"n cook" bravely wrote to "All" (23 Jan 06 17:22:52)
--- on the heady topic of " Marshall 4140 Amp question"

nc From: "n cook"
nc Xref: core-easynews sci.electronics.repair:356259

nc Swapping each pair over, improved matters a lot.
nc The bias stayed about 6 volts more negative and much less noise for
nc the same time intervals from switch on. So to save buying new
nc bottles , a matter of switching off for 2 minutes every hour or so.
nc I can only assume it is due to over driven valves - well something had
nc to burn up those resistors and knock out the HT fuses.
nc Is there a name for this leakage? effect


The input grid on a tube can become "poisoned" by deposited material
coming from the plate and this diverts some cathode current. As it
starts to get hot it starts to emit its own electrons and loses some
control over the plate current and it increases. It is especially
noticeable in tubes that are driven very hard.

I saw an extreme of this effect in a 12AX7 phase splitter where 3K3
cathode and plate resistors were mistakenly installed at the factory,
instead of 33K. Must have been a colour blind worker. Anyway, the
12AX7 was practically a short. Just for fun I arced the input grid
with some very high voltage (5KV) and it cleared the junk on it enough
that after this it worked as a tube again.

A*s*i*m*o*v

.... You mean 15" sparks are SUPPOSED to come out of this thing?!?