I am doing something dumb, but I can't figure out what.
My new amp is a pair of 813's, class C. On 20 meters, it loads to 500 ma at
2050 volts and
produces about 800 watts output. The tuning capacitor and tank inductance are
close to the
calculated values (from GM3SEK's spreadsheet). With power off I connected a
2050 ohm
resistor -- (2050v x 0.5a)/2 -- from the plates to ground and used my MFJ
analyzer
connected to the output to get a 1:1 SWR at the same point that it tunes up
under power.
Everything is perfect on 20 meters. But on 160, it's not.
When I used a tank coil of the calculated inductance, the loading was much too
heavy with
a practical value of output capacitance. I added several turns to the coil. Now
it loads
properly, but the tuning capacity is considerably less than the calculated
value. Also, I
get only 750 watts with the same input. I expected that it would be more
efficient on 160
than 20.
On 10 and 15 meters, the tuning capacity required for a match is considerably
more than
the calculated values (and of course the inductance is lower). I haven't
checked the
efficiency yet.
Here is one fact that might be related: instead of the usual RF choke of about
200 uh,
I've made the 10-20 meter tank coil out of tubing with a wire through the
center. The wire
is connected to the plate side of the blocking capacitor. After it exits the
tubing it
goes to a large 1.7 mh choke.
The idea (courtesy WC6W) is that the inner conductor of the coaxial tank coil
will be
close to ground potential on 20-10 meters, so it's possible to use a choke that
will do a
good job on 160 meters without worrying about series resonances in or near the
higher bands.
But I have a feeling the choke has become part of the tank circuit!
Any thoughts? Should I just order a choke from Ameritron and use the
conventional system?
Or is something else going on?
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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