Twin Reverb; Reverb issues troubleshooting

[WhopperPlate]

Hello yall,

I have an old silver face twin that is a real work horse (backline rental for 30+ years!) and playing great, except for this fizzy sizzle distortion that rides the reverb decay. Reverb on zero sounds like any decent working dry twin. Normal channel of course has no issues.

I have swapped in new tubes with no change , so I am assuming it’s either the reverb tank itself failing or a failing component in the circuit itself

Before I spend the time to tear into the thing, or possibly order a new reverb tank, I thought I would ask the experts first what y’all thoughts were on the matter and hopefully diminish any tail chasing headaches or unnecessary expenses .

Appreciate the help. Thank you for the time

Charlie

[Colossal]

A good way to rule out a failing part(s) would be to take voltages at the plate and cathodes on the driver and recovery tubes and see that they match the expected value (within 10-20% or so). Also check coupling caps for DC leakage. Tank could be starting to go though, as you mention.

[ChopSauce]

Also check coupling caps for DC leakage...

About that: I've read many things from "there must be no voltage at all" to "there may be some". Could anyone clarify, please?

[Stevem]

One way to rule out a bad tank is to just disconnect it’s send and return cables and then jump a 470k resistor across what would be the tips of the RCA jacks.

Doing this will just make the reverb circuit a volume boost circuit.
If this fails I would change out both the plate load and cathode resistor in the recovery gain stage and also that 3.3 meg mixer / level matching resistor.

I would also just shot gun the 2.2 K cathode resistor and the 1meg grid resistor on the 12AT7 pan driver.

[TUBEDUDE]

We're assuming you've already deoxidized the pan, chassis and cable connections.

[WhopperPlate]

Thank you all for the education.

Yes, All connections have been thoroughly cleaned .

Thanks steveM , The 470k resistor across the jack caused the volume increase with reverb knob , so it must be the reverb tank (?)

Gonna order up a new tank

[Stevem]

Yes, the new tank will get you back to good!