marshall slx power amp oscillating

[swt]

HI!. a client want his jcm900 slx to be modified to something like a soldano. It's a similar topology, so nothing dificult here, but the power amp produces some sort of distortion when volume is past two, it seems like a parasitic oscillation. I have 5k6 directly on the sockets, bias ok, voltages ok, and moving cables does nothing...any ideas how to cure this in a pcb amp??. thanks a lot!!

[Enzo]

First i would not assume it was the power amp. Is this distortion at the power amp grids? I would scope stage by stage to see where it comes in.

[swt]

Hi enzo! thanks for the reply. I have more info. It only does it in pentode mode. In triode it doesn't do it. I have isolated the problem to the power amp. I've tested the preamp by using another power amp, and it work perfectly. Also tested the power amp by using another preamp, and there is the problem. If i use the half power switch, it stops the distortion/oscillation problem. Checked changing the screen resistors, changing the whole screens/ps with caps/etc, and nothing happens. It's driving me nuts. Sounds like a distortion riding with the note, and it's worst the louder you play. Also changed power tubes, and driver, and nothing happens. When the note decays, it gradually fades. This amp has EL34s. Any ideas?? Thanks a lot for your help!!