I originally thought that as well. It is an intermittent "rustling" sound, like a bad solder joint or a resistor that has gone noisy. Doesn't sound like anything like rectified RF - unless of course RF is getting in only when the resistor or solder joint opens up. There are no grid stoppers on either channel, and yet this sound is coming from one channel only.
This kind of very low level instability can be frustrating to locate, but if I have to completely replace a half-dozen resistors and a coupe of caps, I will! The circuit is fortunately very simple.
Before ripping apart the amp remove ALL unwanted RF radiators from the area.
Cell phones will emit random bursts of RF energy and my wife's iPhone on the floor above me turned out to be the source of a sound like you describe. WiFi and Bluetooth devices can do it too, but Bluetooth is a fairly low power, so it must be close to cause trouble. The cable TV box is a WiFi emission source, as are most "smart TV's."
Cell phones will emit random bursts of RF energy and my wife's iPhone on the floor above me turned out to be the source of a sound like you describe. WiFi and Bluetooth devices can do it too, but Bluetooth is a fairly low power, so it must be close to cause trouble. The cable TV box is a WiFi emission source, as are most "smart TV's."
My amp has started doing something similar as well, and only in one channel. Sounds just like what you are describing. I'm going to have to go on a similar hunt.
> calculations for 2a3....
> Cin = 8.2pF + (17.2pF *5.2) = ..... 100pf
> 6sl7 ...make it 22.5k Ohm.
> f = 1/ (2*pi*R*C) = 1/ (2*pi*22500*(1x10 to the power of -10))
> f = 7.07 kHz
> My maths say your circuit starts rolling off at around 7Khz.
100pFd against 22k is 70,771 Hz.
Check: 10,000pFd (0.01uFd) against 1k is 16kHz. 100p is 100X higher, 1,600kHz. 22kohms is 22 times lower, about 72kHz.
You have a slipped decimal point somewhere.
Besides that, 2A3 pulling a load will have gain less than Mu, datasheet rp is rarely near where we run an audio circuit, and resistance-fed/loaded amplifier Zout is less than rp.
Anyway the amp is said to be "A beautiful sounding amp!". And me personally, I rarely objected to a clean 7kHz limit unless I was mastering flute or disco.
And anyway a hypothesized top-cut has nothing to do with The Problem: "a very low-level noise".
> remove ALL unwanted RF radiators
Often wise advice. The one-side-only argues against it, but could-be one grid wire length is perfect for the iFone, baby-monitor, etc.
One-side-only reminds me of when I noticed I heard background hiss only in one speaker. It wasn't anything in the audio system.
But I favor a bad connection. Interconnects obviously though this is usually changed by frustrated wiggling. Bad solder joints are universal.
> Cin = 8.2pF + (17.2pF *5.2) = ..... 100pf
> 6sl7 ...make it 22.5k Ohm.
> f = 1/ (2*pi*R*C) = 1/ (2*pi*22500*(1x10 to the power of -10))
> f = 7.07 kHz
> My maths say your circuit starts rolling off at around 7Khz.
100pFd against 22k is 70,771 Hz.
Check: 10,000pFd (0.01uFd) against 1k is 16kHz. 100p is 100X higher, 1,600kHz. 22kohms is 22 times lower, about 72kHz.
You have a slipped decimal point somewhere.
Besides that, 2A3 pulling a load will have gain less than Mu, datasheet rp is rarely near where we run an audio circuit, and resistance-fed/loaded amplifier Zout is less than rp.
Anyway the amp is said to be "A beautiful sounding amp!". And me personally, I rarely objected to a clean 7kHz limit unless I was mastering flute or disco.
And anyway a hypothesized top-cut has nothing to do with The Problem: "a very low-level noise".
> remove ALL unwanted RF radiators
Often wise advice. The one-side-only argues against it, but could-be one grid wire length is perfect for the iFone, baby-monitor, etc.
One-side-only reminds me of when I noticed I heard background hiss only in one speaker. It wasn't anything in the audio system.
