I have an idle SB-200 here so I ran some tests to see what 20 watts drive
would do. Found several interesting and maybe discouraging things.
Forget about ALC. 20 watts will not drive the amp to anywhere near the
power where ALC would be useful. No problem there.
I used an Argosy at 20 watts, heath cantenna, Bird 43 wattmeter and Drake
W4 wattmeter. 40 meters.
My tubes are in pretty good condition.
20 watts drive gave 115 watts out. Little lower than I expected but useable
gain and in line with normal operation where the amp gives approx 600
watts out with 100 watts drive. BUT, effeciency is horrible! Standing plate
current is just under 100mA. With 20 watts drive, plate current is 340 mA
at 2300 volts. That is about 782 watts input for 115 watts output or 667
watts plate dissipation!!!! Not good. As I recall, the two tubes together
have 320 watts max plate dissipation. Under normal operation with 100
watts drive, the amp actually has slightly lower plate dissipation than when
being driven with 20 watts.
I suspect, part of the reason for the very poor effeciency is because the
plate impedance is considerbly different than design target because of the
low drive. For instance, I have the load control full clockwise and still
cannot peak output with the load. Plate tune will peak OK.
While the setup will work, I'm not so sure it's the best way to go. What
other options do you have?
On keying, RF sensing is a poor way to go. It works by sensing RF from
the transmitter and switching the amplifier. The problem with that is that all
timing and waveform shaping is removed to be replaced by contact bounce
as the antenna relay in the amp switches from RX to TX. You will not likely
notice it but users of adjacent frequencies might not be too happy and the
Argonaut will see an unstable load during the switching. Better to use a
separate PTT switch and switch the amp to TX before transmitting with the
Argonaut. I have a friend who used that technique successfully for many
years.
73,
-Lee-
WA3FIY
On 10 Oct 2006 at 6:01, Bob Scott wrote:
> I am considering simplifying my shack a little. I have an SB-200
> currently not in use and was thinking of trying to drive it with my
> Argonaut for those times when I need the power (Air Force MARS). Does
> anyone know of any issues with doing this and what sort of output could
> I expect with only 20 watts max of drive from the amplifier. Are there
> better options to accomplish this?
>
> Thanks and 73,
> Bob
> W4ZY
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