Here is a link to W8JI's article on the subject. He explains it pretty well.
Bill Orr has a little on it in his handbook also.
http://www.w8ji.com/tuned_input_circuit.htm
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of jeff millar
> Sent: Sunday, October 10, 2010 2:35 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Which LDG Autotuner for Amp Input?
>
> On 10/10/2010 12:42 PM, Gary Schafer wrote:
> > Look at the WD7S tuned input board.
> > http://home.earthlink.net/~wd7s/TU-6B.htm
> >
> > An external tuner is not the best way to go as the output capacitor of
> the
> > pi net needs to be very close to the cathode of the tube.
> >
> That's probably true in the sense that the the line from cathode to
> tuner should be much shorter than a wavelength at the operating
> frequency...or else the capacitor starts to look like an inductor. So
> how about putting enough capacitance close to the tube cathode to cover
> say 10 through 20 meters and then use a relatively short line to connect
> to the tuner?
>
> jeff, wa1hco
>
> > The WD7S board solves all problems.
> >
> > 73
> > Gary K4FM
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