunately, this new trend toward "euphonics" is not hi fidelity. Amps that distort belong in the musical instrument end of the spectrum, IMHO. When you're MAKING music, shaping the tone is part of the sound. But reproduction should be agnostic and faithful to the original sound.
I generally agree about communication audio, but there is a HAM near me, K1KBW, who runs a Class E rig on AM that sounds better than broadcast quality. His modulation positive peaks are around 180% and he really cuts through when other AM'ers are barely modulating the noise floor. He's the new modulation king, even well beyond WA1HLR (Timtron), whose transmissions impressed me in the early 1970s.
YayM is still the best transmission mode for enjoyment of HAM radio. Slopbucket never appealed to me. I suppose it's because building a filter that doesn't kill the lows below 300Hz is so difficult that almost no rigs have a decent audio response on sideband.
Nice to hear you're still doing okay, Dave!
your amps always turn out so good
Those transformers are beautiful. The amp looks awesome!
Thanks David. I remember hearing the local AM stations through my tube-driven Heathkit phonograph - good reception. Your videos are always packed with information. I am self-taught in all things vacuum-tube and your experience is invaluable to me. Beautiful workmanship, as always.
Thanks, Gary. I am self taught in vacuum tubes also. I have looked back at my college books of electronics and there were some vacuum tube circuits but I think they were there only as principles of ideas rather than circuits of the day. Transistors were the rage in the late 1960's and even IC's were available. When I went to work as a contractor in January 1970, our mainframes were called third-generation as they had IC circuits in the main arithmetic section of the computer. There were Thousands and thousands of RT-176 (2N2222) transistors everywhere. We called them ICR for integrated circuit registers and had a cycle time of 125 nano seconds. OMG... could it possibly be any faster (that is 8 MHZ clock frequency). Can you imagine but it was a 36 bit (not 8 or 32) word that could be double precision at 72 bits. We were definitely in the big-time 50 years ago and had just gone to the moon six months earlier. It was a good time and lots of work. Wish I could do it all over again.
Another magnificent build...love your videos keep them coming!
What a beautiful design of a amp
Love it
The quality of your work is astonishing, well done on another beautiful build! 🍻
Pro tip: you can watch movies at KaldroStream. I've been using it for watching loads of movies these days.
@Messiah Theo Definitely, I have been using KaldroStream for months myself =)
@Messiah Theo Definitely, I've been watching on kaldrostream for since november myself =)
Fantastic work mate
this is a well executed build. thank you for the view!
Really interesting. Particularly the heater hum pot, I never knew how important that is....thanks
Nice build, I like the way you layout your amps. Beginners should take a close look, they could learn a hell of a lot. Grand video as always.....DA
Beautiful job sir!
beautiful!!!! Magnifico!!!
I'm ABSOLUTELY including a mystery hole in my next build. 😎
I have enough mystery holes in some of my chassis to get good air flow ventilation... I do recycle chassis sometimes if the holes are not such that it prevents me from properly mounting the new project components. It is not too hard to lay out the major components from input RCA connectors to speaker connections and tube sockets and transformers - all the common components, but it is difficult to determine all the best places for the solder terminals for the under chassis resistors and capacitors. They usually have to be drilled and mounted after all the major components are installed. Also, laying out tube sockets for best visual appeal from the top side of the chassis is often not the best layout for component placement under the chassis. Everything is a trade-off between visual appeal and performance, right down to a compromise of THD vs square wave response vs stability and on and on. I recently read an article that decades ago when guys like us did not have any test equipment other than a voltmeter/ammeter, they called amplifier building a "black art". I kind of understood that. How would one know if the amplifier had a parasitic at 70 KHz with out a spectrum analyzer or THD was most anything below 5% and clipping such that it could be heard? A whole lot of guess work was part of the home building art back then. It still amazes me the level of lab quality equipment we hobbyist can own these days. They made some good stuff back in the '70's/80's that still performs properly and is affordable. Stay safe and sane in these trying times.
