[HomeBrew] 833 grouned grid?

[Eric Hughes]

----- Original Message -----
From: <N2EY@aol.com>
To: <w4chq@inetgenesis.com>; <homebrew@mailman.qth.net>
Sent: Tuesday, January 22, 2002 9:48 PM
Subject: Re: [HomeBrew] 833 grouned grid?


Jim Wrote:
> GG operation is impractical with the 833A for reasons given by others. But
> don't give up!
>
> My 1979 ARRL Handbook has a design for an amplifier using a single 833A in
> grounded cathode. The design is pretty simple - the input circuit is just
a
> 50 ohm noninductive resistor (50 watts) from grid to ground, and the bias
> network is just a pair of power Zener diodes (37V 50W in series with 23V
50
> W). The output circuit
> is the usual pi network and the plate RFC is homemade. Neutralization is
> unnecessary because the grid circuit is so heavily swamped.
>
> This amp will easily take a kW input at plate voltages from 2500 to 3500.
> Pi-net component values are given for 2500, 3000 and 3500 volts. Other
tubes
> like the 4-400 and 304TH can be used in the design with slight
modification.
> Operation is class AB2 and efficiency of 55 to 65 percent is reported.
>
> The design shown only covers 160, 80 and 40 because of the high
capacitances
> of the 833A. Two tubes in parallel? Wow!
>
> I don't know if this design ever appeared in QST (I think it did), nor how
> well it works, but it might be worth checking out if you have the tubes
and
> associated goodies. It's no more complex than a GG amp of the same rating,
> and if you don't like it most of the parts can be reused.
>
> 73 de Jim, N2EY

 Jim, Thanks for the info! I now have the circuit you are describing and
have decided to
go with that design, except use two tubes instead of one. Since I only want
the low bands
anyway, I think the limitations on frequency will not be a problem. The only
parts I may have
trouble finding is the high power zeners. I already have the High power
transmitting caps
mounted vertically in a three foot bud cabinet with right angle drives
through the front panel.
Also have a very heavy duty ceramic band switch for the tank circuit. I have
been looking
around for a good tank inductor. I have some 2 inch coil stock but would
like to put something
more substantial . I guess I could wind my own out of copper tubing if I had
to. The filiament
transformer was homebrew by a guy who was into am stuff. It has a secondary
that looks like
it could handle three or four 833 tubes! Anyway, I plan to put about 2550
volts on the plates
or put a variac on it to keep it just at the legal limit.  Thanks again for
your assistance!
73, de Eric W4CHQ