See my recent post about reverb tank.  For the instant cure you need to wrap the reverb tank in a vinyl bag and reinstall it with new rubber grommets.

I hope this helps someone out there...Regarding the Pro185 212 issue....This O.P. along with many other older AMPs have had this oscillation/noise problem... even with inputs removed and even after volumes were re-set down to zero. I had the exact same problem and traced it to the reverb tank. I removed the tank & stock rubber mounts (which were in a state of disintegration) and then wrapped the tank in a vinyl/nogahide coat which was glued & clamped for 24 hours. I then reinstalled the tank using new rubber bushings. Tada... no more problem.
BTW shifting the mounting position slightly is called for in order to install new/tighter self tapping wood screws. use a screw with length about 1 cm in order to grab enough but not protrude to other side of bottom panel.