Radio Moscow was a favourite station to break though on amps back in the 70s in the UK
I remember listening to Radio Moscow and Voice of America as a ham radio operator back then. The last thing I remember from the Russians was the "Russian woodpecker" which I think was an over-the-horizon radar system. Glad that thing is gone. I guess I am a little too far away across the pond for Radio Moscow to get into my stereo but the little 1 KW local AM radio station, WRAB, in the town of Arab, Alabama where I grew up could sure get into my equipment. That was the first radio station I got to see inside and I still remember those four beautiful 833A tubes glowing and probably some blue MV rectifiers at the bottom of the transmitter.
that's so interesting. thanks for the walk around your amp. I wish I could run my altec 604c with that.
Your Altec's would sound so good. 20W per channel might not impress many these days where advertisers claim hundreds of watts output of a plastic box weighing 5 pounds but you guys know better. I drive Dahlquist DQ-20 speakers with this amplifier that are terrible inefficient at 86 dB SPL. compared to Klipschorn at 105 dB. That is 19 db difference almost exactly 4 times louder on the Klipschorns and still these Dahlquist speakers will run you out of the room and sound Great on this little amplifier. Hard to believe but I love it.
@@ElPasoTubeAmps I think they're rated at 30 watts and I bet a 20w amp like yours would
absolutely sound great. my understanding is you don't want to exceed the rated watts on these EVER! but they sound so good so why go there! plenty loud!
I LOVE how your amp is built! great goin !
Nice amp ! Would love to won that beast
Your microphone audio quality is improved to much higher degree. Camera looks good, too. I learned a few things in this video and I thank you.
That is good to hear. Thank you for letting me know. My usual camera is a small Leica camera made in Japan and this camera is a Sony, also made in Japan but a lot more camera and as mentioned 42 MP. I can see a difference in the quality of the video. Video files are huge but they combine in Movie Maker just as fast and upload plenty fast enough. Thanks again and stay safe.
WOW! great work surely it must take a lifetime to be as skilled as you
Well I'm 23 and I have a long way before I will be as good as you
I think it just takes a passion for what we do and always thinking about technology bordering on obsession. It is what keeps me alive today at 70 and I have been doing it in earnest since 13.
The Williamson phase splitter actually unbalances at supersonic frequency because the cathode and anode behave differently. The cathode coupled phase splitter when balanced is better at supersonic frequency and gives improved stability. Less correction is needed across the feedback resistor and the rc network across the plate load in stage 1 can be eliminated.
Beautiful work amazing video question do you have any for sale
Howdy. Great demo.
As You say. The 10 kHz square wave response is superior.
My experience is that using ultralinear mode greatly improves the lower bands THD.
Regards.
Looks beautiful....sound demo...please.
Beautiful! I've been trying to get in touch with you for a year or so sir. What's the best way to do so? Have a pair of amplifiers I think you'll be interested in :) Thanks,
Justin
I’ve had to add an RC circuit on my Ultra Linear KT88 amplifier between plate and screen. It was oscillating at 111khz and although it still sounded fantastic, It was visible on the scope at a reasonably high level. After trying all manner of solutions I eventually came across a recommendation documented in a GEC tube datasheet giving a UL KT88 circuit example and gave resistor/capacitor values that may be required for this RC circuit. It solved my issue immediately. Im not exactly sure what I’ve traded off by doing this but looking forward to taking some comparative readings on the 8903! Interesting about the Mhz filter network on the front end. I would not have guessed the reason behind that one!
Isn't it great to actually catch these anomalies? Do the math and check the 3 dB point of the RC circuit and you will see that many, from days of old, are in the frequency range of AM broadcast stations (somewhere between 500KHz and 1.6 MHz) to help keep out your local AM radio station. Not much of that left nowadays but 50 years ago it was serious.
