On Thu, 12 Jul 2007 14:42:35 +0100, "Keith G"
wrote:


(As opposed to *pinged off*?? :-)

As you're aboot Don Old...

I've emailed Nick Gorham to no effect (inconsiderate sod has probably
buggered off on holiday), so perhaps you could rummage your 'valve
savvy' and assist me with a problem?

I've got Tone The Clone's 300B monoblocs back again. I have previously
removed the volume pots and they were fine; then he put a 300B in in 90
degrees wrong and did one of them no favours at all. Swapping out a
suspicious-looking cap seemed to solve that one, but he has since
managed to accidentally interchange the rectifier and driver valves at
some stage - although the amp was (seemed) perfectly OK when I swapped
them back!!

Anyway, the amps have been superbly good for some weeks, but now the
'damaged' one of them is playing up again - the bias meter (s/b 75 mA
??) moves all the way up to max as the thing warms up and bad distortion
kicks in I'm told. Any immediate ideas where to look as to cause? I have
both amps here and can make comparison measurements - I'm told all the
passives measure OK...??

Here they are *again*

http://www.apah69.dsl.pipex.com/show...yAmpsAgain.JPG


If (with my zero knowledge) I can't bash 'em straight *with some
guidance*, they will end up at the local (shiny Nigel's) hifi shop for
the odious techie *Andrew* to sort them out and I don't think I could
stand the 'loss of face'...!!

:-)




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.

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

d

--
Pearce Consulting
http://www.pearce.uk.com