Gibson GA-15 RVT Noisy Reverb Recovery

[bobloblaws]

Well, I was determined to figure this one out on my own but I am completely stumped. I'm getting a hum from the reverb recovery stage of this Gibson GA-15 RVT and I've tried everything I can think of to correct it but to no avail.

The amp is pretty much quiet when C15/R15 is grounded, which is how the footswitch turns off the reverb effect. But when that point is ungrounded by pressing the footswitch or disconnecting the footswitch cable, there is a distinct hum present. If I disconnect the reverb tank the hum is still there but also with a lot of hissy, RF type noise. Because the reverb level pot is on the reverb driver side it has no bearing on the amount of hum. I've ruled out the V2 tube and coupling cap, in fact I went as far as wiring in another tube socket, and other things too numerous to mention, still I haven't been able to eliminate it. Suggestions?
Schematic:
GA-15RVT.pdf

[g1]

Is it the stock tank?

[bobloblaws]

Is it the stock tank?

It sure looks like it.

[g1]

Disconnect the tank and short across C13, does the hum remain?
Have you tried moving the tank around to see if the hum is affected?

[Enzo]

Pull the tank, leave it connected, but move it as far as possible from the power transformer, any help?

[bobloblaws]

Disconnect the tank and short across C13, does the hum remain?
Have you tried moving the tank around to see if the hum is affected?

Pull the tank, leave it connected, but move it as far as possible from the power transformer, any help?

I thought I had ruled out the tank, but I'm second guessing that now. When disconnecting the tank I could still hear a hum along with added hiss, but now I think that is probably the same inherent hum that is there even when the reverb circuit is switched out.

In any case, moving the tank around does not affect the hum level and shorting C13 does not eliminate the hum that is there with the tank connected, but it does kill the hiss that is there when the tank is disconnected.

[g1]

Then I would say the hum is coming in after C13. The footswitch (after C15) gets rid of it, so somewhere between those 2 points?

[bobloblaws]

Then I would say the hum is coming in after C13. The footswitch (after C15) gets rid of it, so somewhere between those 2 points?

Right, that's what's driving me mad, what else could it be besides the tube, coupling cap, or the cathode resistor, right? (the 5uF bypass cap is not installed)
And I've tired swapping all those already. Someone told me the unbalanced heater circuit in this amp can introduce hum, I assumed that was in general and that my problem was more specific. I dunno, could it be a factor?

[g1]

Yes it could be a factor. Is it more like 60Hz or 120Hz hum? The filter cap for that node of the power supply (C17A ?) could also be related. I put that question mark in there due to the great variations in Gibson amps vs schematics.

[bobloblaws]

Yes it could be a factor. Is it more like 60Hz or 120Hz hum? The filter cap for that node of the power supply (C17A ?) could also be related. I put that question mark in there due to the great variations in Gibson amps vs schematics.

I think it says C12A. In any case if it was a filter cap issue would it not affect the other gain stages on that node similarly? i.e. V1A and V3A? Maybe it is, the amp in general is not completely free of hum, so maybe it's a matter of hum from V1B getting summed with that of V1A. FWIW the filter caps were changed at some point, although they could be a number of years old. I suppose it's worth a shot to look into that angle.

Is there a way test whether it is hum from the heater circuit?

[g1]

Is there a way test whether it is hum from the heater circuit?

Hook that heater up to a battery.

[Chuck H]

g1 is really being magnanimous to chase this. Noise, especially hum in reverb systems, can be a real PITA.

IMHE examining the possibility of ground loops WRT how the reverb circuit is wired nearly always solves hum issues for me. Are the input and output jack sleeves grounded to the tank while the circuit in the amp is grounded elsewhere? This is actually the case most of the time with old, hummy reverb circuits. It's critical to isolate either the input or the output from the tank and keep either end grounded with their respective amplifier circuits.

[bobloblaws]

Are the input and output jack sleeves grounded to the tank while the circuit in the amp is grounded elsewhere?

Sorry, I don't know what you mean exactly. The in and out sleeves would be chassis grounded at both the amp end of the cable and the tank end, no? I have continuity between both amp and tank chassis with either in or out connected. I don't know what you mean by circuit ground in this context.

[bobloblaws]

Hook that heater up to a battery.

As it happens, I tried the battery, no difference, so we can safely rule out heater circuit I would think. I also swapped in a brand new filter cap for that node, no change there either.

[Enzo]

We cannot assume both pan jacks are grounded to the pan chassis walls. On many if not most, the input jack is not. It wwinds up that way because the cable plugged into it is grounded at the other end, but that can be involved.