>
>> 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.
Like I said - didn't know thge specifics of the 8171... with RL down at
circa 1200R the transform's not that great then... hence the standard
pi-tank
works. If the tube used 20Kv HT @ 0.5A then it would be a different
thing to tame...
>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 wit
h
>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.
Certainly found it to work well. The amp is stable, efficient and remains
tuned
well even when thrashed in contests. It should be noted that if you change
the
4.5 turns to 1.5 turns and reduce the diameter of the loading coil by 1/3rd
the
amp works equally well on 70MHz (UK 4M band).
On both bands it will do > 850W carrier with remarkable aparrent anode
efficiency using a modest power supply. Just goes to show that 30+ year old
designs are still hard to beat today.
>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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