>I just had my 1st QSO with the 4-1000A 6M amp, running 750 out at 50% eff.
>the problem was (is) two high of a load impeadance, I tried lowering the
>plate voltage with a variac and eff came up every step lower, to 60% at
>1500 volts! (675 watts in/400 watts out). I am running 4000 volts now, with
>capacitor at minimum. The next step is a tuned input, maybe the greater
>drive will lower my RL some more. I'm getting there, not bad for not
>knowing algebra and drilling the first hole 10 days ago, (thanks to K5PC
>for shipping Vac Variable before recieving my check!) still 12 days before
>the contest.
Hi Dick,
There is a trick that some of us talked about a few months ago when I had
the same problem with a 4-1K at high voltages on 10M.
The tube does have a 7.6pf output capacitance. However, put that puppy
in socket and add a cabinet around it and your capacitance probably
doubles or tripples. Then you have the problem of matching a large
impedance with a somewhat large tune capacitance. The answer is a
super-high Q output tank. I calculated some possibilities with my amp to
be Qs of around 40 or so! Too much circulating current!
The trick I used and used successfully, was to add an additional coil on
the output of the tube before the tune capacitor. This coil is in series
with the anode output line. Really you can put it between your blocking
cap and tune C. You'll need to experiment with different coil values,
but suffice to say, it will be fairly small. Just a few turns is all
you'll need.
What this coil does is to act as an impedance step down transformer. You
can work out the math using series to parallel conversion, etc. but it
does work. Then you'll be able to use your pi-net to match a lower
impedance to 50 Ohms which is easier to realize.
If you don't feel like doing the math and all, I have a couple of Excel
spreadsheets available that I can e-mail you. They are specifically set
up for designing this sort of circuitry.
Also, check out my web page (www.qsl.net/ke9na) and check my amplifier
pictures to get a good idea of what I am talking about.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
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