18Watt.com

EDIT: Nevermind. It's a grounding issue. Not fixed yet, but I know what to do.
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My newly completed 36W amp is quiet until the Reverb intensity is turned up. The 12AT7 being used to amplify the tank is humming.

[deleted]

Out of ignorance I wonder if reverb is compareable to high gain in a matter that the more you push it, the more noise will rise.
I experience it as a fragile part of an amp, which Leo Fender (sorry folks) seemed to handle quite well. Fender just rules when it comes to reverb. That is beyond discussion

The reverb on my vibro king is the best ever and it's dead quiet. Not bad when you consider that reverb is located between the guitar input and the preamp, not to forget running 3 tubes! If it were noisy that would be amplified like hell. Bruce Zinky is one of my top 3 amp builders. ( don't worry Josh, you are sitting lonely at the top, being the designer of the tremolo TMB, which is the love of my life)

My reverb is also dead quiet now. I decided to use phono jacks at the amp instead of hardwiring, but I initially neglected to isolate the recovery-side jack from the chassis. Grounding it instead to the 12AT7 supply ground totally eliminated the hum.

The only extraneous noise in this amp now consists of microphonics at full volume (it will actually howl) and occasional clicks and pops picked up from the mains.

Jack

TriodeLuvr wrote: ↑

Sat 02/20/21 1:26 pm

My reverb is also dead quiet now. I decided to use phono jacks at the amp instead of hardwiring, but I initially neglected to isolate the recovery-side jack from the chassis. Grounding it instead to the 12AT7 supply ground totally eliminated the hum.

The only extraneous noise in this amp now consists of microphonics at full volume (it will actually howl) and occasional clicks and pops picked up from the mains.

Jack

Howl? Are you sure it's not a ground loop issue? What's your ground scheme look like?

JMPGuitars wrote: ↑

Sat 02/20/21 2:56 pm

TriodeLuvr wrote: ↑

Sat 02/20/21 1:26 pm

My reverb is also dead quiet now. I decided to use phono jacks at the amp instead of hardwiring, but I initially neglected to isolate the recovery-side jack from the chassis. Grounding it instead to the 12AT7 supply ground totally eliminated the hum.

The only extraneous noise in this amp now consists of microphonics at full volume (it will actually howl) and occasional clicks and pops picked up from the mains.

Jack

Howl? Are you sure it's not a ground loop issue? What's your ground scheme look like?

Pretty sure it's acoustic feedback (doesn't do it with a dummy load). At some point, I'll try to isolate it, could be one of the tubes. BTW, this is on the test bench with a 12" speaker about a foot away. It might not do this in the cabinet with the amp chassis dampened by the wood. Aluminum tends to ring, and I can hear that in the speaker when I tap near the rear of the chassis.

Jack

TriodeLuvr wrote: ↑

Sat 02/20/21 3:54 pm

Pretty sure it's acoustic feedback (doesn't do it with a dummy load). At some point, I'll try to isolate it, could be one of the tubes. BTW, this is on the test bench with a 12" speaker about a foot away. It might not do this in the cabinet with the amp chassis dampened by the wood. Aluminum tends to ring, and I can hear that in the speaker when I tap near the rear of the chassis.

Jack

I don't think a ground loop would be really audible with a dummy load either. How many ground points do you have in the amp?

It doesn't need to be audible to know whether there's an oscillation. That's what scopes are for.

Every stage has its own ground with its own filter capacitor going back to the central star. No signal ground uses the chassis, and no signal ground has current pulled across it by any other stage.

Jack

I've never tested to see if it was an oscillation that causes this (since I don't do this, I have nothing to test), but there is a relatively common issue with guitar amps using a single star ground that the amp squeals when the volume and gain are dimed.

If you'd like to see how we prefer to ground, check out the ground scheme thread in my signature.

JMPGuitars wrote: ↑

Sun 02/21/21 8:44 am

I've never tested to see if it was an oscillation that causes this (since I don't do this, I have nothing to test), but there is a relatively common issue with guitar amps using a single star ground that the amp squeals when the volume and gain are dimed.

If you'd like to see how we prefer to ground, check out the ground scheme thread in my signature.

I agree. It's how I do grounds on any amp. That avoids me worrying if a problem is grounding layout related. It just works perfectly doing it like that... why reinvent it?

JMPGuitars wrote: ↑

Sun 02/21/21 8:44 am

I've never tested to see if it was an oscillation that causes this

If it howls, screeches, whistles, screams or motorboats, it's an oscillation. And if it does any of that with a speaker but not a dummy load (as viewed with a scope), it's oscillating due to acoustic feedback.

If you'd like to see how we prefer to ground, check out the ground scheme thread in my signature.

Sheesh, you guys are stubborn. Its acoustic feedback. LOL, discovered today that it's stable if I move the guitar into the next room. This thing just has too much gain!

One last problem, and I think I'll be done. It's got a raspy buzz in the background at full MV. No, it's not a ground problem. The MV is picking it up from the HV secondary. I've already determined that a small shield behind the pot totally cures it. I'll route the secondary wiring farther from the pot first to see if that's enough.

Jack

OK, moved all the HV wiring farther away from the MV pot. The biggest contributor turned out to be the Bias tap. It was connected to the turret board only an inch or so from the pot, and I suspect noise from the bias diode was radiating back into that area. I reversed the layout so the turret closest to the pot now carries the filtered DC, rather than the raw AC. Less than 10mV rms of hum at the 8 ohm output now with the MV at max. Can't hear it unless my ear is almost on the speaker. I think this is the last issue that needed attention. The debugging phase is over.

Jack