"Don Pearce" wrote in message
...
On Thu, 12 Jul 2007 14:42:35 +0100, "Keith G"
wrote:

a sorry tale


Swapping those valves could easily have damaged something, because
they would have both been going flat out with nothing to limit the
anode current (neither grid would have been connected). Best I can
suggest is swapping valves between the two amps to see which one
carries the fault with it, then get a new one of those.


No, all that's been tried - all/any of the valves (and the meter) can be
swapped to the good amp willy nilly and it continues to work perfectly.



I haven't looked at the pinouts of the two to see what connects to
what when you swap them, but this is my best guess.

Passives damaged by too much current tend to show the evidence pretty
dramatically. But if it is a passive, measure the value of the cathode
resistor


Seems OK/same on both amps.


and make sure it hasn't gone low. Also measure the resistance
from grid to ground to check it isn't open circuit.


OK, with no power on (?):

Good amp = 100 kOhm

'Bad' amp = 40 kOHm

??