[AMRadio] W9INN antenna & Central Electronics 20A

[Gary Schafer]

Hi Lynn,

I also have a pair of 813s that I thought of doing the same thing with. But the
tubes are mounted on a small sub chassis that doesn't have much room left under
it for a grid tank circuit.  The B&W amp as I recall also has the 813s on a sub
chassis. Probably more room under it than I have. Still might be a tight fit
for a tank circuit, band switch and tuning cap though. Another problem that you
might have with it is getting a good enough ground on everything so that there
is no feed back to the grid which would make it oscillate.

I don't think that you could get away with just a balun and grid resistor on
the 813s as with smaller tubes. The 813s require a much higher grid drive
voltage which means keeping the grid impedance higher. Then if you put in
enough resistance for swamping to keep it stable you will probably run out of
drive with the 20A. The 813s need to be run in AB2 to get good power out which
takes more drive. Also keeping in mind that if you want to run them in class C
that requires even more drive. So a tuned grid circuit and neutralization is
probably the only way to go.
Then consider the power supply size and the size of the plate tank components
for plate modulation too. I know that they use the B&W tank coil. That thing is
a brute. Not sure about the caps though.  Would be nice if you can fit the grid
tank in there.

73
Gary  K4FMX


Lynn Fisk wrote:

> At 08:26 PM 01/02/2002 -0500, you wrote:
> I found that by making a small 4:1 balun and loading the grid with a
> resistor I could drive it with no grid tuning. Simplifies the amp.
>
> Hi Gary.  You may recall that I built one of these for the 6HF5 amp that I
> built for my 20A.  I worked great on 75m but not on 20m.  I may have not
> had the right core.  I just don't know.
>
> Anyway, I want to try again, this time to convert a set of B&W 813s (the
> L1000A, I think) from GG to grid driven.  I could then use the 20A to drive
> these, and I could also plate modulate them for AM.  That is the grand plan
> anyway.  Do you see any problems with my thinking?
>
> 73, Lynn.
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