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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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