"Phil Allison" wrote in message
...

"Gareth Magennis"


Coca-cola spilt through the grille?!


** Not tricks here - I have detailed all the faults and there is no sign
of any such damage.




... Phil




Well, one last try before I give up then.


The "lost vacuum" tube probably just went out of circuit, leaving one valve
vs. two.
I think I'm right in saying that this will produce a net DC field in the OPT
primary that would push it towards saturation and overheating.

The amp was kept in use at high power with this fault.

So, the "two valve" primary winding got very hot, shorted some turns, then
went open circuit. This meant that its Screen Grids became the Anodes,
which got overloaded until the resistors burnt or fell off the PCB.
Half of the Bridge rectifier may well have burnt out in the process of
burning out the screen grid resistors - at some point the HT fuse would
finally blow, halting the whole process.
(Or the customer could have replaced the blown HT fuse with a higher rated
one, then put the original back in before handing it in for repair).

There might be other problems due to disconnection of speaker, or arcing
inside the OPT connecting secondary and primary, that might force HT back to
the output circuitry, I don't see any "flyback protection diodes" or
whatever they are properly called, in the schematic.


That's just about the limit of my knowledge of failure modes of valve amps
I'm afraid.


Gareth.