But I favor a bad connection. Interconnects obviously though this is usually changed by frustrated wiggling. Bad solder joints are universal.
Good advice! All RF emitters were turned off and there is no change! Plus, this is a dual mono amp built symmetrically on a single chassis, sharing only the power transformer. Yet, this damned noise is only in ONE channel!
Tonight I changed the plate and cathode resistors in the single-stage driver to run a bit more current, so those two parts are totally new. I also replaced the 2A3 grid leak resistor on the bad channel. Guess what? NO CHANGE!!! The faint, randomly-occurring sputtering sound is still there!
There really isn't much else in passives left to change except the .47uF coupling cap between the driver and 2A3. If that doesn't solve it, it has to be either the power supply electrolytics in one channel acting up...or a bad UX4 tube socket. I already changed that channel tube socket a few months ago with an exact same NOS Cinch replacement. Maybe the entire lot of them are just too tired, but I kind of doubt it. I cleaned them very well and there is a lot of contact tension on the pins. Plus...the "good" channel has no such issues and it uses the exact same NOS Cinch socket.
Basically, I'm rebuilding the entire left (bad) channel of the amp. There's not much left to do! Man, this sucks!
PRR, we posted simultaneously. See what transpired this evening.
As I mentioned before, my SPICE sims show a low enough drive Z to goose this thing to 20 kHz. And my MEASUREMENTS verify that: -0.25 dB both channels at 20 kHz.
I'm trying to find this darn sputtering noise. The amp truly sounds beautiful otherwise...!
Ok sleuths...NEW CLUES!
Tonight I replaced the grid leak resistor on the 2A3, and changed the values of the plate and cathode resistors in the driver stage to run the driver a bit hotter. This has lowered the voltage appearing on the driver tube plate a bit, and consequently - also changed the voltage appearing on the single 0.47 uF coupling cap to the 2A3 grid.
The noise has changed it's characteristics a bit! Now, I hear the noise when the amp is turned on from cold. Once it warms up and stabilizes, the noise goes away and has not randomly appeared as it did before on a warm amp.
I'd normally think this is a thermally-related, expansion/contraction issue inside of a tube - but swapping tubes out has no effect. So it must be a charge/discharge cycle-related issue. My #1 guess right now is the 0.47 uF coupling cap, 2nd guess is one of the electrolytics in the power supply for that channel.
Tomorrow I'll put it on the bench again and try the coupling cap. No result...I'll move over to the 'lytics. After that, I will have essentially rebuilt the entire channel!
Tonight I replaced the grid leak resistor on the 2A3, and changed the values of the plate and cathode resistors in the driver stage to run the driver a bit hotter. This has lowered the voltage appearing on the driver tube plate a bit, and consequently - also changed the voltage appearing on the single 0.47 uF coupling cap to the 2A3 grid.
The noise has changed it's characteristics a bit! Now, I hear the noise when the amp is turned on from cold. Once it warms up and stabilizes, the noise goes away and has not randomly appeared as it did before on a warm amp.
I'd normally think this is a thermally-related, expansion/contraction issue inside of a tube - but swapping tubes out has no effect. So it must be a charge/discharge cycle-related issue. My #1 guess right now is the 0.47 uF coupling cap, 2nd guess is one of the electrolytics in the power supply for that channel.
Tomorrow I'll put it on the bench again and try the coupling cap. No result...I'll move over to the 'lytics. After that, I will have essentially rebuilt the entire channel!
Thanks PSRR, my maths suck, which is why I usually do them 3x. It was 1 to the power of -10 (not 1x10 to the power of -10) going from 100 pF to 0.0000000001 F.
I was kind of hoping to get to the other problem though, since that one is bigger. Properly driving that 100pF 2a3 input capacitance at full voltage swing takes some real current. The high value load resistor on the input of the following stage helps a bit, but it still won't be enough.
You can't drain a few mA's off of a driver stage that only draws a few mA's in the first place. This is why the "wimpy" driver circuits are so "wimpy" sounding (compared to ones driven with a source that can spare more current).