You will find that it does affect the HF response up at 20KHz and beyond but it is usually a good idea to put one in usually on the output of the first stage as you will see in virtually all commercial made amplifiers. Your amplifier may be perfectly stable with out it (during any particular test) but you can change the tubes, components can age, your cat can walk in front of you, and some will not need it and some will. I add them in now.
Really nice amp. Pretty much what my next build is going to be. Not too sure on the output tubes though. Those EL34s are great tubes, but I may go with 6L6s. Definitely going with the 6sn7 Williamson design. Hopefully I can find some output transformers as good as those UTC. That UV glue is pretty amazing. I have some and use it every once in a while. My go to is usually Loctite instant epoxy or Goop. Depends wether I can let it sit all night or not. I definitely need to get an Audio Discovery. Take care!
I am definitely a 6L6 fan but it seems the EL34 has a little brighter high end and so does the UTC output transformers over the Acrosound transformers. I think they are both great OPT but the UTC transformers seem to hold a special place in the audio world. It seemed only appropriate to finally build a stereo amp with them. I truly do like the sound of this amplifier. Of course, I have to know it performs well on the test bench and then it also sounds great to my ears and music so that is a match made in heaven for me. I like the quick epoxy also. I think I mentioned I already had to replace one of the voltage doubler caps and by sticking it on with Goop, it was nice and easy to remove and clean off the residue and stick another one back in its place. I don't think the UV glue would be the best for sticking on the electrolytic capacitors. It is a little light-weight in my opinion. Great to hear from you. I hope all is well.
I have always had a liking of Williamson design power amps. Weren't they also known as ultralinear amps too?
I know I have said it before, buy you do VERY nice work! Great sounding and Great to look at! I really enjoy seeing your work. Thank You.
Stay Safe!
Thanks Bill,
In the beginning of the Williamson design, I don't think UL existed yet but came along shortly afterwards. That is kind of the interesting about these great sound UTC transformers. They are simple CT primary with on fancy UL taps, etc. I am wondering if they wound the primary wires side by side for better coupling rather than in a bobbin style. They actually do live up to their reputation.
David, thank you for another great video what kind of a preamp can you pair this amp with
Most any preamp should work fine. I am driving it with a McIntosh MX-113 which has a tuner. It really sounds nice and clean with great high end. I attribute that to the UTC transformers and only about 15-16 dB NFB. Too much NFB seems to make them sound more compressed although it does bring down THD. There is always a trade-off in these amplifiers. I have found that over focus on making one parameter perfect often times causes an anomaly we are not aware we have created. In particular, too much NFB or making the NFB bypass capacitor too large may cause a parasitic around 100KHz that we will only see if we use a spectrum analyzer. I hope to make a video on that soon. Thanks for you comments. Stay safe.
Great job, I too like an impartial amp that doesn't discriminate any kind of music be it classical, rock, punk, jazz or what have you.😁😎
Even though it doesn't do 20Hz, it is not a light-weight on the low end and I do, like yourself, like all kinds of music. Thanks for your comments.
Hey great video. ? Do you say any of your amps
Igor s. Popovich in audiophile vacuum tube amplifiers vol . 3 suggested a solid state diode on the center tap of the transformer with two vacuum tube diodes would reduce IMD . This would be a fine test bench to test that out. ( ss diode neg to ground making a sort of full wave bridge 3 leg thing . A good name for it exscapes me.
Hi there, Im wondering if you think using two different tubes/transformers for tweeter and midbass is a good idea. IE acrosound for tweeters and UPC for bass.
I have a UTC special series I'm trying to find it and I'm running a little bit more about this from the 1930s can you give me some more information on that if you could please and maybe if you have some pictures
david do you put a mains earth on the chassis
Hi David, I’ve just finished my four year tube amplifier build and have been running some tests on it with my newly acquired HP8903! Really exciting to get to this point. The frequency response is super flat! However the THD+N reads higher than I imagined it would. As it is a Single Ended KT88 Ultra Linear build what would you expect as reasonable THD values at moderate power level at 1Khz? I think i was up at around 3% I have been playing with Hum Balance and a few other ideas but unable to reduce it much. I wonder if the distortion which is present is mainly even order harmonics and if this can be expected with SE or Ive got some issues somewhere. I suppose this means I need to buy a spectrum Analyzer now!
