Joe,
Precision Paper Tube Co. made a lot of those adjustable forms that most used
for variable inductors. You could get these either PC board mount or chassis
mount. The chassis mount goes in a round hole with a small metal clip around
the top neck of the form. You need to tell them what ferrite material you want
for the slug and then wind your own coils. These coils are calculated the same
as a toroidal impedance transformer is. The only difference is by using the
slug, the impedance coupling is changed by moving the slug in and out.
Sometimes Oceanstate Electronics carry these too.
You can change this by using an air wound bi-filar transformer and tune it by a
padder capacitor like an Arco 302 or 304. Generally the 302 is used. If more
capacitance is needed, you can add a silver mica cap at 500V in paralell with
the padder or go up in model number to the 304 or greater. The Arco caps are
mica compression caps and the 302 is around 50-100 pF if I recall. This is the
method I use to tune grid driven amps and works for G-G too. A simple Pi
circuit can be used also with an air coil. You just need to know the input
imprdance you want.
Oceanstate Electronics
http://www.oselectronics.com/
Best,
Will
>
> I'm working on a "classic" W6PO 144 MHz 8877 amp
> (http://web.wt.net/~w5un/8877-1.htm) . The input tuning circuit
> calls for L1 to be wound on a 1/2" diameter form with a white
> tuning slug (CTC 1538-4-3) . Does anyone know where I can find
> this particular form? Or a reasonable replacement? The amp
> currently just has the coil and some unknown ferrite rod stuck in
> the middle and can't get the SWR between the amp and driver below
> 3:1.
>
> Thanks
>
> joe n6kk
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