Noisy Ampeg M12-Q

[drewl]

Replaced cap can and power cord on this amp but when volume and tone are turned up over half way on either channel I get alot of noise, static like hum, 120hz I think.

Tried different tubes.

This amp is a bit different than any M12 schematics, but nothing odd.

The input jacks are open circuit, so something has to be plugged in otherwise you naturally get bad hum.

Now here's the odd part.
They are using the unused ring connector of two jacks as a place to connect the mixing resistors of the two channels after their first amp stage.

I was thinking of moving them, think they could be radiating noise back to the input?

[g1]

They are only unused if nothing is plugged into them, right? If you use it with a mono jack, the ring gets grounded.
Some of them do use a stereo input, I forget how it was supposed to be used, here is a M15 example.

[drewl]

Ring is not grounded and is used as a connection point for mixer resistors.
All but one are mono jacks.

Tried moving the mixing resistors with no change.

[g1]

My point was if you plug a mono cable into the stereo jack, the ring gets grounded (and mixer resistors), so it must have been intended as a TRS input jack, not a tie point.
So maybe someone wanted to use a stereo cable there with the ring going straight to the mixer resistors, maybe for a line level input.

[Enzo]

Stereo accordion plugs into TRS and feeds the two channels independently. Plug a plain TS plug in there and it grounds the other channel input.

[drewl]

Anyway, aside from the funky jacks I'm getting alot of noise from the first stage.