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Old 01-16-2017, 09:53 AM
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dtvmcdonald dtvmcdonald is offline
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I have the latest blown one, which is a 250 mA. I'm out of 200s. I looked at it
and didn't see any remaining element at all. I'll look tonight.

It survived two turnons this morning.

Given that it has blown at a time of rapid change from black to
normal intensity, or rapid change in AGC, maybe we have a clue that
it is the AGC circuit. Other than the screen pots, that's the only thing
(its screen) connected directly to the 375 volt supply.
Of course, there is the possibility of arcing across at the tube socket.
Maybe I should put a 5K resistor in series.


But I see no trace.

Perhaps I should try a filter cap, say 10 uF at 450V
across the "far" side of the fuse.

Note that while all the 450V caps see 500V at a normal turnon
I tried a couple of ones of different capacitance from the
same batch on a tester at 550V and none showed any leakage at all.

But in any case, the fuse never blew except at normal voltage, 365 to 390 volts
depending on Variac setting (barring the event that caused the blow, of course).

Last edited by dtvmcdonald; 01-16-2017 at 10:05 AM.
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