65' twin reverb reissue hum with reverb

[pontiacpete]

I've think I tried just about everything, changed the tank, RCA cable, tubes, moved the tank and chassis out of cabinet, changed out reverb xformer, cleaned all connections and moved wires around the inside of the chassis. Nothing seems to eliminate the hum that I'm getting when increasing the reverb pot. The hum gets louder as the pot is turned.
The only thing that I think I will settle with is reducing the gain on the recovery tube by using an 12AT7 instead of the usual 12AX7. This makes the hum more tolerable.
Anyone ever come across this issue?

[Chuck H]

Is this a new condition or did the amp always do this to your knowledge? And what is the number on the tank?

[pontiacpete]

I don't know. The amp came to me and the reverb pedal switch was bad. Replaced it and now I discover the hum.

[guitician]

If the hum is in the circuity, like ripple, then a bad capacitor could be the issue.

[pontiacpete]

I checked capacitors for ripple and didnt detect any.

[guitician]

The recovery stage is amplifying a 60hz/120hz signal. If it's 120hz then the power supply would be suspect. 60hz could be from ground loops. Are both ends of the RCA cable grounded? Only one end should be on either cable.

[52 Bill]

Ground the return signal cable coming from the tank. Now turn up the reverb control. Any hum?

You need to try and figure out if the problem is in the tank and wiring or in the internal circuitry. Either part can cause the problem.

You mentioned there was a problem with the footswitch. Do you have the reverb and vibrato plugs reversed? The vibrato cable isn't shielded, while the reverb one should be. Try the amp without the footswitch. Any difference?

[pontiacpete]

When I ground the output from inside the tank the hum goes away. What does this mean??

I have the footswitch unplugged from the amp.

[guitician]

Check the continuity of the shields of the cables to see if you have an open ground.

[pontiacpete]

the continuity of the shields tests good

[52 Bill]

the continuity of the shields tests good

Is the tank the correct one? 8 ohm input, 2500 ohm output, isolated input, grounded output, etc.

Is the tank mounted with the output side away from the power transformer?

[pontiacpete]

it's the 4AB3C1B, tried swapping it with another of the same type, no change.

I put it on the bench away from the chassis.

[52 Bill]

it's the 4AB3C1B, tried swapping it with another of the same type, no change.

I put it on the bench away from the chassis.

That's the correct tank.

If you move the tank around can you null out the hum by changing the relationship of the tank to the chassis? It's sort of like moving your guitar around the amp to find a quiet spot.

Have you tried the amp in another location?

[guitician]

When you said you "Grounded" the output, I thought you meant that you supplied a ground to the lead, not that you shorted the output of the tank, which would be the same as grounding the grid of the return stage, thus silencing it.

[Enzo]

Are we having this problem with the footswitches completely disconnected?