1970 Twin Reverb - Reverb Issue

[BiggestThomas]

Greetings, I've got a problem with the reverb on a 1970 (AB763) Twin Reverb. When the reverb is engaged, there is a loud harmonic that cuts through the mix and is really unpleasant. It seems to only be present when notes on the B sting are played. This effect is intensified as the reverb is turned up. This is not an issue on the clean channel and seems to be solely associated with the reverb. I have replaced all preamp and power tubes along with Filter caps, yet the problem persists. Any input would be greatly appreciated. Thank you.

[pdf64]

Have you got a buddy with a similar Fender blackface or silverface amp (anything with reverb, doesn't have to be a twin)?
If so, try plugging their reverb into your amp, and visa versa, and see which amp has the problem then.
If it's not the reverb tank, it could be the cathode bypass cap on the reverb recovery gone bad - it's shared with the mixer stage and can cause oscillation when it fails, at higher reverb settings.
Does the background hiss from the amp sound like it's going through a wahwah set mid way, as the reverb is turned up?

[Bruce / Mission Amps]

Also, look and see what the reverb driver tube's, the 12AT7, cathode resistor is and it's cathode bypass cap.
If you haven't yet, change the cathode resistor to 2k2 and install a new 22uF-50c cap across it.

[BiggestThomas]

I did replace the bypass cap with a 25uf 25v cap (would 22uf be a better choice?), which improved the reverb, but didn't solve the problem. I hadn't thought to replace the cathode resistor. Ill check it out.

Unfortunately, I don't know another with a fender amp, but the reverb tank is as old as the amp. I'll try to find away to isolate the tank or temporarily replace it to see if its the culprit.

Thanks for your time and insight!

[Gibsonman63]

A couple of basic suggestions

You may want to pull the reverb out and do a visual inspection just to make sure the springs and transformers look to be intact and nothing has come loose. I have seen the wires from the transformer to the RCA connnector on the reverb tank break off before, but in that case you get no reverb at all, just a lot of noise. Also, check the cables connecting the reverb tank to the amp and make sure that you have good clean connections on the reverb tank. I assume that you have tried this without the footswitch, but I had to resolder a footswitch a while back. The braid came loose from the RCA connector and it made horrible noises when plugged into the amp. If all else fails, you can get a new accutronics tank for around $25.