----- Original Message -----
From: "Vic K2VCO" <vic@rakefet.com>
To: "Amps reflector" <amps@contesting.com>
Sent: Friday, December 03, 2010 5:54 PM
Subject: [Amps] Sanity check
>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
That method is more a curiosity and far from a design in wide use if at all.
What are you measuring power with? The 2-30 MHz Bird elements are down at
1.8.
Recalculate and use an Ameritron or a B&W 802.
Carl
KM1H
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