I second Derek Atter's comments. My Decca transmitter run for 2 years without any malfunctions in normal CW and QRSS modes. It survived working with accidentally disconnected antenna at 1Kw power level and a melted and shorted coax did no harm. My store of spare fets is undiminshed.
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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