Absolutely... congratulations on your build and measurements. The HP 8903 is a beautiful machine. HP or whatever they are called now, a few years ago, offered to buy back 8903's for almost the price of a new model that did not have the built in oscillator - so they could destroy them - the 8903's are that good and last that long - not like the limited-life instruments of today.
First off, I assume you are not using any GNFB? If that is the case, 3% is to be expected for the 300B at around 6 or 7 watts and probably not any better for a triode connected KT88. I installed a switch in my 300B stereo SET to have no NFB or 5 dB of NFB. A little bit of NFB will allow you another watt or so.
Your most "offending" harmonic is going to be the second at 2 KHz because as you mentioned, it is a characteristic of a SE amplifier - the reason, long ago, engineers went to push pull.
A 50 ohm resistor across the filament winding with the wiper to ground is a good idea for balancing out 60 Hz filament hum. Just by listening, it is often hard to distinguish between 60 and 120 Hz hum where the 120 Hz hum is coming from your full wave HV power supply.
The best FFT audio spectrum analyzer I have and know of is the SpectraPlus software. It only requires an inexpensive USB sound card. You can download it for 30 days, all features, for free.
For years... I had a "Ghost" copy on a CD of a Win 98 SE OS that I would load back on my PC after a quick and full format after a US government full-disk-wipe (the SpectraPlus looked for remaining data after a format so it required a disk-wipe) because the SpectraPlus is so good.
www.spectraplus.com/Downloads.htm
I am only going to this length because the old HP 3580A is likely a bad investment being over 40 years old - even though it is the bench-mark that all of the high-end vintage equipment was measured by. Just my opinion.
I am betting your amplifier sounds great. I just did not believe in the "magic" of the SET amplifier for over 50 years of building until I actually did build one and listen to it. It is still unbelievable and THD at 1 or 2% is to be expected and we love it...
Thanks for letting me know of your projects. Mine have focused on high power AM (amplitude modulated) transmitters lately as my love-affair with electronics flips between audio frequencies and radio frequencies.
Please stay safe and keep in touch.
@@ElPasoTubeAmps Thanks for your Input on the Analyzer. I am totally with you on going for a software based FFT with an audio card. Ive been looking at compatibility between various USB audio interfaces and Arta. Will check out SpectraPlus too!
Actually the Schematic I based the amp around uses GNFB 100K from the 8ohm OPT secondary to first stage Cathode. I havnt calculated how many DB’s this is, But I feel the THD may be a little high for the GNFB in place. The Amp sounds pretty good to me though!
I went on to test another much smaller Chinese SE kit tube amp I Purchased and built years ago and have been listening to in the lounge at home and it read something insane like 10-15% THD Which makes me feel a little better about the KT88 amp. But on comparing to a Solid state Rotel I have lying around the workshop, it reads right down sub 0.1% at full output.
Which assured me the 8903 isnt wildly out of calibration. I did notice while measuring the chinese kit amp that the 8903’s voltage reading across the 8ohm load was reading much lower that it was on my MultiMeter (multimeter reading 4Vrms, Whereas the 8903 was saying like 1.3v) I still can’t understand this, I checked on the Rotel and it was consistent at its 25v output. I wonder if the voltage reading from the chinese amp was too low for the High Level BNC input on the 8903. Perhaps I need to run the tests again connected to the low input/output.
Would love to share some photos and measurements of the finished amplifier. Do you still have a website (Im sure you mentioned one in one of your videos, but couldnt track it down) I will motivate myself to make some videos someday too!
there good david don't worry i did that with the rca
Very nice David!
Just a question, how do you usually adjust the feedback network? Never thought it can be so critical that it might need a variable capacitor.
