Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
I should be able to get back to it in a couple of hours. Only just got
around to building a Dexion support frame so I can toss these sorts of
amps
around with gay abandon, and not risk the bottles.

The valve set that are in here are all 2009 , he took out the new ones
and
replaced these used ones. They look brand new except for simple dragon's
teeth marks, ie not repeatedly moved around.
The Russian markings are quite visible and there is no sign of
overheating
of the bases , still light brown , and the pcb around looks as new.
But the first thing I will do is check for o/p bias drift, then heating
the
pcb with hot air and a 2Gohm megger and then remove V1 and see if hum
returns then replace and monitor DCs around V1 on heating ,

No mention of that Marshall Forum page about hum but it could be these
pcb
problems around V1 the immediate problem




The "Hum" I have associated with the PCB problem is ripple due to the huge
currents the output valves are taking from the PSU.
One pair of valves inevitably draws way more current than the other pair,
which again manifests as a hum. The more current the valves draw, the
hummier it gets, until things start glowing red, then things start to
break
and the hum is gone.

Yours may not actually have this PCB problem, but the bodged in grid
stoppers make me suspicious.



Gareth.




main bias results
monitoring every 5 minutes for the first 15 minutes
rate of increase is falling
passing 10 to 15 minutes in
one side rising 1.0mV per 5 minutes and the other side 1.5mV per 5 minutes
starting from that 15 min reading of 76.9mV and 68.2mV should be about 90mV
over an hour .
locally heating with hot air (low setting) for 20 seconds the voltages shot
up
5 or so mV but soon dropped back to where they were , unlike the problem
around V1 it would seem.

now for the other preliminary tests