I am using a 1kw Bird element. So that might be the explanation for the slight
drop in
power. I have a VFD digital power meter that I can try.
On 12/3/2010 5:28 PM, Carl wrote:
>
> ----- 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
>
>
>
>
>
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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