I have always been taught to ground only one end of a shielded cable within the chassis an instrument at the source end - probably still good advice - but there may be exceptions... Well, this one seems to be tat exception. Changing to an insulated to ground input RCA jack and grounding the input point at various places on the chassis and grounding the far end of the shielded cable at the tube socket feeding the grid of the input stage (and, obviously, playing with the 60 Hz phase by sliding the input ground point at various places on the chassis) I can reduce the 60 Hz and its harmonics to -125 dB whereas it was at best in the video at about -95 dB ..... another 30 db reduction. This is, indeed, at the level of practically counting and smashing electrons but it is even more amazing that we can have this level of test equipment at our finger tips in a home laboratory. Stay safe, my friends.
David excellent detective work in tracking down and solving the spur/harmonic problems. Thanks! You realize that the price of 6SN7 tubes will go up again now.....😉
LOL...yes, I have thought about that. I can see the headline, "NOS" 6SN7's just doubled in price on Ebay... (actually I just bought four more and bid on 17 more them)
@@ElPasoTubeAmps Are you trying to hand pick from the lot of 17? The new PSVane CV181 is the best 6SN7 I have heard. Athought at $80 a pop, they are worth the price.
@@vintageaudio7518 I almost bid a high price to get those tubes but I have discovered, just tonight that lowering the NFB to below 15 dB completely removed all the "grit" and made the amplifier sound so good with any 6SN7. I will post soon to show what I have found,
Fascinating. This is a very important investigation. Much to ponder on. Regarding what happens at 23:00 with the amazing spread of lines 60Hz apart, I'm wondering if severely unbalancing the hum has caused the 60Hz hum wave to clip and so produce square-wave harmonics to IM with the signal. Might be worth a look. Of course the hum wave wasn't all that clean to begin it, it was never a sine wave, just whatever was on the mains transformed down. Maybe LC filter it, or I suppose more to the point go straight to DC heating.
I have toyed with changing to DC filaments but I don't think that is the issue. See my comment at the top. What really puzzles me is the vast spread of 60 Hz harmonics all the way out to the 11th harmonic at 660 Hz. The problem and the solution seems to be simpler that we think. Isn't that often times the case?
Thanks David. You're right. A person can hear the effect of these artifacts even if he or she doesn't hear the actual artifact, itself. These artifacts color the sound and one can be trained to hear their effects on the signal source.
May seem crazy but I rarely listen intently to my own amplifiers. But I have this time. I mentioned a slight "grit" sound as I described it, and someone else commented about the same thing. I can't find or remember who. I also remember hearing about the amount of NFB present in vacuum tube amplifiers vs SS amplifiers. Years ago I built an amplifier with KT90's and put in a variable NFB control (I got the idea from a Fender guitar amplifier). It was amazing how the sound changed as the NFB was increased all the way up to about 20 dB. I labeled the control "sound stage" as the amplifier sounded so alive with little to no NFB and how compressed the sound became as NFB was increased. However, max power level with NFB in was 80 watts or so and with no NFB it was 10 or 15 watts before distortion became too high. Surely there must be a compromise and it seems too much NFB may give an amplifier a "gritty" sound. So, with that in mind, I lowered the NFB in this amplifier to just under 15 dB and, to my ears, it cleaned it right up. Also makes it a little easier to drive which is not a problem. I think about 0.7 volts for full output.
@@ElPasoTubeAmps David, you remind me of those great actors who never watch their own movies :-) If 0.7 volts is all that's needed to drive this beauty, you could probably drive it directly from your CD player if there's a volume-out. They should deliver about 2.0 volts at the output jacks, if they're up to snuff. I drive my tube amps directly from my DAC. It's a Steinberg / Yamaha 4 in / 2 out professional recording DAW that I connect to my computer which then bypasses the computer's sound-card. The DAC is 24 bit at 192 kHz sampling rate so it decodes like butter. No preamplifier needed. Simple sound-chain. Thanks for your excellence in all things tube.
@@radiojet1429 Hi Gary, looking at amplifiers like McIntosh, some have a switch for a 0.7 volt input and a 2.5 volt input which they recommend for their preamps. I have been playing with a ST-70 lately that I have had for many years and it sounds darn good. I rarely listen to my amplifiers. Their life is lived out on a test bench. and I am beginning to realize that is inadequate to evaluate an amplifier for my YT viewers. I try to steer sway from subjective comments like, "how does it sound" because it is too subjective but is the bottom line with all sound equipment. I just got thru measuring my two speakers with the impedance bridge and they both measure the same at 120 Hz and 1KHz. Actually surprised me. I have some very nice CD's of classical music I may be able to play without RUclips keeping the video from being shown internationally because of copy-right laws or whatever. So, when we are trying to evaluate an "amplifier" what about the CD itself, the CD player or DAC or the speakers? Or maybe even my vintage ears... from hearing tests my ears drop off pretty quick after 8 KHz, although I don't think that necessarily disqualifies a person from being able to hear and evaluate good quality sound. You know how people rave over no NFB in 300B SET amplifiers. Years ago I built an amplifier of 80 watts with a pot in the feed back line where I could adjust NFB from basically zero to probably 20 dB or so. Wow... they sound so alive with no or very little NFB and get so compressed sounding as the NFB is cranked in. On the other hand, max output power was something like 15 watts vs 80 watts. I should do that as a video. I bet people will love it. As for being a great actor, I will try to keep that from inflating my ego to where my wife can't live with me... and as for watching my own videos... that was truly one of the intentions when I started doing YT so I could remember how to do things. Often times I do watch them once to make sure I don't make too many bloopers :-) YT took away the speech bubble because it can not be seen on phones and most YT videos are watched on phones, according to the answer I got from them. To my way of thinking, they are not truly interested in promoting any serious science... Thank you for your kind words and stay safe in beautiful Albuquerque where Bugs Bunny should have taken a left (or was it a right?)
