On Oct 2, 2005, at 9:02 AM, Scott Townley wrote:
> Regarding air-variable capacitors:
> Most folks refer to air-variable caps by their max C value, and
> sometimes
> include the plate gap and/or rated working voltage (particularly when
> selling them!)
> But much less frequently mentioned is the minimum C value, which of
> course
> is quite important in Ctune applications.
> It's surprising (to me, the uninitiated) how few "vintage" caps
> (Johnson,
> Millen, etc.) are suitable for say a single 3-500z amp without
> compromising
> the loaded Q at 28MHz.
Good point. This was why Drake used a dual section Tune-C in the L-4B,
with one large section for medium and lower freqs and one small section
for higher freqs, especially 28MHz.
-
Another solution might be to add a fixed L between the anode and the
Tune-C so as to transform RL to a lower value on 28MHz, which would in
turn happily require more C at the input to the Pi. A possible
additional benefit of such a design is that it would decrease the
anode's VHF resonance. If this new resonance was below the grid's
resonant frequency (c. 89MHz for a 3-500Z), the grounded-grid would
start acting more like a shield than a sieve, and VHF stability at the
anode-resonance might improve.
> Or perhaps I am missing something. ...
You didn't Scott.
...
> Scott Townley NX7U
> Gilbert, AZ DM43di
> http://members.cox.net/nx7u
>
>
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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