On Wed, 29 Oct 2003 09:40:27 -0800 (PST), Mark Toussaint
<n1mt@yahoo.com> wrote:
>I'm converting an old sb-220 to work on 50mhz. I need
>a low value tune cap and want to use a Jennings vacuum
>variable of 2-30pf, rating of 5kV or 7.5kV. Does
>anybody have one or know of a good source for these
>that won't break the bank ? I know I'll need a low
>value of C for the tune, any other thought's or
>suggestions ?
_________________________________________________________
There's a much easier way.
Use an L network to step down the existing plate impedance to about
600-800 ohms and you'll be able to use the existing tune and load
capacitors. Details are in any recent ARRL handbook in the HF
amplifiers section. Basically you are using the tube's output
capacitance as the parallel reactance of the L and adding a small coil
as the series reactance. This does the job perfectly and is much
simpler than changing the tune/load capacitors.
In my SB-1000, I used a coil of #12 wire, 3/4 inch diameter, four
turns, about 1 inch long for the L network coil, and a slightly larger
diameter coil of #4 wire (other dimensions the same) for the tank
coil.
I also changed the filament choke and plate choke to avoid resonances
in the 50 MHz region, but this has to be done regardless.
--
Bill, W7TI
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