Still hard to think that this sound would be present without any driver stage present. Without the driver stage, there really should be no current. Resistors just don't make any noise if there is no current (bad ones at least).
I honestly never had a film cap that made a sound. If it was defective, it shorted fully to ground.
As an aside, I did build SE 2a3 amps a few years back using 6SL7. I build many, many different versions. You can check my threads to see some variations. I thought these amplifiers sounded really excellent at the time.
But then for fun I tried a MOSFET follower, like George (TubeLab) does. Wow, huge difference. The music suddenly had a LOT more presence.
These days I use a driver circuit that has more power and can swing the volts with more authority. One that has current to spare. Just sayin' 🙂
My point:
One way to stop such intermittent noises is to use grid stoppers. But with your current circuit, I am thinking you will not like the way a 1k ohm grid stopper "changes" your sound.
So, changing to a driver circuit that has more power will allow the use of grid stoppers (with a possible benefit of improving what you hear, unbelievable as it might seem).
You might not like this advice, but from my experience it works (it did for me).
EDIT:
Absolutely! This is also BEST advice. A friend of mine had this problem with a cordless telephone docking station. Those cordless phones can be horrible RF emitters...
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Soulmerchant,
I think there's another sucker in your math. You assumed a voltage gain of 4.2 in the 2A3 (which could be inapplicable, as PRR mentioned). But why did you multiply Cag by 5.2?
To the original issue: Of which construction is the .47 µF coupling cap? There's some audiophoolish hype on paper capacitors around, but they should better be avoided due to their inevitable tendency of being leaky.
Best regards!
I think there's another sucker in your math. You assumed a voltage gain of 4.2 in the 2A3 (which could be inapplicable, as PRR mentioned). But why did you multiply Cag by 5.2?
To the original issue: Of which construction is the .47 µF coupling cap? There's some audiophoolish hype on paper capacitors around, but they should better be avoided due to their inevitable tendency of being leaky.
Best regards!
1) Miller effect is defined by grid-plate capacitance, 16.5 pf for a 2A3. That multiplied by mu (4.2) = 69 pF equivalent. Add 13 pF more for other capacitance = 82 pF total. Assuming my driver has a 12.5 k output impedance, the -3 dB point is 151 kHz. Even adding a 1k grid stop resistor at the 2A3 just lowers -3 dB to 140 kHz.
Note: I cannot hear above 12 kHz, and my transformers surely don't have 100 kHz BW. So much for my "wimpy" driver circuit.
2) Even with the driver tube removed, there is a potential leakage path for a bad coupling cap: Driver B+ > driver plate resistor > 0.47 coupling cap > 470k grid leak resistor on 2A3. ground. Never say never!
I get it that somebody does not like my parallel 6SL7 driver, as I've read this numerous times now. Really - I DO get it. However, for the moment I think it sounds wonderful. Perhaps someday I will experiment with a different driver circuit, or just leave well enough alone - and move onto the 5 other amplifier projects I have on the bench: 2x Nelson Pass FET, 2x other SE tube projects + 1x P-P tube amp.
Right now my goal is to remove the noise I'm hearing in THIS amplifier with THIS driver.
I appreciate all of the advice given by everybody. Thanks.
Note: I cannot hear above 12 kHz, and my transformers surely don't have 100 kHz BW. So much for my "wimpy" driver circuit.
2) Even with the driver tube removed, there is a potential leakage path for a bad coupling cap: Driver B+ > driver plate resistor > 0.47 coupling cap > 470k grid leak resistor on 2A3. ground. Never say never!
I get it that somebody does not like my parallel 6SL7 driver, as I've read this numerous times now. Really - I DO get it. However, for the moment I think it sounds wonderful. Perhaps someday I will experiment with a different driver circuit, or just leave well enough alone - and move onto the 5 other amplifier projects I have on the bench: 2x Nelson Pass FET, 2x other SE tube projects + 1x P-P tube amp.
Right now my goal is to remove the noise I'm hearing in THIS amplifier with THIS driver.