@@ElPasoTubeAmps Thanks for the great reply, David. I have been intrigued by the sound changes that occur with NFB adjustment. It's true that lower NFB gives an openness to the sound source albeit while sacrificing power and distortion levels. I am sure I am not the only one who would like to see your NFB adjustment circuits. :-) Be safe.
If you are getting that problem with a 60Hz heater hum. That must mean ANY other frequency will give you intermod distortion which would probably be an amplifier design issue. Might be interesting to do a normal test for intermod!!
Michael Beeny Or maybe it is just caused by the heater modulating the cathode voltage, probably at the input stage, which is an IM effect of its own that doesn't apply elsewhere in the amplifier.
what about a sampling rate of 44150KHZ or 88300KHZ, If you take the FFT of the amplifier and then use a DSP system that can process the incoming signal with the FFT transform you have a digital amp with the signature of a vacuum tube amp, can you not decouple the heaters to ground with large value high voltage capacitors.
It has been suggested to raise the filament above ground but simply insulating the input RCA connector from ground and using shielded cable all the way to the tube grid did amazing things for the 60 Hz noise.
@@ElPasoTubeAmps good thing to keep in the old tool box. Its funny that you can build 1000 amps and every one finds a new way to pick up 60hz, but has never been a problem in the previous 999.
Can the hum balance be controlled/adjusted separately between 60Hz line side and the power supply 120Hz side? Would it help to adjust these balances when running with much higher power just under the clipping point so that you could see the range of its effectiveness?
Howdy! I ran into the same “grit” problem both in push-pull and SE amps. Now I don’t know if that’s the case for your amp but it always went back to modulation distortion from the power supply, sometimes it’s like the ripple wants to modulate the fundamental and for me, it does. I only noticed in the spectrum when I moved back from Brazil (60Hz mains with 120Hz hum) to France (50Hz mains with 100Hz hum)then it kinda dawned on me that it was a B+ power supply problem. Usually that came about on amplifiers that had a somewhat undersized power transformer and I remember you saying that the power transformer on the acrosound ran kinda hot so.... I went crazy looking for filament hum, even ran dc regulated filaments and in the end it was just the B+ power supply. Thanks for sharing.
We are finding the same thing. I can remove the output tubes from one side and reduce the current draw by 100 mA so that the transformer is not "taxed" and see how that turns out. I really appreciate you sharing your experience.
@@ElPasoTubeAmps I might experiment with that 12-400 dc-dc converter from eBay you showed. I'll feed the filaments from the 6.3V windings on a wall-plugged power transformer and the B+ from the DC-DC converter. If the filaments are to blame, then I should see it on the scope around 50Hz, not around 50kHz or whatever the switching frequency of that thing is.
I really appreciate your information. I have it hooked up and is seems nothing has changed although some music is stunning good. Maybe my speakers are "gritty". I am thinking of putting an oscillator on them one at a time and see if there are any harsh points. I have Klipschorns and years ago I could hear a harshness in one of them on vocals so I did put the oscillator on and found it was around 500 Hz or so. I took the midrange horn apart and "adjusted" the voice coil and put it back together and fixed it. Whewww.. that could have gone the other way, so I am going to do the same thing later tonight on the speakers I use on this amp and then put it back on the bench with one channel removed (just remove the tubes) and see how that measures out. I am just surprised it took me so long to notice and measure the frequency difference to identify the 60 and 120 Hz modulation on the fundamental and the first few harmonics.
ElPaso TubeAmps yeah, the klipschorns are amazing speakers but they need to be fed a pretty clean signal otherwise they can be kinda unforgiving. They’ll show every detail, including all the artifacts created by the amp. After seing the spurs on spectrum though, and the fact that they are sort of symmetrical to the left and to the right of the fundamental, I’m willing to bet it does not come from the speakers. Now there could also be a leakage from cathode to filament on the lower leg of the phase splitter because of the elevated cathode... that would explain the improvement when you changed the 6sn7’s.
I vaguely remember but I just did it for you. Power On Press red SHIFT Press dB Ref (ohm symbol) to set impedance Press up and down arrow above and below RANGE to get to the desired impedance (8 ohms?) Press and hold RANGE for 3-4 seconds until it beeps to set impedance above Press AC V (to go to AC volts) Press dB Ref (ohm symbol) until you get to Po
Very good information. I'd like to see the spectrum analyser with no fundamental applied. When you have the 1khz fundamental applied, those spers are actually riding on that frequency. 1060 hz and 1120 hz. Very interesting. I need to get that program also. I seen you posted a link to it. I sometimes use the fft on my Rigol scope, but it's not nearly as accurate as a dedicated one. I just acquired a BPI auto null signal analyser. I need to go over it a little. I think some electrolytic caps need replaced. It won't be nearly as good as your HP or Tektronix. It's supposed to be good down to. 001% THD. As always, Great video!
See my comment above and I think you have an excellent point of analyzing the 60 Hz noise and harmonics with no input. Great idea, in my opinion, and I will try it. Work on one thing at a time without the distraction of a driving signal and the complications/harmonics generated. There is just so much to analyze and be distracted by data that is not part of the solution. It just goes to show how confirmation bias is such a hard thing to get past and get to the real data and an unbiased solution.
Yup, looking at the spectrum analysis, I could hear the difference.... 😉 Seriously, though, I do think that those are differences that a trained ear can detect, at least that has been my experience. Hard to believe that it was the tubes themselves that were introducing the noise and not something else within the circuit. Blew my mind.
for the third amplifier, is it a possibility that it can be poor power supply rejection? That means the design is not as good as the others or requires regulated supply.
