My own message bounced back...not sure why.
Sorry if this a dupe.
73 Carl KM1H
On Wed, 12 Nov 1997 08:59:59 km1h writes:
>
>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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>>
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