I read all of Sevicks articles in CQ as well as throwing money away for his
book which was filled with errors.
I understand corrections have been issued in a later edition.
Working in a RF lab at that time I tested many of those designs and can only
surmise that they were built for photos and to fill up article word count
and not actually tested in a lab enviroment.
It is possible to cover 1.8-30MHz with a reasonably flat response but not in
a single step with anything over a 4:1 step up. The 4X1's were driven thru
25 Ohm Globars at each grid which were figured into the load. Those and
Globar based parasitic suppressors at each plate resulted in complete
stability. Awhile after the amp was picked up the owner upgraded to a TS-930
if I remember and reported no input VSWR problems at any point in the grid
swing.
I think I will give some 813's and 814's some Class C testing with swamped
grid.
OK on Radiokit. That was fun while it lasted but it pretty much died with
all the nocoders and CBers that were swarming in at the time. I just got
tired of dealing with people that didnt know which end of a soldering iron
to grab. I held on to a lot of the inventory for my own use and went back to
a real engineering lab job until I recently retired.
Carl
KM1H
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: "Discussion of AM Radio in the Amateur Service"
<[email protected]>
Sent: Tuesday, December 04, 2007 4:55 PM
Subject: Re: [AMRadio] Building
----- Original Message -----
Subject: [AMRadio] Building
Date: Tue, December 4, 2007 13:53
From: "jeremy-ca" <[EMAIL PROTECTED]>
Ive been wondering, besides the drive requirement, why swamped grid cant
be
used in Class C? It would eliminate the neutralization requirement with
some
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.
73,
-Larry/NE1S
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