[AMRadio] Building

ne1s at neandertech.com ne1s at neandertech.com
Tue Dec 4 16:55:50 EST 2007

----- Original Message -----
Subject: [AMRadio] Building
Date: Tue, December 4, 2007 13:53
From: "jeremy-ca" <km1h at jeremy.mv.com>

> Ive been wondering, besides the drive requirement, why swamped grid cant be
> used in Class C? It would eliminate the neutralization requirement with
> tubes as well as the hassle of a bandswitching input.

I was attracted to this design in the original incarnation of the RF deck
for my PDM transmitter - at the time it was a class C final using a single
4-400. To get enough grid drive I was trying to use (IIRC) a 16:1 Guanella
transformer design out of Jerry Sevick's book, for a 4X voltage step-up
from a 50 ohm source. Worked acceptably except at the edges of the
intended frequency coverage (160M & 10M), where the VSWR was out-to-lunch,
even into a purely resistive load. Also, as I recall, 15M was sort of
marginal. Once the parasitic reactance of the grid circuit was added, of
coarse, things got even worse. I'm not saying it can't be done, but I
couldn't get it to do what I wanted with a reasonable amount of effort
expended. So I went to a variable T-network input and swamped the grid
with about 3000 ohms non-X (in fact I bought the resistors from
Radiokit!). More knobs to twiddle, but worked much better for me. Same
input circuitry is in place now with a pair of 7527A's.


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