If it squeals, it's oscillating at a frequency low enough to hear.
The single most common cause of this is a tube going microphonic.
Try swapping tubes first; it's most often the first preamp tube if
it actually squeals solidly instead of only when you hit a note that
excites the resonance.
In most other cases, the squeal starts just after some pivotal event -
the amp has just been repaired or modified, or new tubes put in, or has been
dropped. That event is a clue. Think about what changed, then un-change
it or tinker with whatever was changed.
Some causes of squeal:
- Tube going microphonic - most often first preamp tubes.
- Shorting contact on input jack (esp #1) not making contact; the
"squeal" is from sound vibrations vibrating the chassis and the contact
making and breaking contact repeatedly, making a little 'click' each
- Power tube shorted (this only happens for a short time - the amp
squeals and then dies.)
- Lead dress - the leads carrying the signal around inside the amp
have been moved around somehow so that the signal is causing internal
electrical feedback. You can find this by running the amp with the chassis
open and moving the wires around (gently! with a wooden stick) to see
if the squeal changes or
goes away. Once you locate the critical wire(s) you can figure out where
they have to be to keep this from happening and tie them there. Another
option can be to substituted shielded wire for the sensitive ones, with the
shield connected at one end of the run only.
- If the amp has been modified, the squeal may be caused by poor lead
dress in the modification, improper grounding in the modification,
parts layout too close,
that the new (usually higher) gain has pushed things over the edge. Higher
gain makes a lot of things more critical, including grounding, bypassing,
lead dress, and signal shielding.
- Wrong polarity/ incorrect hookup of a replacement output transformer.