"Phil Allison" wrote in message
...

Gareth Magennis wrote:

"Phil Allison"

Come on Phil, give us a clue, I'm stuck here.


** Most times I've seen such a problem, it was due to lack of shielding
between the output valve plate wiring and input signal wiring. Maybe a
shielded signal wire really isn't or the plate wires need to be twisted,
tidied and pushed against the chassis.

Everything in the JCM600 is crammed together making unwanted coupling more
likely.

Other times it has been due to high resistance grounds on pots or jacks -
or because some fool decided to put the OT next to the inputs jacks.



Yep, the plate wiring wasn't tidy or hard on the chassis, and one of the
PCB
interconnects was way too close. (Some cable ties had not been replaced)




** Found this pic of the insides of one:

http://gitaradiy.pl/uploads/2_974_JCM600_6.jpg

The OT is on the far left ( right behind the input jacks ) and primary wires
run next to all the pre-amp valve wiring on their way to the valve sockets
on the right.

That is a *really* poor layout for a valve guitar amp and sitting duck for
oscillation.

Be worthwhile twisting the three primary wires and moving them further back.


..... Phil




The multitude of grey interconnects are not actually screened cable, just a
signal wire lying beside an earth wire.
I don't know what level of protection that actually offers.



Cheers,


Gareth.