John Heath wrote:

Phil Allison wrote:



The layout relies on the two plate wires being in close
proximity so the out of phase electric fields cancel at a distance.




There is a third option. Stop being a hero.
You can not fix everything. Phone the customer
and say this is a tricky problem and that they
are better off taking it to the manufacturer for
service.



** The OP fixed the oscillation problem, after acting on my
advice about cable dress. A previous repairer had been careless.




I can see how wire dressing would cause this with high gain of a guitar
amp. However this is Monday night quarterbacking. One does not know it
is wire dressing ,



** Moving the input signal and output plate wires BACK to where they were originally and having all oscillation stop is enough proof for anyone familiar with tube guitar amps - but apparently not you.

AC supply transformer orientation can be critical in sensitive devices, graphic equalisers and tape machines are good examples - plus CRT based scopes.

In tube amps, AC and output transformers are normally mounted close together with the cores at right angles to prevent direct injection of 50/60 Hz hum.

In case of the OP's JCM600, Marshall elected to mount the two at opposite ends of the chassis, maybe to balance the weight, but doing so can easily lead to instability.


..... Phil