"Phil Allison" wrote in message
...

"Gareth Magennis"

Well, one last try before I give up then.


The "lost vacuum" tube probably just went out of circuit, leaving one
valve vs. two.


** When a power tube loses vacuum, it usually conducts very heavily
regardless of grid bias setting. The *low pressure* gas inside ionises
with applied DC and it is lights up like a neon tube.


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.


** This is a large transformer and it would normally have almost no temp
rise in operation - the primary resistance is 20 ohms per side. Even under
fault conditions, the 1 amp HT fuse protects it from damage.


So, the "two valve" primary winding got very hot, shorted some turns,
then went open circuit.

** With any large OT - this is a very unlikely scenario.


This meant that its Screen Grids became the Anodes, which got overloaded
until the resistors burnt or fell off the PCB.

** This is correct.


Half of the Bridge rectifier may well have burnt out

** Read the thread again. There are no faulty bridges.


(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).

** I figure more than one HT fuse has blown, but the one in the holder
looked like the original.


There might be other problems due to disconnection of speaker, or arcing
inside the OPT

** Now you are getting warm...

connecting secondary and primary

** No way is that the case here.



... Phil








Well, are you going to tell us then?



Final answer - some problem with the speaker connection fed back 2 x HT
voltage all the way back to the mains transformer.

The customer DID replace the HT fuse with one of higher rating, ran the amp
again, and caused lots more damage. He put the original blown fuse back in
place before handing it in, in the hope you wouldn't realise what a Numpty
he is.







Gareth.