> 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 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. 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
|