I appreciate all of the advice given by everybody. Thanks.
I just changed the coupling cap in the noisy channel. No change. The sputtering noise always arrives when I turn the power off and lasts for a few seconds until the power supply bleeds down.
This noise appears to be a charge/discharge or change in current-related phenomenon. I'll try a grid stopper resistor at the 2A3 next, and from there it is onto the electrolytic power supply caps specific to that channel.
This noise appears to be a charge/discharge or change in current-related phenomenon. I'll try a grid stopper resistor at the 2A3 next, and from there it is onto the electrolytic power supply caps specific to that channel.
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I would 1st swap out the grid Load Resistor on your 2a3.
My advice wasn't to swap out the coupling cap...
My guess if your Load resistor is noisy, then there must be some trickle current on it.
Ian
ps. Sorry if I was suggesting too strongly in previous posts.
My advice wasn't to swap out the coupling cap...
My guess if your Load resistor is noisy, then there must be some trickle current on it.
Ian
ps. Sorry if I was suggesting too strongly in previous posts.
Yes, terrible math, I admit. This is why I am a physicist and not an engineer.
I have not encountered caps that make intermittent sounds, but maybe I just do not use low quality enough ones? If they leak, then they are faulty and short to ground (like I mentioned).
Of course, the shortest path would be through the Load Resistor..
I have already swapped out the "load resistor" (what is more commonly known in USA as the "grid-leak" resistor) and there was NO effect! I also changed the coupling capacitor - NO effect!
I use only high-quality parts. The cap is a RelCap Alpha film and foil. Regardless, defects can occur in any manufacturing process, capacitors included.
The next (and perhaps last) parts for me to swap are the two electrolytics that serve this channel only. One of the electrolytics is bypassed with a film cap...I'll replace that one too. After that, the ONLY components left that have not been swapped are the filament and output transformers!
Frustrating!
(Mis-)Calculating the alleged "poor drive" is not fixing the stray noise.
> two electrolytics that serve this channel only.
So put clip leads from right ch caps to left ch caps. Either they both get clean or they both get stray noise. Or no-change which means yet another wild turkey chase yet L/R e-caps exonerated.
> two electrolytics that serve this channel only.
So put clip leads from right ch caps to left ch caps. Either they both get clean or they both get stray noise. Or no-change which means yet another wild turkey chase yet L/R e-caps exonerated.
For anyone still curious as to what the intermittent, faint "sputtering sound" was - I finally found it after two LONG weeks - and 20+ times moving the 40-lb (18 kg) amp from my listening room to the workbench and back again to listen.
I started by resoldering all joints in the bad channel - no change.
Next step was to change the cathode resistor on the 2A3 - no change.
After that - I replaced the cathode and plate resistors on the single-stage 6SL7 driver, AND the coupling cap to the 2A3 - no change.
Replaced grid leak resistor on the 2A3 - no change.
Replaced the 100-ohm AC humbucker pot across the 2A3 filament - no change.
Replaced all power supply caps on the bad channel...no change.
Added grid stopper resistors to the driver and the 2A3, thinking it might be RF...no change.
At this point, the bad channel has essentially been 100% rebuilt - leaving just ONE probable source of the noise...and a very unlikely one - the output transformer. But "sputtering?" I'd think that a transformer would either short or go open, not spontaneously exhibit signs of instability - especially when it sounds so darn good!
I plugged the amp in and found the "bad" channel was now dead. A-HA -it must be the transformer after all, right? Just to verify, I took the good channel transformer and swapped it to the bad channel. But a BIG SURPRISE...the bad channel is STILL DEAD!!! What the heck is going on?
Onto the bench, connected signal generator and starting probing with an o'scope. No signal right at the input RCA jack! WHAT? Just COINCIDENTALLY - the RCA jack had an internal failure and wasn't making contact. Replaced both RCA jacks and now that "bad channel" was up and running...and NO "sputtering" sound either. So it still points to that output transformer - or was it the failing RCA jack causing the sputtering?
