[Amps] What am I missing?

[Phil Clements]

> > January QST has an article on old amplifiers that
> > advocates replacing the input circuits on an SB-221 with
> > simple parallel-tuned circuits for each band. How can
> > these circuits possibly achieve matching between a 50-ohm
> > exciter and the tubes?
> 
> They can't if the impedance of the tubes is anything other
> than 50 j something.

Nothing new here; the Alpha 77DX and others, use simple L/C networks about 3
inches from the cathode. That is also 50 j something. The SX kit that adds a
second 8877 includes a 2:1 un-un to make the input Z 50 j something also.

The QST article was supposedly inspired by two Eimac bulletins; one for
strapping the grids to chassis, and one for using the L/C networks instead
of a pi for 3-500Z amp design. I wish someone could dig up these bulletins. 

Following the QST article verbatim, I would think that running "Z" tube(s)
at zero bias as shown in the diagram is a much worse idea that the other
two.

(((73)))
Phil Clements, K5PC