[AMPS] Another Stupid Question

[2]

>
>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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