Reverb honking on Silverface Super

[phsyconoodler]

I have a silverface master volume reverb that honks when it gets turned up.I mean a real honk or honky squeal.I've remove the master and boost silliness and redid all the grounds,essentially making it a blackface reverb and tone stack.
  I've tried tubes,lowering the gain on the recovery,tank,cables,chopsticking,coupling caps,tec.
 The only thing tht stops the honk is a 12AU7 in the recovery position.
I've never run into this issue before and any insight would be welcome!

[HotBluePlates]

Does it sound to you like it could be acoustic feedback? I would wonder about and look for microphonic parts or better ways of isolating the tank from vibration.

[phsyconoodler]

I don't think so.The amp is on the bench and the tank is far away from the speakers.
  I'm trying another reverb transformer today.We'll see if that does it.I've pretty much replaced everything else.
 It sounds a lot like an amp that has the plate wires reversed on the OT.I wonder if I reversed the reverb transformer wires.....???

[HotBluePlates]

Well, that would require a feedback loop from the recovery stage to the driver stage, right?

Do you think there is either some kind of capacitive coupling through wiring, or maybe through the fiberboard, which could be creating a feedback loop of sorts? I know that's really reaching, but the symptoms you describe remind me of mechanical or electrical feedback.

Since you've eliminated the mechanical to a degree (coupling caps can sometimes be microphonic, but they're usually not bad enough to do this), maybe there's electrical feedback. That would tend to explain why a lower gain tube can nix the problem. Look around for some way for signal to get from the output of the recovery stage back to the driver stage. It might be something as silly as shared grounding or a shared bad filter cap.

[phsyconoodler]

"It might be something as silly as shared grounding or a shared bad filter cap."
  Now that may be a possibility!
It does have a shared filter cap and they are the original mallory silver aluminum types.