[AMRadio] Building


jeremy-ca km1h at jeremy.mv.com
Tue Dec 4 20:05:57 EST 2007


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: <ne1s at neandertech.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
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" <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
> 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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