Guitar amp oscillation

Hi everyone, I have an EICO signal tracer I'm converting to a guitar amp. I have been trying different speakers, and all has been well until the other day. This amp started to oscillate with an external speaker when the gain is set to about halfway or further with no input signal applied. It works fine with the internal speaker. Let me desribe the circuit. Two stages of 12ax7 feeding a single 6AQ5. Could a bad output Tranny cause this? Any suggestions?

Thanks

When you connect the external speaker, are you disconnecting the internal speaker or running it in parallel with the extenal speaker?

Is the oscillation a "motorboating" sound or higher-pitched?

I have tried it both ways. It will oscillate either way starting at about 600hz and then go slightly higher as the gain is increased.

Hi,

Check following:

Does it oscillate with input shorted ?
with guitar connected ?
with guitar connected and vol on guitar turned down ?
where is the input jack located relative to output transformer ?
any input signal wiring running close to, or parallel with power or output wiring ?
Does the external spkr have same impedance as the internal one ?
With impedance mismatch you may have a situation where the 6AQ5 falls into an operating point where it has higher voltage gain than with the internal speaker, then the total gain may be just enough to cause oscillation .
How high is the combined gain of the 2 12XA7 stages ?

Oscillation at 600Hz might, but doesn't have to, suggest acoustic feedback, have you checked 12AX7 for microphonics - tap & listen.

Hope these questions may help to pinpoint the source of oscillation.