Neil wrote:
The amp (Valvestate VS100R with ECC83 Glass Valve) packed in a few
months ago. Two of the 4 darlington transistors (2xBDV64 & 2xBDV65)
had gone short circuit, also taking out the 1A anti surge fuse. I
replaced them (using heat sink paste as well) and it happened again,
so I replaced them once more and left the amp on (guitar & effects
unit unplugged / overdrive off) to eliminate external possibilities.
After about 15 mins I measured the temperature of the transistor heat
sinks - 3 were 40-50deg C and 1 was cooler. After an hour I went out
of the room for 5 minutes and returned to find it had blown again. The
heat sinks were hot - one of them was 100 deg C. Any ideas what could
be causing this? The amp is a few years old but hasnt had a great deal
of use. Thanks in anticipation.

Summats up with the claas B bias circuitry.

No direct experience of this design, but you may find a preset pot on
the board somewhere in that general area.

Assuming nothing else is blown, the off signal current draw should be in
the tens of mA range. You can check that by using a meter instead of the
fuse.

If it has a bias preset pot, adjusting that should adjust the queiscent
current smoothly. If it jumps around, replace the pot.