The FINAL verification was to reinstall the suspect transformer in the "good" channel. I now had a functioning stereo amplifier again...and that sputtering sound followed that one transformer! FINALLY...I found it! A BAD OPT!!!
A word about these transformers. These were push-pull Tamuras (reputedly super high quality) taken out of a junked, vintage Sansui 500 tube receiver. I took the transformers apart, separated the laminations into "E" and "I" and reassembled them with an air gap - essentially making SE transformers out of them. They sound incredible, and have been serving me for the past 15 years. I have never heard this "sputtering" sound before. It is way down at the ambient hum level and can't be heard when the faintest of music is being played. Regardless, it indicates some type of instability and I don't like that. Perhaps in due time, it will turn into a more catastrophic failure?
I've decided to retire the rebuilt Tamuras and ordered a pair of Edcor 10W 3.5k SE transformers. I hope they sound nearly as nice as these old Tamuras.
This has been a very long and difficult troubleshoot, especially after that RCA jack decided to let go and send me off to a potentially false conclusion!
I'm now VERY relieved and can move onto other audio projects. I want to sincerely thank each and everyone who offered their advice here, it was all very helpful.
-D
I started by resoldering all joints in the bad channel - no change.
Next step was to change the cathode resistor on the 2A3 - no change.
After that - I replaced the cathode and plate resistors on the single-stage 6SL7 driver, AND the coupling cap to the 2A3 - no change.
Replaced grid leak resistor on the 2A3 - no change.
Replaced the 100-ohm AC humbucker pot across the 2A3 filament - no change.
Replaced all power supply caps on the bad channel...no change.
Added grid stopper resistors to the driver and the 2A3, thinking it might be RF...no change.
At this point, the bad channel has essentially been 100% rebuilt - leaving just ONE probable source of the noise...and a very unlikely one - the output transformer. But "sputtering?" I'd think that a transformer would either short or go open, not spontaneously exhibit signs of instability - especially when it sounds so darn good!
I plugged the amp in and found the "bad" channel was now dead. A-HA -it must be the transformer after all, right? Just to verify, I took the good channel transformer and swapped it to the bad channel. But a BIG SURPRISE...the bad channel is STILL DEAD!!! What the heck is going on?
Onto the bench, connected signal generator and starting probing with an o'scope. No signal right at the input RCA jack! WHAT? Just COINCIDENTALLY - the RCA jack had an internal failure and wasn't making contact. Replaced both RCA jacks and now that "bad channel" was up and running...and NO "sputtering" sound either. So it still points to that output transformer - or was it the failing RCA jack causing the sputtering?
The FINAL verification was to reinstall the suspect transformer in the "good" channel. I now had a functioning stereo amplifier again...and that sputtering sound followed that one transformer! FINALLY...I found it! A BAD OPT!!!
A word about these transformers. These were push-pull Tamuras (reputedly super high quality) taken out of a junked, vintage Sansui 500 tube receiver. I took the transformers apart, separated the laminations into "E" and "I" and reassembled them with an air gap - essentially making SE transformers out of them. They sound incredible, and have been serving me for the past 15 years. I have never heard this "sputtering" sound before. It is way down at the ambient hum level and can't be heard when the faintest of music is being played. Regardless, it indicates some type of instability and I don't like that. Perhaps in due time, it will turn into a more catastrophic failure?
I've decided to retire the rebuilt Tamuras and ordered a pair of Edcor 10W 3.5k SE transformers. I hope they sound nearly as nice as these old Tamuras.
This has been a very long and difficult troubleshoot, especially after that RCA jack decided to let go and send me off to a potentially false conclusion!
I'm now VERY relieved and can move onto other audio projects. I want to sincerely thank each and everyone who offered their advice here, it was all very helpful.
-D
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It's hard to believe that the laminations were the real cause of the issues you reported. I guess that in the former life of these re-purposed OPT's someone incidentally has pulled one channel's speaker plug during operation and caused arcing between two adjacent layers with the consequence that this tranny has got some severe insulation issue.
Best regards!
Best regards!