Reverb Feedback Issues SOLVED

[makinrose]

Hi

Is the output stage hum-balanced? If the cabinet is not stiff enough it will transmit the vibration of the PT and OT to the reverb tank.

A simple yet awkward test to do is to get the tank to howl then lift the tank away from the speaker and hear if it quiets down (after the crashing). If it does, it suggests that there is a hum field within the cabinet - a sympathetic vibration due to shae or size. If the cabinet is square or close to it standing waves are easy to set up.

The output stage is hum balanced so that is covered. 

The cabinet is stock Deluxe Reverb repro cab.  I'd imagine it's stiff enough---it's professionally built but it could be a problematic factor. The feedback issue is directly related to the proximity of the speaker to the tank.  If I move the tank outside of the cab I don't get any feedback and the reverb is very quiet.  Inside the cab after the reverb is turned about half-ways up the feedback begins to build. 

I have tried a few positions for the tanks and padding the inside of the reverb bag without success. Somewhere along the line I'm sure this is some small error on my part. Prior to this I've built a number of 60's Fender type amps without issue.  Those amps were randomly grounded and used a traditional layout.  This was meant to be an improvement over those builds and in all other respects it sounds better and has less noise.