[AMPS] Another Stupid Question

[Steve Thompson]

Sorry, attribution marks up the spout again. Good old MS.
-----Original Message-----
From: Ian White, G3SEK <G3SEK@ifwtech.com>
To: amps@contesting.com <amps@contesting.com>
Date: 03 December 2001 11:13
Subject: Re: [AMPS] Another Stupid Question



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).


Steve: I'd agree that's the better way of looking at how it works.

Ian:
>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.


Steve: Can't find a Smith chart do do the doodles, but in a mental picture I
think a low value cap also raises the loaded Q of the network for any given
anode load.

Steve



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