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[AMPS] Input matching experiment

To: <amps@contesting.com>
Subject: [AMPS] Input matching experiment
From: W8JI@contesting.com (Tom Rauch)
Date: Fri, 18 Jan 2002 20:08:41 -0500
> In reference to the subject matter. I have an article here written by
> a ham in spain and published by Svetlana several years ago. This
> gentleman built an amp, single 572B, using a home made rf choke and
> input transformer combined, and using a large swamping resistor ,
> about  75 ohms at 40 watts, untuned input..

With low enough swamping resistance it will certainly work OK. 
The swamping resistance simply has to be a fraction of the lowest 
expected impedance of the load throughout each portion of the RF 
cycle. That way non-linearity over the RF cycle and harmonics are 
minimized.

There also are cases where you can "get away" without a tuned 
input or any special considerations in an amplifier drawing grid 
current or a less than 360-degree conduction angle amplifier, but 
they are specific to a particular set of circumstances such as 
exciter type and cable lengths. The problem is such systems are 
not reliably reproduced. 

If you want to build a linear GG amplifier (or grid driven amp that 
draws grid current) with near optimum IMD performance, you either 
better swamp the heck out of the input or use a proper tuned input.

Eimac and others aren't just joking when they suggest a tuned 
input or swamping resistance as a good design suggestion. There 
is a valid technical reason for their suggestions.
73, Tom W8JI
W8JI@contesting.com 

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