Paul Herber wrote:
On Thu, 16 Oct 2008 15:44:46 -0700 (PDT), 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.

Are you getting some parasitic oscillation? If any wires have moved or
capacitors are failing then some HF oscillation could cause this.



Yes. That is the other possibility. There is normally something akin to
a resistor and inductor, in series with the output, and a resistr of
about 5-10 ohms in series with a capacitor of about 0.1uf across te
output. If that capacitor is bust, you may well get oscillation,
especially with no load.