Hum in new AB763 DR build Reverb circuit - increases as reverb pot is turned up

[Helmholtz]

IIRC it was nickb who once reported considerable improvement with a hum problem by grounding the reverb recovery stage at the same point as the PI.

[prongster]

IIRC it was nickb who once reported considerable improvement with a hum problem by grounding the reverb recovery stage at the same point as the PI.

I've still got a little hum in the reverb circuit, which is grounded a few inches down stream from the PI, on the ground bus bar that touches the chassis only at the input jack. Do you think moving that connection to the same spot as the PI would make any difference?

And as an update, I added a 10uF cap to ground on the bias pot - which is a SF feature I believe, and that quieted the amp substantially. I'm very happy with it now. If I can eradicate this last bit from the reverb circuit (that is only noticeable with the Reverb cranked up) I would have zero complaints.

[Helmholtz]

I've still got a little hum in the reverb circuit, which is grounded a few inches down stream from the PI, on the ground bus bar that touches the chassis only at the input jack. Do you think moving that connection to the same spot as the PI would make any difference?

It might. Hard to predict.
I would try it.

[The Dude]

IMO: Few, if any, tank reverb circuits are dead quiet when cranked to max. You might be chasing a ghost.

[g1]

And as an update, I added a 10uF cap to ground on the bias pot - which is a SF feature I believe, and that quieted the amp substantially.

This makes me question the condition of the regular bias filter cap. Generally they introduce 60Hz hum when there is something wrong with them.

[Chuck H]

IMO: Few, if any, tank reverb circuits are dead quiet when cranked to max. You might be chasing a ghost.

This^^^^^^^^^^^^^^^^^^^^^^^^^

But... Reverb circuits are EXTREMELY high gain with a VERY high noise floor. The reason is that the signal from the reverb tank is only a few mV and must be amplified up to mix level in only two triodes (the reverb recovery and dry/wet mix stages). So there's a heap of sensitivity at the reverb recovery grid. It doesn't help that there are often grounding anomalies with reverb circuits. The reverb actuation circuit is operating with a small signal bled from the pre amp, BUT, the recovery stage is mixing with the pre amp before being sent to the PI. That said, it's usually best to ground reverb circuits with their respective filters. That is, the drive circuit grid should be grounded with the preamp since that's where the signal is being derived. The drive circuit cathode and reverb transformer common should be grounded with the screen supply filter. Which in this case should be where the power for the drive circuit is derived. But, even though it "seems" further down the chain, the recovery circuit is actually part of the pre amp. And all it's 0V references should be aligned that way.

JM2C

[prongster]

The drive circuit cathode and reverb transformer common should be grounded with the screen supply filter. Which in this case should be where the power for the drive circuit is derived.

JM2C

I read this a couple of different ways...does what you're proposing differ from the typical AB763 grounding scheme?

And I appreciate that I may be well past the point of diminishing returns. Mostly I'm interested in exploring this for my own education.

[mikepukmel]

It's an old thread, but sincere thanks to everyone, the O.P. for asking, and all the contributors for suggestions and debugging tips. (This is such a great 'place' ) The only amp build I completed an AB763 deluxe revert type thing, has hum issues, and I was scanning through posts to see what others have run into and what they did to fix the issues. This one I built has multiple issues, after reading and re-reading, Im sure its due to 1) non stock unintended mods, 2) ground loops introduced from 1).

I have a ground "bar", about 1/16" brass round stock soldered to some of the turret board pins on the pot side. There is a terrible looking messy ground thing (mea culpa) bolted to the first input jack side of the chassis. I will remove that and try to get it as 'stock' as possible.

Question for anyone who might be 'listening', is it better, for getting rid of ground loop issues, to remove the brass rod, and use the brass plate similar to what the stock amps had? I do have a brass plate, the pot drilling didn't come out very nice (I did not have a drill press at the time), and the metal is just a little too thick. The threaded part of the pot shafts had just barely enough thread meat for the nut to grab. But it does fit. My original attempt (attempt 0) did have that plate in, but I screwed up the grounding and had too many long wires (bad lead dress) caused all kinds of terrible things to happen. Now that I know what other issues were fixed, neatening up the lead dress etc, I think I could put the plate back in. Thanks.