Hi Manuel, I first start off by disabling all NFB and drive the amplifier up to a power output level somewhere at least half of what I expect to get at full power out of the amplifier. In my 20W amplifiers I drive them to 15 watts. I know how much NFB I want in dB and start lowering the NFB resistor (usually starting around 10K) until I get the power output level for 16 db NFB (or whatever you want and calculate) which would be 0.38 watts for 16 dB attenuation from 15 watts. You know how to do all the math. The capacitor is determined (if needed) by placing a variable capacitor across the resistor and tweaking for best square wave leading edge. Hope this makes sense.
@@ElPasoTubeAmps yes it makes a ton of sense,never found a good explanation on how the capacitor is adjusted!
So your feedback amount depends upon how much drive voltage you want to apply and the capacitor is for best frequency response right?
@@manuelminutello3027 I think that is the right idea. The resistor is going to determine how much FB voltage goes back to the cathode of the first stage out of phase. If it goes into a square wave oscillation when you apply NFB then you must reverse the phase somewhere along the line - reverse drive to the final tubes or reverse the primary leads of the OPT on the plates of the tubes. The resistor method and calculating FB dB output with no NFB and then with NFB, as far as I can see, if flawless but the capacitor thing is not that cut and dry from my experience. I have worked hours trying to get the capacitor value determination down to real science, pretty much always with the Williamson circuit but with different output transformers and I can not guarantee any result with the capacitor. Values can vary greatly and if they are to large will do very strange things to the LF response of the amplifier.
I have read that using the square wave test and trying to clean up the leading edge is the thing to do.
In this case it worked perfectly but in other cases it is very confusing and nothing seems to go the way we want it to. That is my experience and I am likely just to leave the capacitor off.
If you look at some of the old Williamson designs, they put in a formula that the FB resistor should be 1200*sqrt(Z), Z being the impedance tap on the transformer. I really do not buy that but that might give you a place to start and they do not even show a capacitor.
BTW - I do not use any of the series F(3dB) RC circuits unless I have to especially the first one that I believe is for the suppression of AM stations. It only seems to degrade HF performance in the amplifier. You may have to use the 70 KHz circuits if you can see a parasitic out around 70 KHz or so.
Those old Williamsons remind me of some of the old stromberg carlson equipment ive seen. Wonder if there's any connection?
The only Stromberg Carlson schematic I remember is more like the Dynaco ST70 using a 7199 driver. There may be designs that others can identify but in the simplest sense, there are three basic designs of the vintage amplifiers - the Williamson design, the Dynaco design and the McIntosh design. There are also some differences in phase inversion and the driver stages but these are the classic designs that are recognized.
You using Big Red like LMG are using.
Nice performing amplifier and great video. I have a pair of UTC LS55's toutputs that I would love to build an amp with, and have been wanting to do a Williamson build. Would you be willing to share a schematic or at least let know what your feedback values are. Thanks for any help you can give.
Hi David - yes, I will be happy to share anything I know (or think I know...). As for NFB values and the resistors necessary to get the values we desire in feedback, I start out by disconnecting all NFB then run the amplifier up to some level of power that is still very good, meaning not distorted, monitor with an oscilloscope. If it is driven into clipping the measurements don't work. For example, run a 20 watt amplifier up to 10 watts or so with no NFB. I then decide how much NFB I want and put a pot, something like a 25K or so where the original feedback resistor was and turn down the power to the level that the amount of feedback would give. I am thinking of a classic Williamson design where we connect a resistor between the 16 ohm tap of the output transformer back to the cathode of the first stage of the 6SN7 or 12AU7, etc. For example if your amplifier is running at 10 watts with no NFB and you want 10 dB of feedback, you would adjust your resistor for 1 watt output. That would be 10 dB. Then disconnect and read the value of resistor with your ohm meter and install that value. 16 dB of NFB is common in Williamson era amplifiers and as much as 20 dB may start causing instabilities at the low end causing the scope trace to oscillate up and down at about 1 HZ. I have also found that the more NFB introduced into an amplifier, the more compressed it may sound although (within reason - up to around 16 dB) the more NFB the more power you will get with lower distortion. Typical values I have found for Williamson amplifiers are between 4.7K and 6.2K - just as a starting point... Hope this help.
