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"?
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.
--
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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