Dear Richard, Andy, LF Group,
I once had a similar problem where one FET blew up in an
amplifier, and it's replacement immediately failed too. This turned
out to be because the other FET, although not catastrophically
shorted, no longer had any gain, and the amplifier was trying to
operate in a single-ended mode without much success. Replacing
both FETs solved the problem; I suppose the moral of the story is,
if one FET goes, functionally check the others in the circuit as
thorughly as possible.
As far as short-circuit protection for Andy's amplifier goes, the
Decca "guard" circuit does this very nicely - when testing, I ran
mine into a dead short at full power for the best part of an hour
without it getting particularly hot.
I'm all set up to record CFH, and hopefully get a reasonable
calibration of the actual field strength too, but it looks like Murphy
has intervened, and it will have to wait....
Cheers, Jim Moritz
73 de M0BMU
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