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Mal,
I am glad to say that your comment regarding
experience of frequent failures of FETs in the 1Kw Decca 5501
amplifiers in use on the LF bands (including 73Khz) is as ever, a load
of codswollop!
In all the years they have been used since
they became available for use on the Ham bands, I have only ever had heard
one report of FETs been blown. and that is despite plenty of instances of short circuits and open circuits etc.being
reported on the antenna output. at full power. They have proved to be almost
bomb-proof.and that includes long periods of QRSS at 10sec dots at which
duty cycle they barely get warm! They were
after all designed to operate at continuous carrier ioutput n the Racal-Decca
Navigator system.
At least 15 of the
amplifiers went through our hands at the Crawley Amateur Radio Club to various
destinations including EU and North America and we still carry a
quantity of spare FETs but have only ever had one request for
replacements.
73,
Derek Atter G3GRO
----- Original Message -----
Sent: Thursday, September 29, 2005 5:56
PM
Subject: LF: LF AMPS
For trouble free and rugged amplifiers
on LF a tube is the only reliable approach. The type of antennas used by the
majority of LF radio amateurs are totally unreliable, poor insulators,
antenna moving around in the wind, variations in wx dry or wet and
other considerations. These conditions cause considerable variations in
antenna impedance, resulting in sudden mis matches, high swr etc and except
you have a high degree of protection the FETS will not survive. Tubes also
suffer but seldom are destroyed.
Those using the rugged Decca Jeep fet
amps (5501 amps) also suffer the same fate. The majority of
users have blown the fets.
Using fet amps into a dummy load is a totally
different story, no variations in load and total reliability, but connecting
to the average amateur LF antenna will produce loud bangs and
smoke.
Using QRS especially above 3 sec dots
duration is just waiting for a disaster to happen!! 60 or 120 sec dot
duration is a killer for prolonged QSO'S
Go for a tube but watch those very
high anode voltages like 3k. YOU could expire
prematurely.
73 de Mal/G3KEV
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