> Phil Clements wrote:
>
> > If one wants to depart from the traditional Pi input network a parallel
> > L/C configuration can be utilized. A tapped fixed inductor from the
> > cathode(s) to ground, shunted by 300 pf Arco trimmers for each band.
> This
> > seems to cover all bases, is easy to construct, install, and tune. This
> is
> > the scheme utilized in the Alpha 77 series amps that results in a clean
> > signal and a happy exciter.
>
> This would work fine when the input Z is near 50 Ohms (which most are).
> I think you would need a lot more than 300 pf to get the proper Q on
> the lower bands.
The two tube kit for the Alpha 77SX included a 2:1 unun transformer to be
inserted in series with the input line. This matches the 50 ohms out of the
input tank to the 25 ohm impedance of a pair of 8877's. A proper unun can be
constructed to handle most of the popular tubes. Yes you need an additional
1500 pf on 160m shunted across the trimmer. Al the other bands work fine
with 300 pf on the Alpha's.
(((73)))
Phil, K5PC
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