Fender Tone-master Problem

[rf7]

This amp's output goes crazy as you bring it up past clipping. It looks like oscillations on the valleys of the output sine wave. Is this most likely a power supply issue or an OT problem?

I've tried new tubes, and they don't make a difference. The amp is over 15 years old and has it's original filter caps.

It does not do this with only 2 power tubes in. Always does it with 4 power tubes.

Has anyone seen this type of thing before?

[Enzo]

It is most likely parasitic oscillation problems. Dress the leads from the power tube grids away from other wirs. Dress the PI wires away from others.

[jrfrond]

If you are bringing it up past clipping, then it's no longer a sine wave, so exactly where is the ringing (oscillation) occurring? Leading edge? Trailing edge? What's your frequency? Does it occur uniformly throughout frequency sweeps?

This might actually NOT be a problem. A lot of parasitic garbage disappears when operated into an inductive load e.g. speaker. In fact, some newer Class D designs DEPEND on this to get rid of the last of the digital switching artifacts.

If you are cooking it at peak power, I suspect you are using resistive loads, unless it is a speaker sim load.

[rf7]

Thanks Enzo, good call! The grid resistors just needed to be located at the power tube sockets, as opposed to across the chassis. This is an early hand wired custom shop version of the amp. I hope they changed that in later versions.

John, the ringing could be induced by switching channels repeatedly with no input. It was trailing edge triggered with a signal.