Please see below a pic of my long suffering 20W P.A. It's had most of the usual ailments that go along with an early 70's Marshall but one has been the bane of my existence for a long time now. The buzz. Substituted a lot of components to track it down but still no luck. Finally tracked it down to the 0.022 coupling cap coming from pin 1 on V2. 290 ish volts DC on the pin side and about 0.6vdc on other side. Shame as it's a mustard but needs must. Popped in the cap you can see in the pic and still reads like a leaky cap 0.6vdc on the wrong side. Tried another, same result. Dug out a pulled Mustard, same result. Went to bed. Woke at about half 2 in the morning and started thinking about conductive Fender boards (as you do). Must be watching too many youtube tech videos, and had a thought. Remembered the jumper wires across the board on this amp are a waxy cloth rather than pvc insulated like on many others. Anyhow, to the point. As you can see in the pic, there are two brown cloth covered wires running left to right along the centre of the board, underneath the 0.1 cap. One is for the plates on V1 (290ish vdc) and the other comes from what should be the 0vdc from my cap. I put the meter from chassis to the plate wire cloth and got about 9vdc. Tried the same thing on the other wire and bingo, 0.6v. After giving all the cloth covered jumpers a good soak in isopropyl and a lot of cotton bud rubbing, i separated the two in question as they were touching and i fired up the amp and tentatively stuck the meter on the end of the 'leaky' cap. 20mv dc, it worked and the buzz is gone. Feeling very happy and a bit techy. Oh, i will be putting the mustard back.