I took your thought serious and did change out the power transformer for a 350 mA transformer and that did not change anything but it does run cooler...
@Phil Allison Phil, I agree with your thoughts on the spurs. When I first tested the amplifier, setting the level of NFB, and the hum-pot was not installed, the spurs were enormous and the IMD as shown on the SA was something like 100+%. I wish I could show all this in a video but it is such a dynamic movement thru the process it would be hard to capture. Anyway, NFB reduced these spurs on the SA dramatically and then again, adjustment of the filament hum pot did the rest and it dropped to equal to or below the THD. While I have never been overly focused on huge power supply capacitors, the large one I put in (470uf/450V and a 7H 150mA choke) certainly did make a difference. Speaking of separately screen supplies, a friend of mine thought about using a capacitor input filter for the plate supply (as usual) and a separate rectifier into a choke input supply for the screen. Think about it. Properly loaded, the Pi filter will deliver about 1.3 times half the secondary voltage and the choke input will deliver about 90%. Might be worth looking into if one wants to run these tubes as tetrodes and not triodes or UL. Thanks for your comments.
Very interesting note on input cable grounding. The intermodulation products are an interesting issue, but the harmonic distortion products are also interesting, with the Mac amps looking considerably better than the Williamson design on this respect.
I need that FFT.. My Spectral Dynamics died some time ago.. What hardware/software package is it?? My SG505 is the balanced output, IMD option version. Fantastic bit-o-kit. I have 2 of the AAs, a AA501and a AA501A. The 5000 series generators I have are almost the equal of the 505. Please let us know about the FFT system in that laptop.
Here is the website to get the spectrum analyzer: www.spectraplus.com/Downloads.htm I have tried several different USB sound cards and stuck with the smallest one called a Focusrite Scarlette Solo. They all about the same but this one was a single channel which is all I wanted. I use a 110K pot (any high resistance pot) to cut the voltage down to a level that does not overdrive the input of the Focusrite. I do not use frequency compensation on the attenuator although I do have it available but it doesn't seem to make any difference. I have made videos on break-out boxes with 10x frequency compensated ports. I used to use the HP 3580A but it is unimaginably slow on low frequencies and from what I can tell and what you can see in this video, the PC FFT software is good. There is another free FFT program called ARTA that can be downloaded also.
i would never thought that hum balance could have such an effect, very interesting to see. but now since we are splitting electrons i wonder, in that dual channel amplifier does the second channel ( if operative) intermodulate the first one by the plate supply voltage variation? i mean, when the second channel is working the current draw between main cycles is provided by the capacitors but this current draw of the second channel is at double the amplified frequency ( push pull operation) and it does generate a little voltage swing in the voltage rail that could get reflected on the first channel, right? might worth investigating! ( and i have to get myself that sound card, it works incredibly well ) as always great job!
I am sure you are right about each channel modulating the other thru the power supply but I suppose that is just something we have to accept in a stereo amp with a shared PS. I have not tried to measure it yet. Indeed, the splitting of electrons quickly leads to chasing gremlins. Not sure there is a difference in most cases. Actually lowering the NFB made it sound better. Becoming blinded-by-the-light of reducing THD or IMD to the best possible level make make us aware of other things creeping in that will actually have a "sound" in the speakers that we are not aware of on the test bench. Maybe actually listening to the amplifier occasionally is a good sanity check. Stay safe.
I have a vintage set of 5881 and I think a set of Genalex KT66 I could send you to try out. Also a vintage set of 5692 (6SN7). I also have some 1960's Russian 6SN7 equivalents and they are great.
I would like to see a Western electric 350B set in there. Or Mullard EL37. They are the 2 best sounding 6L6's I have ever heard. I bet 807 might even do a smidge better
hello best radio and amplifier builder I looked at your measurements with great interest and there is one small thing I might be able to contribute third order intermodulation distortion differs on measurements in HF compared to second and third harmonic distortion namely, as the output signal increases, the intermodulation increases tree times more this way you can quickly see whether it is harmonic distortion or intermodulation distortion i think this is the same in audio but you have the measuring equipment to determine this Best regards 73,s PA3EKA Met vriendelijke groet , Bert
Bert, Thank you. I did not know that about IMD vs THD but I will pay attention to that next time. Sometimes things are staring me right in the face but until I focus on a specific detail, it goes unnoticed. I am amazed that I have overlooked the spurs in the output as being 60 Hz and 120 Hz intervals for so long and only used the 60 Hz display for adjusting the hum balance. Thanks for your comment.
Just curious, could you measure the frequency difference between the fundamental and the spurs to see if it’s a multiple of 60? Edit: Ha, I wrote that comment a few seconds before I got to the point where you measured it. I just got a Heathkit distortion analyzer that measures intermodulation distortion instead of THD, but I haven’t fired it up yet. I wonder if that could help find these sorts of issues.
I have to admit, I am charmed by the idea of DC on filaments. But I have found that is usually is not necessary even on direct filament tubes. If there is significant hum and it can not be dealt with with a simple hum balance type system, the problem is likely somewhere else. In my case, insulating the RCA input jack from ground and careful routing of shielded cable with the shield attached at both ends, lowered my 60 Hz hum to as much as -120 dB. It just simply went away. I also changed the power transformer to a larger one as the original one was getting too hot and moving the ground point right next to the AC line input may have helped also. I have found it not necessary to use a floating buss or star ground for all components and it may even be a bad idea in some cases but I have found that grounding all power supply components at one point on the chassis as far away from the amplifier section usually does a good job of putting the S/N ratio down to -80 dB or better.
Thanks for the videos. Always learning from watching them. Im realising the benefit of having some form of FFT from watching your videos. Im looking at upgrading my scope to one with fft built in. Looks like your setup with the fft software works well though.. Do you need a special sound card or just a good PC audio interface? By attenuating the speaker level signal down to a mic or line level, would this introduce more distortion? Is it achievable with any old potentiometer? Thanks again!!
