[Amps] Sanity check

[Carl]

----- Original Message ----- 
From: "Vic K2VCO" <vic at rakefet.com>
To: "Amps reflector" <amps at 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