w8jitom@postoffice.worldnet.att.net wrote:
>
> > From: Martin Ellis <jmellis@ihug.co.nz>
>
> > Good morning Carl,
> > The best looking homebrew 6m linear I have seen was made by
> > Jim WA7JCO in Phoenix AZ. He showed me >1.5kw keydown on 6m
> > with an 8877. There was no parallel plate capacitor - instead
> > he used a vacuum variable as a tunable series capacitor.
> > This replaced the blocking capacitor, and by placing a
> > capacitor in series with the <large> anode capaciance
> > it enabled an efficient tank Q to be achieved.
>
> Bad theory there Martin.
>
> Adding a series C does NOT reduce thank Q, contrary to what Jim
> might claim. It actually increases tank Q, because of the extra
> stray L and C of the vacuum cap.
>
> What happens is this...
>
> The tube out C STILL remains the tank input C. All the
> additional series C of the vacuum cap does is cancel some of the
> inductive reactance of the tank inductor, requiring the designer to
> add more inductance for the same resonant frequency.
Hello,
I have never tried a circuit like described with the capacitor acting
as a blocking and plate tune. It sounds a little screwy to me too. I
think I would do this, put in a fixed value blocking cap, with enough
capacitance to have good transfer of power. At 6 meters 100pf will
probably be fine. Dont add any more capacitance to the circuit by adding
a plate tune cap just simply put enought L in the tank coil to resonate
at the desired frequency with the existing tube capacitance. On 6m your
not moving around so tuning will not be required often except by the
loading cap, when changing antenna or loads. You can find the exact
amout of L a couple of ways. Spread the coil by hand small amounts at a
time. Temporarly add a plate tune cap and spread or compress the coil
to you achive minimum capacitance on the cap. Make the L mechanically
adjustable, nice article in one of the handbooks shows details. I built
a 4cx1500 about 18 yrs ago for 6 meters using a adjustable tank coil,
leaving out the plate tune cap. Amp is still in use today and works
great. Never needs retuning for normal ssb cw band requirments and you
will find the tank coil compression deal to not be very sharp when
tuning. I used 1/4 soft copper, heat treated it again to make it softer
and used a teflon screw and ends to make the spreading action of the
coil. It was all silver plated afterwards.
Ron W4WA
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