OK, let me eat a little crow and try the numbers again. :-) Seems there
are two peaks, on 40 meters anyway, and of course I got the wrong one
first time around. The load control is now near max CCW.
Same test conditions as first reported but with much different results.
With 20 watts drive, 200 watts out, 576 watts input, 376 watts dissipation,
approx 35% effeciency. Not exactly great but much better than my first set
of readings.
I would think with these numbers, the SB-200 would be quite useful at 20
watts drive.
I tried tuning up at full power, 1000 watts input and then reducing drive to
20 watts. Although that gave almost 200 watts output, tuning up at 20 watts
gave the most output and best effeciency. So Bob, since you already have
the SB-200 give it a try. Hopefully it will tune the MARS frequency OK and
get you around 200 watts and not be abused in the process.
73,
-Lee-
WA3FIY
On 10 Oct 2006 at 18:41, wa3fiy@radioadv.com wrote:
> 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
>
>
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