[AMPS] Another Stupid Question

[Ian White, G3SEK]

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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