Hello Tom
I'm not convinced that the 572 or the 811 was ever intended
for reliable operation above 20mhz and would never use
either of them there. Didn't you post here a few months ago
telling us that improper grid grounding was the cause of the
3cx1200's(d7 & z7)instability and you had found a method to
ground it correctly? What sockets are you using to get the
tube to oscillate?
Paul
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://wd7s.home.att.net
----- Original Message -----
>
> > and see. You really have to make several mistakes in
layout
> > to require neutralization in a GG amp.
> > Paul
> >
>
> Not true in the least Paul.
>
> A layout has to be extremely poor to require
neutralization, but
> many tubes have so much anode/cathode feedthrough they
require
> neutralization no matter what the layout.
>
> A GG pair of 572B's, perfectly shielded and laid out, are
actually
> not unconditionally stable above 15-25 MHz because of the
tube's
> internal feedthrough. Four 572's are a real major problem.
>
> While you can sometimes get away with three 811's in
parallel GG,
> four 811A's are always a problem at higher frequencies.
The Collins
> 30L1 is one of the few unneutralized 811 amps, and has
stability
> problems above 15 MHz.
>
> 3CX1200A7 and D7's have feedthrough problems above 25MHz,
> and require neutralization if you expect the anode dip to
be
> anywhere near maximum grid current, although the Z7 cures
that
> with a modified grid.
>
> .
> 73, Tom W8JI
> W8JI@contesting.com
>
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