Replaced all electrolytic caps, new glass, new reverb tank, sounds fine until you get gain over "7" and then it squeals, like a self generated feedback loop. I would not think this is "normal", I must be missing something somewhere.... Suggestions? Thanks!
See the Service Bulletin #1…
See the Service Bulletin #1 available in the Downloads & Resource Section of this Site. You may need to adjust the output tube cathode bias. There is a small trim-potentiometer on the circuit board. What you describe sounds like it might be a cross-over distortion/feedback.