I use an outboard USB sound card called Focusrite Scarlett Solo which is a single channel unit and I do use just a regular pot to attenuate the signal so as not to overdrive the input of the sound card which is not frequency compensated. I have a small Tektronix scope with FFT but it doesn't do well at audio frequencies. Can't remember the model but I have posted videos on it using it at audio that you might like. ruclips.net/video/uZ8JJZ4aUyU/видео.html
Great comparison of the the 3 amps, and tubes, and the effect on THD. I was wondering if you ran the filaments with DC rather than AC if the 120Hz spurs around the fundamental would be affected? Or is there ripple on the B+ to the outputs and/or driver/inverters? So many place to check and possible sources of unwanted spurs! Thanks for taking the time to show us!
I have still been working on it and I changed out the power transformer for a much larger one, same voltage, as the transformer ran much too hot and it was mentioned that an undersized transformer could cause some of these problems (?) Anyway, the bigger transformer did not change anything. I added insulated RCA input jacks with shielded cable grounded at both ends in the amplifier and that took care of a lot of the 60Hz. After carefully listening to it on two different sets of speakers, I determined it had a little "grit" as I called it on the highs so I remembered that too much NFB might cause this so I lowered the NFB to about 14.5 db and it did go away. Sounds very good and you might have noticed that I rarely get into detailed listening to my own amplifiers. Many of my amplifiers live out their life on a test bench. This one really does sound very good.
Are these push-pull amplifiers? Kind of interesting to see so much 2nd order, since PP stages cancel those out. The IMD (spurs) are from the 60Hz humm. There must be some ground loop or something. These levels are way to high
Actually, the harmonic levels are pretty normal. Remember that we are on a log scale so they are 60 or 70 dB down and all add up to a total harmonic distortion around 0.05% at 15 watts. What makes this amplifier interesting is how the spurs on the harmonics appeared at either 60 Hz or 120 Hz intervals around the harmonics and how they were attenuated with the filament hum balance, adding capacitance to the PS filter and different driver tubes (6SN7). The next thing now that was suggested is to lessen the load on the PS by pulling out the tubes of one channel. I am going to test hat and if it works, try to find another transformer with a higher HV current rating to replace the one in the amplifier at this time.
@@ElPasoTubeAmps What i was trying to say is that with a push-pull design, even harmonics should be basically absent, since they are cancelled out. You could also use a DC heater current. Than you're pretty sure you won't see any harmonics.
Piet Muijs They are cancelled out to the extent that the PP is balanced, which can never be perfect. Normally in a cathode-biased stage this is assisted by the cathode bypass capacitor, as if you think about it any AC at the cathode can only be even-order HD, but when you add the balance pot you lose a bit of that, in proportion to the track resistance vs the tail resistance, and now imbalance AC can develop across the pot. Which makes me wonder whether the pot halves shouldn't be bypassed as well.
@@EJP286CRSKW I did try bypassing the pot and its halves. It made no difference. But I totally agree with your first statement that nothing is perfect. After eleven years of this RUclips channel and what I have learned from my own experience combined with what I have learned from comments from viewers, there is no doubt in my mind that we like and even seek the artifacts that a tube amplifier adds to the music. Remember the comments we made to each other some time back of how one of the notes of a violin has a second harmonic with an amplitude greater than the fundamental? How would a violin string with a fundamental frequency that had the purity of a Tektronix oscillator sound? It would be like a strange version of Morse code or something we would think a robot would like. The tube amplifier actually enhances the complexity of the music (without 60 or 50 Hz harmonic noise...)
Sane people do not work with tube amps . Sane people do not learn and never achieve greatness . It is a hobby . Try some choke load in the power supply or a number of chokes . Heavy regulation of heaters and raise the reference off ground about 45 volts. So far My finding have been that the heater to cathode cap. value is often very different for the new vs nos . The 45 volt lift is from the RCA red book , and the Valley and Wallman (MIT series) vacuum tube amplifier books.
I have over 500 uF of capacitance on each side of the PS choke now and it definitely improved the 120 HZ issue with spurs. I also lowered the NFB to get rid of the bit of gritty sound at high frequencies. Sounding good now.
Hi Mark - I always wish I had shown a little more. In particular, the dramatic difference in THD by balancing the output tube cathode currents and balancing the dynamic drive to the grids of the output tubes. I think there is another message here, when we want to get down to measuring the "electron smashing" level, and that is, the driver tubes, i.e. the 6SN7's are every bit as important as the output tubes which get all the attention. Please stay safe.
@@ElPasoTubeAmps thank you for the comment. Would be interested in seeing other drivers and phase inverters like transformers and split load perhaps. Also distortion on your 6b4g amps which have higher distortion but subjectively sound better.
I have always been taught to ground only one end of a shielded cable within the chassis an instrument at the source end - probably still good advice - but there may be exceptions...
Well, this one seems to be tat exception.
Changing to an insulated to ground input RCA jack and grounding the input point at various places on the chassis and grounding the far end of the shielded cable at the tube socket feeding the grid of the input stage (and, obviously, playing with the 60 Hz phase by sliding the input ground point at various places on the chassis) I can reduce the 60 Hz and its harmonics to -125 dB whereas it was at best in the video at about -95 dB ..... another 30 db reduction. This is, indeed, at the level of practically counting and smashing electrons but it is even more amazing that we can have this level of test equipment at our finger tips in a home laboratory. Stay safe, my friends.
i realize I am kinda randomly asking but does anybody know of a good place to stream newly released tv shows online ?
@Steven Roger lately I have been using flixzone. You can find it on google :)
Just a note sent your way David with wishes for continued health - stay safe, young man!
