Message-ID: <6690.200012181217@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Mon, 18 Dec 2000 12:20:57 +0000
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Subject: LF: FETs / Overload Protection/CFH
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Reply-To: rsgb_lf_group@blacksheep.org
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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