@@ElPasoTubeAmps Thanks, that does help. The LS55 outputs are the same as the LS57s you used, excepting the line output terminals, so should perform similar to your amp if keeping to el34 triode mode, similar voltage, feedback, etc. You said you found a 150 pf cap across the feedback resistor worked for you. Realize that will be dependent on the feedback resistor, but do you know the frequency you found worked best of the tuned rc combo? Thanks!
@@davidmcgown9425 I know I did the capacitor tuning on a square wave and if I remember correctly, it would have been at 1 KHz and probably checked also at 10KHz square wave. It was amazingly sensitive as I went above about 180 pF, if I remember correctly, it got a big hump in the square wave and THD went up quickly.
Just realized you may be talking about the RC snubber. I think I used 220 pf in series with 4.7K (I think...) that would be about 154 KHz. It seems almost anything in the upper KHz range should be good. It definitely stopped and smoothed out all the parasitics. If you raise the capacitor to 470 pF it will have a pole at 72 KHz or you can leave the capacitor at 220 pF and change the resistor to 10K for 72 KHz pole if you prefer. I chose to keep the capacitor small as it does have some effect of raising THD at the 20KHz level that I can measure.
This is going to be "off the chart" so to speak, but I am considering using the UTC LS-61's that I have in the same manner I used the James transformers - that is, used them in a para-feed SET as the matching transformer between the choke which is the plate load for the 300B and the speaker. I know that sounds strange but it is exactly the same thing I did with the James transformers and the results are truly amazing. There is no current flowing thru the primary of the matching output transformer. You would have to hear it to believe it.
@@ElPasoTubeAmps Thanks again. Your video was so clear, I was able to read the resistor code off your feedback resistor off the output winding (6.5K) and your discussion on square wave response stated you found 150 pF was the optimum value in your build. I assume you took the feedback off the 7.5 ohm tap because of the way the LS transformer windings are connected. Again the video and your discussion of the schematics really tell it all after reviewing it again. Thanks again for sharing your experience with us!
lol, same here. I have 6 williamson amps and an st70, some will at some point be used for parts. Not the best ones though. i hope. Nice vid. learnt some new things.
We like to think that some of our best works will not just be parted out after we are gone. On the other hand, how many home brew amplifiers have you picked up over the years and how many have you restored rather than just using parts like the OPT or power transformer, tube sockets, etc? I can't think of a single one.
@@ElPasoTubeAmps none indeed. although i picked up one. Only have the tubes left. the rest was chinese stuff that whas just not up to par. Have you noticed by the way that if you look at the signal when switching from say 30 to 40 Herz, the signal bumps up and down a bit before stabilizing?
@@kuda_71 I have noticed the scope trace bounce up and down on lots of amplifiers. I think that indicates that a maximum level of safe NFB has been reached. I find instabilities caused by excessive NFB show up on a 20 Hz signal often times. NFB level and performance is, in every case I have dealt with, dependent on the output transformer. Acrosound and UTC transformers do not perform the same. Same for the Chicago Standard BO-13 and the Triad and the Dynaco amplifiers. That interface we call the output transformer is where things get passed on accurately from the Williamson voltage amplifier section (the 6SN7's) or the signal gets lots of artifacts added in by a less-than-perfect output transformer. With that said, it is my conviction that the NFB resistor and shunt capacitor should be very carefully selected when building these amplifiers. If an amplifier is built exact to a schematic with the exact output transformer, I would assume it is safe to use the values assigned in the schematic but often times we can have everything the same, i.e. capacitors, resistors, etc but the output transformer is likely to not be the one prescribed in the schematic - so, we have to do some serious investigation and testing to get a stable amplifier.
Interesting, so it actually beats your previous favorite, the 6B4G amp?