David excellent detective work in tracking down and solving the spur/harmonic problems. Thanks! You realize that the price of 6SN7 tubes will go up again now.....😉
LOL...yes, I have thought about that. I can see the headline, "NOS" 6SN7's just doubled in price on Ebay... (actually I just bought four more and bid on 17 more them)
@@ElPasoTubeAmps Are you trying to hand pick from the lot of 17? The new PSVane CV181 is the best 6SN7 I have heard. Athought at $80 a pop, they are worth the price.
@@vintageaudio7518 I almost bid a high price to get those tubes but I have discovered, just tonight that lowering the NFB to below 15 dB completely removed all the "grit" and made the amplifier sound so good with any 6SN7. I will post soon to show what I have found,
Fascinating. This is a very important investigation. Much to ponder on. Regarding what happens at 23:00 with the amazing spread of lines 60Hz apart, I'm wondering if severely unbalancing the hum has caused the 60Hz hum wave to clip and so produce square-wave harmonics to IM with the signal. Might be worth a look. Of course the hum wave wasn't all that clean to begin it, it was never a sine wave, just whatever was on the mains transformed down. Maybe LC filter it, or I suppose more to the point go straight to DC heating.
I have toyed with changing to DC filaments but I don't think that is the issue. See my comment at the top. What really puzzles me is the vast spread of 60 Hz harmonics all the way out to the 11th harmonic at 660 Hz. The problem and the solution seems to be simpler that we think. Isn't that often times the case?
Thanks David. You're right. A person can hear the effect of these artifacts even if he or she doesn't hear the actual artifact, itself. These artifacts color the sound and one can be trained to hear their effects on the signal source.
May seem crazy but I rarely listen intently to my own amplifiers. But I have this time. I mentioned a slight "grit" sound as I described it, and someone else commented about the same thing. I can't find or remember who. I also remember hearing about the amount of NFB present in vacuum tube amplifiers vs SS amplifiers. Years ago I built an amplifier with KT90's and put in a variable NFB control (I got the idea from a Fender guitar amplifier). It was amazing how the sound changed as the NFB was increased all the way up to about 20 dB. I labeled the control "sound stage" as the amplifier sounded so alive with little to no NFB and how compressed the sound became as NFB was increased. However, max power level with NFB in was 80 watts or so and with no NFB it was 10 or 15 watts before distortion became too high. Surely there must be a compromise and it seems too much NFB may give an amplifier a "gritty" sound. So, with that in mind, I lowered the NFB in this amplifier to just under 15 dB and, to my ears, it cleaned it right up. Also makes it a little easier to drive which is not a problem. I think about 0.7 volts for full output.
@@ElPasoTubeAmps David, you remind me of those great actors who never watch their own movies :-) If 0.7 volts is all that's needed to drive this beauty, you could probably drive it directly from your CD player if there's a volume-out. They should deliver about 2.0 volts at the output jacks, if they're up to snuff. I drive my tube amps directly from my DAC. It's a Steinberg / Yamaha 4 in / 2 out professional recording DAW that I connect to my computer which then bypasses the computer's sound-card. The DAC is 24 bit at 192 kHz sampling rate so it decodes like butter. No preamplifier needed. Simple sound-chain. Thanks for your excellence in all things tube.
@@radiojet1429 Hi Gary, looking at amplifiers like McIntosh, some have a switch for a 0.7 volt input and a 2.5 volt input which they recommend for their preamps. I have been playing with a ST-70 lately that I have had for many years and it sounds darn good. I rarely listen to my amplifiers. Their life is lived out on a test bench. and I am beginning to realize that is inadequate to evaluate an amplifier for my YT viewers. I try to steer sway from subjective comments like, "how does it sound" because it is too subjective but is the bottom line with all sound equipment. I just got thru measuring my two speakers with the impedance bridge and they both measure the same at 120 Hz and 1KHz. Actually surprised me. I have some very nice CD's of classical music I may be able to play without RUclips keeping the video from being shown internationally because of copy-right laws or whatever. So, when we are trying to evaluate an "amplifier" what about the CD itself, the CD player or DAC or the speakers? Or maybe even my vintage ears... from hearing tests my ears drop off pretty quick after 8 KHz, although I don't think that necessarily disqualifies a person from being able to hear and evaluate good quality sound. You know how people rave over no NFB in 300B SET amplifiers. Years ago I built an amplifier of 80 watts with a pot in the feed back line where I could adjust NFB from basically zero to probably 20 dB or so. Wow... they sound so alive with no or very little NFB and get so compressed sounding as the NFB is cranked in. On the other hand, max output power was something like 15 watts vs 80 watts. I should do that as a video. I bet people will love it.
As for being a great actor, I will try to keep that from inflating my ego to where my wife can't live with me... and as for watching my own videos... that was truly one of the intentions when I started doing YT so I could remember how to do things. Often times I do watch them once to make sure I don't make too many bloopers :-) YT took away the speech bubble because it can not be seen on phones and most YT videos are watched on phones, according to the answer I got from them. To my way of thinking, they are not truly interested in promoting any serious science... Thank you for your kind words and stay safe in beautiful Albuquerque where Bugs Bunny should have taken a left (or was it a right?)
@@ElPasoTubeAmps Thanks for the great reply, David. I have been intrigued by the sound changes that occur with NFB adjustment. It's true that lower NFB gives an openness to the sound source albeit while sacrificing power and distortion levels. I am sure I am not the only one who would like to see your NFB adjustment circuits. :-) Be safe.
If you are getting that problem with a 60Hz heater hum. That must mean ANY other frequency will give you intermod distortion which would probably be an amplifier design issue. Might be interesting to do a normal test for intermod!!
