>
>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"?
>
Yes, however the resistance presented to the electron-tube is the main
game. Reactance can be tweaked with C-tune and C-load.
>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.
>
// Which is no problem when C-tune and C-load are adjustable.
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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