6U10 hummm

[drewl]

Got an Ampeg Reverborocket, GS12R and the reverb return stage has some hum to it.
I don't think it's very noticeable, and is not as loud as the typical background white noise when the amp is cranked up.
But it's too loud for the owner when playing quietly.

I tried a spare 6U10 I have and the hum was gone, but that tube is very microphonic and I keep it for testing since these are getting rare.
Now the owner just picked up 5 more and they all have this hum.

I reworked that amp stage, everything looks good and I relocated the footswitch ground connection to after the reverb tank so you can kill the noise when not using the reverb.

I had replaced the power supply filter caps when I repaired this amp, I tried a couple other reverb tanks, still have the hum.

Any other ideas?

[Enzo]

Assuming that Piazza-drawn schematic is accurate - always risky - tack solder a piece of wire directly across that 22k at the reverb pan return. Any effect on the hum?

[drewl]

In other words ground the input to that stage, which is where I initially moved the footswitch to ground connection and - no - there was no effect, the hum was still there.

If you turn the reverb pot all the way down the hum is gone.
So something in that tube is picking up hum from somewhere.
I tried adding another B+ node with another filter cap just for that stage but that did not kill the hum either.
I moved the footswitch connection to the pot so the hum can be turned off with the reverb off.

And the schematic is pretty accurate except for the pinout of the 6U10, everything else looks okay.

[Jazz P Bass]

I am not sure if this is the amp, but I had an Ampeg 'hummer' like this.
The resolution was in reworking the grounding scheme.
The reverb circuit was off to the left side of the chassis.
I added a dedicated ground over there.
I think I came off the input jack.
That cured it.

[Enzo]

I think Jazz is probably right. Other AMpeg hummers I have fixed needed ground attention too.

It is because of that I suggested grounding the input in the way I did. By grounding it right where it lives, or better yet, grounding pin 7 (in the drawing) directly to the bottom of the 3.3k resistor, we hopefully will skirt the issues of the grounds enough to eliminate anything about the input circuit.