----- Original Message -----
Sent: Sunday, October 02, 2005 7:37
PM
Subject: Re: LF: Re: LF AMPS
Mal,
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.
de Lech, G3KAU
Its nice to know you and Derek are still about. I have not heard both of
you for years, so its no surprise your Decca is lasting well, it is never USED
!!!!!
I recall your old friend Peter up the road that used one describing how
to change blown fets. Did I imagine this ????????
I have two of these Decca machines that I
modified for 137 khz but still use my old home built FET tx. When I get more
time I will fire the old Decca Jeep up.
Have you boys from the deep south all given
up on LF?
de G3KEV
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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