It does and it delivers quite a bit more power and punch. I still love triodes but it is hard to beat the beam power tubes and the pentodes. I think I would need a quad of 300B or 6B4G for each channel to out-do this amplifier. But then that would be a bit too much power for these LS-57 and I would need bigger transformers. Actually I have two larger UTC transformers I hope to start on with Svetlana 572-10 triodes soon. That should do the trick.
Did you try your amp at 10W but at 20Hz to see if distortion disappear? AM station signal is reduced by a cap(+res) at input not in output stage.
I think I did test it at 10W but I don't remember the results. I will have to try it again and let you know. I agree that the AM station reduction is on the front part of the amplifier but it seems the 70KHz region of parasitics are around the output tubes.
What are you using as spectrum analyzer software these days?
I use the PC based software that can be gotten off the Internet. The HP 3580A is a beautiful lab grade instrument but it is so slow at 60 Hz it is almost unusable, unless you have lots of time to watch it plot... over and over again and again... I do not recommend spending the type of money most people want for a 40+ year old 3580A. If it breaks, and you can't fix it, you know the answer... I use an inexpensive Tektronix scope with FFT function if I need to look for harmonics or parasitics above 20 HKz which is not uncommon in the Williamson design, often times with a parasitic in the 100 KHz range. There are a number of PC based audio spectrum analyzer software packages available on the Internet easily found with a search.
Hi i look on your amp and id like , you explain how you set your 6.3 v heather with heater pot with a shematic and the value of the pot .thanks
The pot is 100 ohms and is across the 6.3 volt filament line with the wiper to ground. It is used to adjust out any possible 60 Hz hum that might be induced by the filament supply.
I'd be great if you could provide schematics of your builds. Share the knowledge man!
I agree but it is a lot of work to draw the schematics. However, I have gotten into using LTSpice to model them before the final build and I print and tape a schematic on the bottom cover for one channel. Usually I do not draw the full power supply. In many of my amplifiers, I use the Williamson Design which can easily be found with an Internet search.
How about building a matching preamplifier.
I have never gotten into that level of detail for audio. I don't think a single channel preamp with tone controls would be that hard but if we wanted multiple inputs, the input selector switches, etc would be quite a challenge. I think I will stick with the store-bought preamps. :-)
What does a 20H choke do to the frequencies?
Audio amplifiers of good quality, those having a frequency response that goes down to 30 or even 20 Hz, needs to have an output transformer with a high inductance (like 20 Henry's or more) in the primary so that the inductive reactance remains relatively high at these low frequencies and they are not attenuated. Most quality vacuum tube output transformers in the 20 watt range have a primary inductance around 20 to 40 Henry's of inductance.
RC circuit across the output transformer: According to the Audio Cyclopedia (page 615), quote "A limiting circuit consisting of a resistor and capacitor connected in series and in parallel with the output transformer, prevents damage to high-frequency speaker units caused by transient disturbances."
It will not affect the frequency response.
It is not a Zobel circuit. A Zobel circuit are seen in solid state output circuit to cancel out the reactive portion of a speaker (inductive load).
As for the capacitor & resistor in parallel in the feedback circuit, it is there to help maintain the phase of the feedback at higher frequencies.
True . It acts like a rc damper to keep the circuit from hf ringing excited by transients. Looks like it improves the phase margin in the transient response.
@@johnwilliamson467 I have become a believer in the RC damper circuits. Maybe in a particular amp with a certain set of tubes, etc... measurements may be perfect and it may be decided that they are not necessary but the slightest bit of change and we may end up with a parasitic at 75KHz... and we will never know it unless we take the amplifier back to the bench and do lots of testing and look way out past the audio spectrum with a spectrum analyzer. I can see that they are a real necessity in a mass production environment calming down the variations in circuit components and especially ageing of component values. I have finally decided that the little RC snubbing circuits are our friends.
Doamne ajuta !
what sort of cost would it be to build 1 of these amps?
