>
>> Sometime ago, I bought an 8171 with socket, chimney and filament transformer
>> with the idea of building a 6M amplifier. This project is not intended to
>> be for the greatest power output, but for an amplifier that would withstand
>> the strain of considerable EME operation. and be easily cooled with a QUIET
>> blower.
>>
>> I've looked at the specs on the tube, and at various tetrode designs for 6
>> meter operation, and I'm completely stumped.
>>
>> To start with (while I enjoy drilling, hammering, filing and soldering) I'm
>> the furthest thing from an amplifier designer.
>>
>> The most significant problem is that the large input and output capacitances
>> are hard to deal with at 6M. At least that is my conclusion.
>>
>> A while ago, on this reflector, someone was dealing with a similar problem
>> (although not with the 8171), and suggested using the W6PO circuit, scaled
>> to 6M.
>>
>> Could that be an option? What are my options?
>>
>> Thanks, Barrie, W7ALW
>>
>
>Barrie,
>
>I don't know the specifics of the 8171 (4CX10000 ?) but I have played with
>8877, 3-500 and 3CX800 amps on 6m.
>
? The RL of an 8171 is c. 1200?, so, at 50MHz, a total C-Tune of roughly
80pF is needed for a Q of 15. Since the anode-screen C, stray C, and the
neutralizing bridge total c. 30pF, a 50pF Tune-C should do the job.
>The relatively large anode capacitance makes the conventional pi-tank rather
>"touchy" up at 50MHz - my 8877 amp copied directly from the well known
>GJ4ICD/G3WOS design would tune but wouldn't load at about about 1100W
>output until we had it on a network analyser to find out what was going on had
>to change the L by about 5-10% to fix it.
>
>You might find a better approach would be to use the L/L-C tank circuit with
>a rotating shorted-turn loop to vary the L - my 3-500 6m amp (copy of the
>1971 QST design) uses this technique to great effect.
? An excellent solution for higher RL tubes, Mike.
>I got the tank circuit to
>work first time by putting a 3K3 resistor across the tube and sweeping the
>output socket and adusting (bending) the coil around until it covered
>48-52MHz when rotating the shorted-turn loop.
>
>Something like this should 'scale up' for the 8171...
>
>Mike G8TIC/M0VHF
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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