Silverface Twin Reverb hum in Reverb drive cathode circuit

[jvm]

1970 Twin Reverb with what sound slike filament hum at the cathode of the reverb drive 12at7. I assumed filament hum and elevated the heaters . No affect on the hum at all. Checked/cleaned grounds, replaced cathode resistor, bypass cap is new.I've chop sticked the wiring. I've never had this problem not be remedied by elevating the heaters. Anyone seen this and found a cure? Not my first rodeo but I'm out of ideas.
fender twin_reverb_aa769_schem.pdf

[pdf64]

So if the valve in V3 is removed, what happens to the hum (I've found leaky h-k insulation usually causes a buzz type hum)?

[jvm]

pdf64, removing V3 hum is still present in th speaker but I don't have it at the cathode. Hum is present at V4 pin 1 and Reverb pot increases hum through the speaker. This is with or without the reverb tank connected so I think that rules out cables or tank.

[jvm]

I also just realized that my cathode is at 3 volts rather than the 10 volts shown on the schematic. I'm used to just looking for 2-3 volts on cathodes and didn't realize it was low until now.

[Helmholtz]

Possibilities:
1) Reverse connected or leaky bypass cap.
2) Low value cathode resistor.
3) Bad V3.

Not sure this explains your hum problem.
A scope pic of the hum might give some clue.

[g1]

:
2) Low value cathode resistor.

Yes, perhaps it is not the exact schematic. The one posted shows 2200, some SF twins as low as 470 ohms there.

[Chuck H]

It seems to me that if removing V3 doesn't eliminate the hum and the hum increases with the reverb level control then the hum is on the recovery side.


EDIT: Or it could be a ground failure in the jacks, cable or tank output.

EDIT2: This provided the amp was known to work properly and this is a new problem unrelated and not analogous to a repair or modification. Any possible cause outside of random failure should always be disclosed. Though they often are not.