On 23/12/2015 12:52, John Heath wrote:
On Tuesday, December 15, 2015 at 7:00:52 PM UTC-5, Phil Allison wrote:
Gareth Magennis wrote:



** 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.





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.


** Close to none.

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



... Phil

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. You will lose money but gain some cookie points for being honest. Better still the manufacturer is now spinning their wheels trying to fix it not you which the way it should be if it is a design problem.


When an owner finds out how much it costs in carrier fees plus insurance
to a main agent for repair sometime in the next 3 months(even if
manufacturer pays the return cost), then finds out a good chance of said
item being repaired locally for much the same cost , in half a week,
they always tend to one option, as they have a gig next week.