So I went with my buddy to buy a Silverface Twin yesterday for quite the steal, and its having a crazy reverb issue. I can hear the reverb working up until about 3 on the knob, then it just starts to squeal. Super high pitched and incredibly obnoxious, it's so loud that I can't even hear the guitar. I tried every obvious fix, new RCA cables, different reverb tank, all new tubes (pre-amp and power) all to no avail. I am now 100 percent positive that its something with the circuit because it even oscillates when the cables are unplugged, and pressing the footswitch to turn off the reverb doesn't stop the squeal. Other than not having working reverb, everythig sounds great, and the amp is every bit as loud as it should be. Oh, also, turning the reverb knob will adjust the frequency of the oscillation. Surely this has to be some type of feedback loop, but where do I start looking?