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N_Cook N_Cook is offline
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Default Marshall JCM 2000, DSL of 2003

No E number of board supplier found but I would say, by trying to press a
needle into the board, it is epoxy rather than polyester composite.
Megger showed nothing untoward
With no V1 in place , the grid socket pins measure about 10 or 20mV DC, wave
hot air over the valve base and the readings shoot up to 100 to 200mV, just
like applying a magic wand.
The cathode lines have 2/3 orders of magnitude lower resistance to ground so
any such effect not so obvious there

These are the small bottles not the hotter big bottles, something to do with
that metal shield plate for them? I'm assuming the effect is at the valve
base through board , hot pins rather than where the anode dropper leads pass
through the pcb, and passing through rather than along the surface . I will
explore this, as hard wiring the HTs to isolated valve bases ,only, is quite
different to hard-wiring all HT traces.
Now what is the physics /chemistry of all this ? I assume something
hygroscopic is grabbing moisture then forming a conductive salt that stays
within the micropores of the surface of the glass fibres, but why
temperature increasing the conductivity , what salt has highly temp
dependent conductivity, we're only talking 50 degree C or so