[TenTec] Argonaut V and an amplifier

[Dr. Gerald N. Johnson]

On Tue, 2006-10-10 at 18:41 -0400, wa3fiy at 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 usable 
> gain and in line with normal operation where the amp gives approx 600 
> watts out with 100 watts drive.   BUT, efficiency 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 efficiency is because the 
> plate impedance is considerably 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
> 
I think you might find better efficiency if the amplifier was tuned with
normal drive levels and those tuning capacitor positions were used for
the low drive condition. At least for the loading.

As for amplifier control, if you use manual control, you can probably
use a double pole switch to control the Argo V and the PA at the same
time with one pole used for each. That way vintage vacuum tube control
voltages don't fry the Argo. One could create a PA control by finding
the voltage in the Argo that is only present in transmit mode and use
that to drive a MOSfet (presuming T is positive 8 or 10 volts) that keys
the PA.
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer