This. Bad capacitors, bad tubes, bad connections can all destabilize an amplifier. It doesn't take much when you're dealing with the kind of power that an SVT manages. I remember sometime long ago hearing the advice that "if you want to build an amplifier, design an oscillator; if you want to build an oscillator, design an amplifier."
If my SVT was doing this, here's what I'd try:
- Try a different cabinet - a failure in a crossover, for example, could cause the load presented to the output transformer to have a nasty resonance at a high frequency.
- Isolate the preamp from the power amp (using the pre-out and power amp in jacks) while the problem is apparent. Does the power amp behave properly (stop squealing, cool off) if it is isolated from the preamp? If the preamp is routed to an external amp, is it squealing?
- Replace the three small tubes in power amp with known good ones; this is easy and safe.
