Hello, I have seen other posts around this site related to fender amp reverb "squeals" but not sure any of them match my issue. Please direct to appropriate forum if this question has been answered. Here is my issue... while playing at moderate volume/settings my amp suddenly started to have a loud, oscillating squeal. I took it to the bench and here is what I learned. 1. No squeal when reverb knob is at zero, but it kicks in around 3 on the reverb pot, squeal stays the same volume and it is still present with the actual volume knob at zero. 2. I notice that the oscillating squeal fluctuates with the tremolo speed. 3. The squeal is present with or without the reverb pan plugged in. 4. I pulled the tubes one at a time and lose the squeal when I remove V3, so I replaced that tube with a known working tube, no change, so I replaced the other preamp tubes one at a time with known working tubes and still no change. 5. Visual inspection inside shows nothing obvious, and I measured everything possible (while attached to the board) and show nothing unusual as well. 6. With reverb at zero amp functions perfectly normal. So... any thoughts where to look next? Thanks!