[AMPS] 6M Linear designs
km1h @ juno.com
km1h@juno.com
Wed, 12 Nov 1997 08:51:12 EST
On Wed, 12 Nov 1997 14:13:53 +1300 Martin Ellis <jmellis@ihug.co.nz>
writes:
>Carl wrote
>> The 6M amps I build use a regular Pi-Network, same as an HF amp.
>> The only difference is that more of the Tune C is in the tube
>> than at 10M. Since the parasitic frequencies of the 572B is
>> close to 6M it becomes harder to suppress without excessive
>> loss in the suppressor.
>
>Good morning Carl,
>The best looking homebrew 6m linear I have seen was made by
>Jim WA7JCO in Phoenix AZ. He showed me >1.5kw keydown on 6m
>with an 8877. There was no parallel plate capacitor - instead
>he used a vacuum variable as a tunable series capacitor.
>This replaced the blocking capacitor, and by placing a
>capacitor in series with the <large> anode capaciance
>it enabled an efficient tank Q to be achieved.
>I believe he told me that no suppressor circuit was required
>in this case.
>Hope my rough drawing helps.
Thats an interesting approach Martin. The drawback however is that the
cap must be able to sustain DC and the RF and being a low C value it
would have a somewhat high reactance at 50 MHz. Not an easy task for an
air variable at QRO levels.
The 8877 is not as bad as some for output C. The trick is to build or
modify an air variable that has a very low minimum C and use in a
conventional Pi-Net. Vacuums are nice but complicate the mechanical
layout.
I have even modified a few LK-800A's for 6M and that uses three
3CX800A7's. There is not much variable C available for that mod but
enough to completely tune and push 2800W out the pipe with 100W of drive.
No parasitic suppressors either.
My conversions are of existing HF amps so I am limited in what I can
do...... and also stay within budget! I have been very pleased with the
results since the efficiencies obtained are every bit as good as when
stock HF (on the best band).
Getting a suppressor design that worked for the glass tubes was a
challenge since the RF deck layouts were not the best to start with.
73 Carl KM1H
>
>
> Tune L
> Cap
> --------||-|--^^^^^^^^^-|--------- output
> | | |
> | ( |
>--- ( --- Load Cap
>Anode ( ---
>8877 RFC ( |
> ( |
> | Gnd
> |-----|
> | ---
> | --- bypass
> | |
> HV Gnd
>
>Regards
>Martin ZL1ANJ
>
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