Michael Beeny Or maybe it is just caused by the heater modulating the cathode voltage, probably at the input stage, which is an IM effect of its own that doesn't apply elsewhere in the amplifier.
His software is displaying THD+N and IMD (intermodulation).
So would be easy to do (I’ll bet the SW has the facility to set multiple test frequencies). I’ll bet the Mac amps do better than the Williamson.
what about a sampling rate of 44150KHZ or 88300KHZ, If you take the FFT of the amplifier and then use a DSP system that can process the incoming signal with the FFT transform you have a digital amp with the signature of a vacuum tube amp, can you not decouple the heaters to ground with large value high voltage capacitors.
Thanks for the eye opener And stay safe
I wonder if there is a way to put a hum balance on the HT before the rectifier.
It has been suggested to raise the filament above ground but simply insulating the input RCA connector from ground and using shielded cable all the way to the tube grid did amazing things for the 60 Hz noise.
@@ElPasoTubeAmps good thing to keep in the old tool box. Its funny that you can build 1000 amps and every one finds a new way to pick up 60hz, but has never been a problem in the previous 999.
Can the hum balance be controlled/adjusted separately between 60Hz line side and the power supply 120Hz side? Would it help to adjust these balances when running with much higher power just under the clipping point so that you could see the range of its effectiveness?
Howdy! I ran into the same “grit” problem both in push-pull and SE amps. Now I don’t know if that’s the case for your amp but it always went back to modulation distortion from the power supply, sometimes it’s like the ripple wants to modulate the fundamental and for me, it does. I only noticed in the spectrum when I moved back from Brazil (60Hz mains with 120Hz hum) to France (50Hz mains with 100Hz hum)then it kinda dawned on me that it was a B+ power supply problem. Usually that came about on amplifiers that had a somewhat undersized power transformer and I remember you saying that the power transformer on the acrosound ran kinda hot so.... I went crazy looking for filament hum, even ran dc regulated filaments and in the end it was just the B+ power supply. Thanks for sharing.
We are finding the same thing. I can remove the output tubes from one side and reduce the current draw by 100 mA so that the transformer is not "taxed" and see how that turns out. I really appreciate you sharing your experience.
@@ElPasoTubeAmps I might experiment with that 12-400 dc-dc converter from eBay you showed. I'll feed the filaments from the 6.3V windings on a wall-plugged power transformer and the B+ from the DC-DC converter. If the filaments are to blame, then I should see it on the scope around 50Hz, not around 50kHz or whatever the switching frequency of that thing is.
I really appreciate your information. I have it hooked up and is seems nothing has changed although some music is stunning good. Maybe my speakers are "gritty". I am thinking of putting an oscillator on them one at a time and see if there are any harsh points. I have Klipschorns and years ago I could hear a harshness in one of them on vocals so I did put the oscillator on and found it was around 500 Hz or so. I took the midrange horn apart and "adjusted" the voice coil and put it back together and fixed it. Whewww.. that could have gone the other way, so I am going to do the same thing later tonight on the speakers I use on this amp and then put it back on the bench with one channel removed (just remove the tubes) and see how that measures out. I am just surprised it took me so long to notice and measure the frequency difference to identify the 60 and 120 Hz modulation on the fundamental and the first few harmonics.
ElPaso TubeAmps yeah, the klipschorns are amazing speakers but they need to be fed a pretty clean signal otherwise they can be kinda unforgiving. They’ll show every detail, including all the artifacts created by the amp. After seing the spurs on spectrum though, and the fact that they are sort of symmetrical to the left and to the right of the fundamental, I’m willing to bet it does not come from the speakers. Now there could also be a leakage from cathode to filament on the lower leg of the phase splitter because of the elevated cathode... that would explain the improvement when you changed the 6sn7’s.
Hello! There was a video you made where you showed how to setup the dmm 4020 to measure power. Do you recall which video was it? Thanks!
I vaguely remember but I just did it for you.
Power On
Press red SHIFT
Press dB Ref (ohm symbol) to set impedance
Press up and down arrow above and below RANGE to get to the desired impedance (8 ohms?)
Press and hold RANGE for 3-4 seconds until it beeps to set impedance above
Press AC V (to go to AC volts)
Press dB Ref (ohm symbol) until you get to Po
@@ElPasoTubeAmps thank you very much! :)
Very good information. I'd like to see the spectrum analyser with no fundamental applied. When you have the 1khz fundamental applied, those spers are actually riding on that frequency. 1060 hz and 1120 hz. Very interesting. I need to get that program also. I seen you posted a link to it. I sometimes use the fft on my Rigol scope, but it's not nearly as accurate as a dedicated one. I just acquired a BPI auto null signal analyser. I need to go over it a little. I think some electrolytic caps need replaced. It won't be nearly as good as your HP or Tektronix. It's supposed to be good down to. 001% THD. As always, Great video!
See my comment above and I think you have an excellent point of analyzing the 60 Hz noise and harmonics with no input. Great idea, in my opinion, and I will try it.
Work on one thing at a time without the distraction of a driving signal and the complications/harmonics generated.
There is just so much to analyze and be distracted by data that is not part of the solution.
It just goes to show how confirmation bias is such a hard thing to get past and get to the real data and an unbiased solution.
Yup, looking at the spectrum analysis, I could hear the difference.... 😉
Seriously, though, I do think that those are differences that a trained ear can detect, at least that has been my experience. Hard to believe that it was the tubes themselves that were introducing the noise and not something else within the circuit. Blew my mind.
for the third amplifier, is it a possibility that it can be poor power supply rejection?
That means the design is not as good as the others or requires regulated supply.
I took your thought serious and did change out the power transformer for a 350 mA transformer and that did not change anything but it does run cooler...
