Weird valve amp oscillation

[jan.didden]

Sounds like a scope job.
Just follow stages through until discrepancy occurs.
Guitar amps don't usually have feedback so are easier to debug.
Its possibly a faulty coupling capacitor if the amp is of old age.

Finding the cause of oscillations in a feedback amp by scoping successive stages is very unlikely to be successful. (It is a feedback amp otherwise it wouldn't oscillate).
You'll see lots of weird things in this stage or that, but that does not point to the actual cause, because it is all interconnected in a round robin way.
This is one of those cases where studying the circuit and do some thinking is indicated ;-)

Jan

 

[Mark Tillotson]

One sneaky possibility is called "squeeging", bursts of supersonic oscillation that increase to the point where the oscillating stage(s) is driven towards cutoff, so then relaxes, normal DC conditions are restored, so then resumes oscillating, repeating at an audible rate.

That was my immediate thought from the description of the noise. It's squegging, not squeeging, there's a wikipedia article even. However there are

other ways for relaxation oscillators to form and a 'scope would be the most illuminating way to investigate. Squegging will often respond to slight changes in parastics, but that's tricky to check safely in high-voltage circuitry, but touching a high-voltage meter probe to each node in the circuit might be one way.

 

[wg_ski]

I didn’t know there was a specific name for it other than “szzzhmp szzzhmp szzzhmp”. Solid state amps have a nasty habit of doing this, but I suppose it can in tube amps too. Usually caused when the high frequency oscillation is power supply voltage dependent. Draws excessive current when oscillating (often causing audible buzz or hum) power supply drops enough for it to stop (goes quiet), supply comes back up, repeat. If it happens at a higher rate it can count like a “purr”. If you used a variac you can find the voltage where it stars up. Tube amps do have the added complication of the heaters - they have sort of a minimum voltage for proper start up (and a long wilt time). Powering the heaters separately can help with that kind of diagnosis.

Most high frequency oscillations I have seen personally in tube amps didn’t cause excessive current draw which will cause this, but it’s not outside the realm of possibility. If it’s at a frequency where the OPT impedance goes very low from excess capacitance between the two halves of the primary. Almost ALL class AB push pull solid stare amps will draw too much current when driven at several hundred kHz (and burn up) unless are specifically designed to run that high. It won’t make a tube amp explode or even overheat.

 

[duncan2]

As this subject is continuing I looked up various old UK radio manuals three of them mentioned motorboating ,all agreed it was due to a faulty capacitor so I will be interested to see if it turns out to be something else?