Wyn,
I have an antique Hallicrafters HT33A with a PL172 tube, and I
modified it so it will accept lower drive. Right now when I drive it
with about 20 watts from my K2/100 in PSK31, I get about 400 watts
out. That's a gain of 20 times, or 13 dB. The tube input is swamped
with a 50 ohm resistor to help in its stability. The PL172 is a
power pentode with 1000 watts plate dissipation, operated in the
class AB1 mode (no control grid current) when amplifying SSB. On CW,
it operates in the class AB2 mode, with a little higher
efficiency, A 4-1000A is the nearest new(er) design tube. I
thought about how I could make the old guy put out something exciting
when driven by my K2. I could swamp the input with a 200 ohm
resistor and use a 4:1 balun and the impedance to the K2 would still
be 50 ohms. Actually, with the K2 automatic antenna tuner, you could
tune the 400 ohms to 50 ohms. Same difference. The tube itself
does not take any power in class AB1, but the input voltage needs to
be so much for a given output. A 4:1 balun would increase the drive
voltage by a factor of 4 times which would increase the output of the
amp to 1600 watts if the drive remained the same, 800 watts with 10
watts drive, or 400 watts PSK31 with the K2 maxed out at 5 watts,
which is respectful. From 20 watts to 1600 watts is a power gain of
19 dB which in my opinion is a little high. I would be more
comfortable with a power gain of 16 dB, but you can try it.
The point is, yes you can find a tube that will take a 10 watt
transmitter to 800 watts or so, but be careful. Those tubes are expensive.
73, Chas W1CG
At 09:17 AM 6/9/2006, Wyn wrote:
Greetings from a newbie K2 builder. I am currently building #5390.
Into phase 3 now, somewhere in the early stage of the rf board.
Don't get too much time for this detailed work and my xyl has made
me clear out the spare bedroom in our apartment (aka construction
area), as she does not seem to like the lead droppings and other
bits and pieces on the floor, while one of our daughters and young
toddler are visiting.
While the build is only a work in progress, and although it passed
the stage 1 and stage 2 tests it's not over till it's over, my mind
is turning to the question how to exploit the K2 when it finally
gets launched. Let me say, I have nothing against qrp. The main
reason I bought the kit (instead of an Argo V), was the desire to
have a low current drain rig, to take on /P. I don't have much of an
antenna sytem at this VR2 location. I have a Icom IC-781 which
serves as the main rig at this station, but if the K2 works, I would
like to try it as a main station rig with decent power as well as a /P rig.
Here's the question. My only amp at this location is a Drale L4. It
is just a 10db gain amp. So it would only produce at most, 150 watts
driven by the K2. In practice, probably less. I have never driven a
grounded grid triode amp sccessfully with such low power with any
success. How does one drive a 10-15 watt output radio to serious
output without an intermediate amp?
I have designed and built many amps and repaired many others but not
before an amp with a projected power gain of 20db. They were all
either grounded grid triode affairs or a G2DAF (pair of 813), which
is pretty similar. My basic thinking is a high mu tetrode amp, grid
driven, preferably with a swamp resistor as input but, if needed, a
tuned input and neutralized. I love glass tubes and am shy of using
a 4cX400a or 4cx800a type tube, as I hated my ex-Alpha 01b, although
I think the tubes would probably work ok but uninspiringly with the
right design. I would really like to try a 4-1000a or even a 4-400a
in that configuration. I have no issues with working on high voltage
and plan to use the 2.5kv supply from the L4, though the 4-1000a
does benefit from a higher anode voltage if available.
I would be interested to hear from anyone else who has gone down
this road and of their experiences.
Meanwhile, best 73s and GL to you all from Hong Kong.
Wyn
VR2AX (ex VS6UK)
GW3YGH
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