@Phil Allison Phil, I agree with your thoughts on the spurs. When I first tested the amplifier, setting the level of NFB, and the hum-pot was not installed, the spurs were enormous and the IMD as shown on the SA was something like 100+%. I wish I could show all this in a video but it is such a dynamic movement thru the process it would be hard to capture. Anyway, NFB reduced these spurs on the SA dramatically and then again, adjustment of the filament hum pot did the rest and it dropped to equal to or below the THD. While I have never been overly focused on huge power supply capacitors, the large one I put in (470uf/450V and a 7H 150mA choke) certainly did make a difference. Speaking of separately screen supplies, a friend of mine thought about using a capacitor input filter for the plate supply (as usual) and a separate rectifier into a choke input supply for the screen. Think about it. Properly loaded, the Pi filter will deliver about 1.3 times half the secondary voltage and the choke input will deliver about 90%. Might be worth looking into if one wants to run these tubes as tetrodes and not triodes or UL. Thanks for your comments.
Very interesting note on input cable grounding. The intermodulation products are an interesting issue, but the harmonic distortion products are also interesting, with the Mac amps looking considerably better than the Williamson design on this respect.
I need that FFT.. My Spectral Dynamics died some time ago.. What hardware/software package is it?? My SG505 is the balanced output, IMD option version. Fantastic bit-o-kit. I have 2 of the AAs, a AA501and a AA501A. The 5000 series generators I have are almost the equal of the 505. Please let us know about the FFT system in that laptop.
Here is the website to get the spectrum analyzer: www.spectraplus.com/Downloads.htm
I have tried several different USB sound cards and stuck with the smallest one called a Focusrite Scarlette Solo. They all about the same but this one was a single channel which is all I wanted. I use a 110K pot (any high resistance pot) to cut the voltage down to a level that does not overdrive the input of the Focusrite. I do not use frequency compensation on the attenuator although I do have it available but it doesn't seem to make any difference. I have made videos on break-out boxes with 10x frequency compensated ports. I used to use the HP 3580A but it is unimaginably slow on low frequencies and from what I can tell and what you can see in this video, the PC FFT software is good. There is another free FFT program called ARTA that can be downloaded also.
Good jobs, sir...
i would never thought that hum balance could have such an effect, very interesting to see.
but now since we are splitting electrons i wonder, in that dual channel amplifier does the second channel ( if operative) intermodulate the first one by the plate supply voltage variation? i mean, when the second channel is working the current draw between main cycles is provided by the capacitors but this current draw of the second channel is at double the amplified frequency ( push pull operation) and it does generate a little voltage swing in the voltage rail that could get reflected on the first channel, right?
might worth investigating! ( and i have to get myself that sound card, it works incredibly well )
as always great job!
I am sure you are right about each channel modulating the other thru the power supply but I suppose that is just something we have to accept in a stereo amp with a shared PS. I have not tried to measure it yet. Indeed, the splitting of electrons quickly leads to chasing gremlins. Not sure there is a difference in most cases. Actually lowering the NFB made it sound better. Becoming blinded-by-the-light of reducing THD or IMD to the best possible level make make us aware of other things creeping in that will actually have a "sound" in the speakers that we are not aware of on the test bench. Maybe actually listening to the amplifier occasionally is a good sanity check. Stay safe.
amazing video ! thank you for sharing
I have a vintage set of 5881 and I think a set of Genalex KT66 I could send you to try out. Also a vintage set of 5692 (6SN7). I also have some 1960's Russian 6SN7 equivalents and they are great.
I would like to see a Western electric 350B set in there. Or Mullard EL37. They are the 2 best sounding 6L6's I have ever heard. I bet 807 might even do a smidge better
hello best radio and amplifier builder
I looked at your measurements with great interest
and there is one small thing I might be able to contribute
third order intermodulation distortion differs on measurements in HF compared to second and third harmonic distortion
namely, as the output signal increases, the intermodulation increases tree times more
this way you can quickly see whether it is harmonic distortion or intermodulation distortion
i think this is the same in audio
but you have the measuring equipment to determine this
Best regards 73,s PA3EKA
Met vriendelijke groet , Bert
Bert,
Thank you. I did not know that about IMD vs THD but I will pay attention to that next time. Sometimes things are staring me right in the face but until I focus on a specific detail, it goes unnoticed. I am amazed that I have overlooked the spurs in the output as being 60 Hz and 120 Hz intervals for so long and only used the 60 Hz display for adjusting the hum balance. Thanks for your comment.
Just curious, could you measure the frequency difference between the fundamental and the spurs to see if it’s a multiple of 60? Edit: Ha, I wrote that comment a few seconds before I got to the point where you measured it. I just got a Heathkit distortion analyzer that measures intermodulation distortion instead of THD, but I haven’t fired it up yet. I wonder if that could help find these sorts of issues.
Great analysis
Notice the difference between around 14:00 and 35:00.
Why not using a DC filament voltage. A 150W 12V AC/DC supply can be be adjusted to 12.6V, max. current 11A.
I have to admit, I am charmed by the idea of DC on filaments. But I have found that is usually is not necessary even on direct filament tubes. If there is significant hum and it can not be dealt with with a simple hum balance type system, the problem is likely somewhere else. In my case, insulating the RCA input jack from ground and careful routing of shielded cable with the shield attached at both ends, lowered my 60 Hz hum to as much as -120 dB. It just simply went away. I also changed the power transformer to a larger one as the original one was getting too hot and moving the ground point right next to the AC line input may have helped also. I have found it not necessary to use a floating buss or star ground for all components and it may even be a bad idea in some cases but I have found that grounding all power supply components at one point on the chassis as far away from the amplifier section usually does a good job of putting the S/N ratio down to -80 dB or better.
What you saw on the FFT, 120Hz from the harmonics, are slight temperature changes during the AC cycle.
what is that software you are using on your laptop
www.spectraplus.com/Downloads.htm
@@ElPasoTubeAmps great, thank you!
Great. Which software do you use?
It is called SpectraPlus. Here is the website: www.spectraplus.com/Downloads.htm
great !!
Thanks for the videos. Always learning from watching them.
Im realising the benefit of having some form of FFT from watching your videos. Im looking at upgrading my scope to one with fft built in. Looks like your setup with the fft software works well though.. Do you need a special sound card or just a good PC audio interface? By attenuating the speaker level signal down to a mic or line level, would this introduce more distortion?
Is it achievable with any old potentiometer?
Thanks again!!
I use an outboard USB sound card called Focusrite Scarlett Solo which is a single channel unit and I do use just a regular pot to attenuate the signal so as not to overdrive the input of the sound card which is not frequency compensated. I have a small Tektronix scope with FFT but it doesn't do well at audio frequencies. Can't remember the model but I have posted videos on it using it at audio that you might like. ruclips.net/video/uZ8JJZ4aUyU/видео.html
Great comparison of the the 3 amps, and tubes, and the effect on THD.
I was wondering if you ran the filaments with DC rather than AC if the 120Hz spurs around the fundamental would be affected?
Or is there ripple on the B+ to the outputs and/or driver/inverters?
So many place to check and possible sources of unwanted spurs!
Thanks for taking the time to show us!
I have still been working on it and I changed out the power transformer for a much larger one, same voltage, as the transformer ran much too hot and it was mentioned that an undersized transformer could cause some of these problems (?) Anyway, the bigger transformer did not change anything. I added insulated RCA input jacks with shielded cable grounded at both ends in the amplifier and that took care of a lot of the 60Hz. After carefully listening to it on two different sets of speakers, I determined it had a little "grit" as I called it on the highs so I remembered that too much NFB might cause this so I lowered the NFB to about 14.5 db and it did go away. Sounds very good and you might have noticed that I rarely get into detailed listening to my own amplifiers. Many of my amplifiers live out their life on a test bench. This one really does sound very good.
Are these push-pull amplifiers? Kind of interesting to see so much 2nd order, since PP stages cancel those out. The IMD (spurs) are from the 60Hz humm. There must be some ground loop or something. These levels are way to high
Actually, the harmonic levels are pretty normal. Remember that we are on a log scale so they are 60 or 70 dB down and all add up to a total harmonic distortion around 0.05% at 15 watts. What makes this amplifier interesting is how the spurs on the harmonics appeared at either 60 Hz or 120 Hz intervals around the harmonics and how they were attenuated with the filament hum balance, adding capacitance to the PS filter and different driver tubes (6SN7). The next thing now that was suggested is to lessen the load on the PS by pulling out the tubes of one channel. I am going to test hat and if it works, try to find another transformer with a higher HV current rating to replace the one in the amplifier at this time.
@@ElPasoTubeAmps What i was trying to say is that with a push-pull design, even harmonics should be basically absent, since they are cancelled out.
You could also use a DC heater current. Than you're pretty sure you won't see any harmonics.
Piet Muijs They are cancelled out to the extent that the PP is balanced, which can never be perfect. Normally in a cathode-biased stage this is assisted by the cathode bypass capacitor, as if you think about it any AC at the cathode can only be even-order HD, but when you add the balance pot you lose a bit of that, in proportion to the track resistance vs the tail resistance, and now imbalance AC can develop across the pot. Which makes me wonder whether the pot halves shouldn't be bypassed as well.
@@EJP286CRSKW I did try bypassing the pot and its halves. It made no difference. But I totally agree with your first statement that nothing is perfect. After eleven years of this RUclips channel and what I have learned from my own experience combined with what I have learned from comments from viewers, there is no doubt in my mind that we like and even seek the artifacts that a tube amplifier adds to the music. Remember the comments we made to each other some time back of how one of the notes of a violin has a second harmonic with an amplitude greater than the fundamental? How would a violin string with a fundamental frequency that had the purity of a Tektronix oscillator sound? It would be like a strange version of Morse code or something we would think a robot would like. The tube amplifier actually enhances the complexity of the music (without 60 or 50 Hz harmonic noise...)
a little bit to much to drink, but doing well, thanks!
Gin and tonic has kept the virus away from me, so far. I may have to ramp up if it gets more dangerous...
Sane people do not work with tube amps . Sane people do not learn and never achieve greatness . It is a hobby . Try some choke load in the power supply or a number of chokes . Heavy regulation of heaters and raise the reference off ground about 45 volts. So far My finding have been that the heater to cathode cap. value is often very different for the new vs nos . The 45 volt lift is from the RCA red book , and the Valley and Wallman (MIT series) vacuum tube amplifier books.
The clipping also shows up on the fft as upper harmonic those are easily heard given their level the spurs are a problem also.
I have over 500 uF of capacitance on each side of the PS choke now and it definitely improved the 120 HZ issue with spurs. I also lowered the NFB to get rid of the bit of gritty sound at high frequencies. Sounding good now.
@@ElPasoTubeAmps Think about a choke on the heaters or CCS the heaters with a lifted ref. of ground to about 45 volts. Enjoy your videos.
Amazing I'm the first viewer!
Hi Mark - I always wish I had shown a little more. In particular, the dramatic difference in THD by balancing the output tube cathode currents and balancing the dynamic drive to the grids of the output tubes. I think there is another message here, when we want to get down to measuring the "electron smashing" level, and that is, the driver tubes, i.e. the 6SN7's are every bit as important as the output tubes which get all the attention. Please stay safe.
@@ElPasoTubeAmps thank you for the comment. Would be interested in seeing other drivers and phase inverters like transformers and split load perhaps. Also distortion on your 6b4g amps which have higher distortion but subjectively sound better.
Very interesting!