Gary:
You didn't say which band?? But, with just a single band, the caps are
less of an issue. Always use a vacuum for the C1, plate tune cap. This
is because there are very high RF currents there, and also high
voltages, due to the high plate impedance of the tube.
The vacuum is a better fit there. For 160 meters, and a single 8877, for
example, you need a 12-500pF, 7.5KV part, for a Q of 10-12. A typical
vacuum variable in this size range handles about 50-70 amperes of RF
current, depending upon frequency.
The following is a good source:
COMET North America, Inc.
Phone: 203 852-1231 and FAX: 203 838-3827
All RF applications
Fixed or variable
3 pf to 6600 pf
Peak test voltages to 100 kv
Current ratings to 1000 amps.
Best of luck.
Vince
WA2PKE
*************************************************
Gary Nieborsky wrote:
>
> I've been collecting parts for my homebrew single band amp project. My
> question is this:
>
> Which is better - air/vacuum variables or fixed value capacitors in the
> output circuit?
>
> My concept is to make up tuned modules for the bands to be operated on and
> use them to change the single band the amp is used on.
>
> I've looked at my antennas and find that r+jx stays reasonably matchable for
> fixed value modules.
>
> Tanks for the output......(sorry).
>
> Gary K7FR
>
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