I am not sure that it works properly when the coaxial part of the tank coil is
tapped.
WC6W has experience with this and it's possible that he only used it in the
highest
frequency section of the tank coil.
What I'm noticing is that funny stuff happens above and below 20 meters, and
the coaxial
part of the tank is the 20 meter part.
Rigid coax has been used for this purpose. One thing I learned is that if you
are making
your own 'coax' with tubing, you must put the center conductor inside the
tubing before
you coil it up! I used a piece of RG/58 with the braid removed inside 1/4" OD
tubing.
On 12/3/2010 5:32 PM, TexasRF@aol.com wrote:
> 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/
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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