On Mon, 3 Dec 2001 09:11:25 +0000 "Ian White, G3SEK" <G3SEK@ifwtech.com>
writes:
>
> Radio WC6W wrote:
> > At higher frequencies, where given a typical tube the plate
> >C represents a significant part of the plate tune cap, the effect is
to
> >significantly raise the plate impedance (as seen by the Pi-Net input)
> >and the balance of the pi-net would need to be adjusted to maintain
> the desired Q & impedance ratio.
>
> Shouldn't that read: "the effect is to raise the load impedance
> presented to the tube"?
>
Hi Ian,
That is perhaps better stated. I was considering that the coupling
cap had become part of the network due its effect on the match.
Also depends whether you are looking at things from the load or source
end...
Congrats on keeping the "train-of-thought" on the tracks!
> As I'd understood it, the function of the pi-network is not to match
> the notional plate impedance of the tube, but to present the tube with
a
> defined load impedance (setting the slope of the load line for the
> tube, when the pi-net is terminated in 50 ohms).
>
> From that opposite viewpoint, the rest of Marv's analysis still
> looks good. The effect of a low-value blocking cap is indeed to raise
the
> load impedance presented to the tube. For example, if a pi-net for
1.8MHz
> is designed to present a 2.000k load impedance to the tube, a 1000pF
> blocking cap raises it to only 2.004k; but a 100pF cap raises it
> significantly, to 2.391k.
>
And it's more interesting when you anaylze before typing!
73 & Good morning,
Marv WCW
> --
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.com/g3sek
>
> --
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>
>
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