A lot would depend on how much a person had to pay for a pair of UTC transformers - probably $600 or so. Power transformer maybe less than $100. Chassis and capacitors and all small parts maybe less than $200 so a reasonable guess might be $900 for all the parts.
Must adjust,on output tubes,katode resistors....
Why did you install Chinese film capacitors? Install vima MKP-10.
zobel network provides a load at high frequencies..
Thank you... I knew one of you guys would know what it was meant for.
It also presents the OPT with a similar load with different speakers. Stops the speakers impedance at different frequencies having as much effect . I use one on all my valve amp builds, start with something like a 0.1u and a big 10r, you have to experiment to get the best result....DA
@@diabolicalartificer I knew you guys would know the best answer. I have used them but I usually don't just like the little RC circuits I mentioned in the video - but, if we feel they are needed it is good to know about all these remedies.
Here in the UK some manufacturers (Naim, Exposure, NVA) preferred not to fit one, along with the output inductor in their solid state designs. As such advised against the use of high capacitance Litz speaker cables which could (and did) burn-out the output stage if used.
@@stephenfurley1025 They're responsible for their own design decisions.. One of the main objectives of the zobel network is to provide load at high frequencies to prevent oscillation.
All that skill, thinking, work time and great vintage parts put altogether on probably great amplifiers but that we won't ear any of them. I's like a chief that shows great skill in preparing a great meal but instead of sharing the results with his audience, he put it into the garbage. That's really frustrating. Maybe having a partner doing it for him would be a great addition.
It is just that there is nothing but the music that TH-cam allows can be played without a copyright warning and I don't like any of it. I understand the copyright stuff but it is frustrating and I don't know how it seems some people get away with it. That is why I hardly every play music on my channel. I once got a copyright warning of a song that sounded like the fans running in my 813 amplifier. Seriously, their music sounded like the cooling fan in my amplifier. What is the world coming to... :-)
I mean 20Hz is not much audible I'll say 30 to 45Hz is fine the output transformer might not like 20Hz
ElPaso TubeAmps could you make a video explaining how a linear Rf amplifier like the one in this link works and any improvements that could be made in this 200 watt grounded grid linear Rf amplifier if used to amplify an AM transmitter signal here is the link www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1955/06/page21/index.html
Parker,
That is a very interesting amplifier. I read the entire article. Very nicely written and he touches on all the important points.
The fact that the base of the tube from the 807 or 1625 must be removed to disconnect the beam forming plates is likely going to be a challenge and will make spare tubes a problem. You could not use the 807 and have a base pin to connect the beam forming plate to as all of the pins are used in the standard 5 pin socket. The 1625 is a 7 pin socket so you could use pin 2 or 5 for this re-connection of the beam forming plates so they could be grounded independently. I can see it working but this is truly a labor-of-love amplifier and complication beyond anything I have seen before. I find it interesting and I did not know the beam forming plate connection was external to the inside of the tube envelope. I don't know if that will every be used by me but it is interesting to know. In my opinion, having to remove the base from the tube is just too much to go thru. I hope you understand my thoughts on it.
Not tuning the input circuit will cause a good bit more distortion in the RF output and make the tube(s) harder to drive. It is a good idea to always tune the cathode circuit in a GG amplifier.
Lastly, using a "linear" to amplify an AM signal is very inefficient but it can be and is done in some cases.
From my perspective, this is a novelty of an amplifier design and interesting to observe the uniqueness and ingenuity of the designer but I would look elsewhere for something that did not require modification of the actual vacuum tube to make it work. Just my thoughts on it. 73 WA4QGA
That's one ugly case but I guess cosmetics isn't everything and its the sound that matters. Thanks for the video.
I have to admit that I am stuck with the 1960's military equipment look.
I did paint my grandson't vacuum tube headphone amplifier blue.
Maybe someday I will paint one black... :-)
@@ElPasoTubeAmps Ha, no big deal especially if you are happy with it. The sound it provides is what's important!