Odd Bb buzz from a JTM 45 copy

[wizard333]

I was working on a Germino JTM 45 copy and it has a very odd problem. The amp is dead quiet when cranked as long as you arent playing, even when the guitar is all the way up (nice filtering). Whenever a note is played, the amp does an odd background hum/buzz at a pitch that would be a Bb on the low E string. It doesnt matter what note is being played. As soon as the note dies, the noise dies. With the amp being so quiet when nothing is being played, the effect is very similar to using a cranked noisegate to eliminate single coil hum (it does it with humbuckers to, just making an analogy). Very odd coming from something as simple as a JTM 45........

Usually phenomena similar to this are the result of speaker cone-cry and not the amp, but in this case it appears to be something going on with the amp. I've not encountered or heard of anything similar, so I'm scratching my head on this one.

Any ideas? Thanks!

[Enzo]

Have you disconnected the speaker in the combo and played the amp chassis through a different speaker? SOunds like voice coil rubbing. COuld also be something microphonic. Changing to an external speaker should make that determination at the same time.

Verify the speaker OK and check for microphonics is the start of my list.

Scope the output to see what the noise looks like. If it looks like ripple tops on top of the signal, you might have a soft filter cap, or something is unballanced in the output stage so the ripple is not canceling like it should.

[Arthur B.]

The fact that you put buzz at (approximately) 120Hz makes me think that you have improper filtering. What's happening is that the ripple is being injected into the stages every time you play a note because the power supply can't maintain a constant voltage.

[wizard333]

The filter caps are only a year or two old, they could be bad but wouldnt that manifest itself in other ways first? Usually with bad caps I hear buzz with the amp up even if the guitar isnt playing or is turned off, pops, crappy general tone, etc. This amp is dead quiet when notes arent sounding and doesnt exibit the other funny behaviors I've come to associate with bad caps.

[Enzo]

Never make up reasons NOT to check something.

New caps can fail, symptoms can masquerade as other things, caps can fail to do their job for reasons other than being a bad part. For example if the connection fails to a cap, it sounds the same as if the cap were open.

[wizard333]

Enzo if the conenections are good, the caps dont show the usual physical signs of having gone bad, and meter out ok, can I assume they are good or would you recommend replacing them or some other test just as a check to see?

Usually I'll replace electrolytics if they are old as a matter of course whether they look good and meter out or not, but on new caps that presumably *shouldnt* be bad, show correct values on a cap meter, and dont show physical problems, I wouldn't replace them to test them. Is there some other indication they may be bad and that given the above they are still a likely culprit? Or is there another direction I should look before replacing them?

[Steve Conner]

I'd bet the filter cap for the bias supply is bad. This would amplitude modulate every note with 120Hz, giving an effect like what you described.

Enzo: The fact that you only hear the ripple when notes are played says to me that the output stage is properly balanced and is cancelling it as best it can.

[Enzo]

Oh maybe. In my minds eye I was seeing B+ that was relatively smooth, but a soft cap was sagging under the wieght of signal. SO in that scenario, the ripple exposes itself when greater current is drawn from the supply. WOuldn't a weak bias filter leave hum all the time?

If everything seems to check out on a cap, then I assume it is OK, but I am not above subbing one to be sure I am not fooling myself. All the test gear in the world won't verify a cap unless they test it at real world working conditions. In other words plenty of caps will check OK on a meter which is probably using a volt or two, but the caps leak like a sieve when several hundred volts are applied.

[wizard333]

The answer, by the by, had nothing to do with filter caps.

It was a bad screen grid resistor on one of the power tubes. Not long after I posted this it crapped out completely and at that point it made itself obvious, but damn it was a hard little sucker to figure out while it was on its way into the grave.

[Bruce / Mission Amps]

I'd bet the filter cap for the bias supply is bad. This would amplitude modulate every note with 120Hz, giving an effect like what you described.

I doubt it is the bias filter cap but wouldn't the bias supply in this be a half wave power supply at 60Hz anyhow?

[wizard333]

Like I posted above, it was the crossover distortion caused by a failing screen grid resistor.

I've never heard that particular symptom before where it wasnt a filtering issue, so if you get an amp in with what should be new caps and you are getting that, check those screen grid resistors and screen grid voltages!

[Satamax]

I've had something similar soundwise in my bandmaster, the 100 Ω res to ground in the PI was removed!