[Amps] Jennings Vacuum Variable Capacitor

[rlm]

On Thursday, October 30, 2003, at 03:18  AM, Bill Turner wrote:

> On Wed, 29 Oct 2003 09:40:27 -0800 (PST), Mark Toussaint
> <n1mt at 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.

***  Clever idea, however, the potential gotcha is that the added L 
simultaneously moves the 3-500Z's VHF anode resonance down into the 
region where 3-500Zs have enough gain to regenerate via C-feedthrough. 
(0.3pF for 2, 3-500Zs)
-   Murphy was right -- Things are usually more complicated than they 
first appear.


>
> 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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>
Rich Measures, AG6K, 805.386.3734.  www.somis.org/