That is an interesting plate choke variation. Can the idea be extended to
include the lower bands as well? If so, the plate choke would be at a
nominal 50 ohm point on all bands and exert very little influence on plate
circuit tuning.
I am wondering if .141 semi-rigid coax would work for making up the low
band inductors.
73,
Gerald K5GW
In a message dated 12/3/2010 4:55:03 P.M. Central Standard Time,
vic@rakefet.com writes:
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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