On Wednesday, December 23, 2015 at 8:14:17 PM UTC-5, Phil Allison wrote:
John Heath 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.


.... Phil

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 , flat electrolytic , cold solder. Oscillation problems are almost as bad as intermittent problems. You can go round and round all day with no clear path to success. I say do not be a hero. If it walks like a dog and it talks like a dog get out of it while you can. Once that repair is billed you have entered a contract for 90 days warranty where the the window of opportunity to get out of it is lost. It can put you out of business.

Related to wire dressing I have noticed in some preamps the power transformer is mounted in an odd orientation. Often at 45 degrees to the chassis frame. I imagine an engineer when prototyping move the power transformer here and there finding the sweet null spot where stray 60 CPS would not couple into the inputs. I have a bench frequency counter that is the same with the power transformer mounted at 45 degrees relative to the chassis frame.