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[AMPS] Input matching experiment

To: <amps@contesting.com>
Subject: [AMPS] Input matching experiment
From: W8JI@contesting.com (Tom Rauch)
Date: Mon, 14 Jan 2002 06:45:23 -0500
> find a reference that the input impedance of a cathode-fed 811A / 812A
> is approx. 300 ohms.

812's have higher driving impedances than 811's, and less gain in 
GG. They also take high bias voltage.
 
> Has anybody ever tried using a broadband matching transformer, the
> kind used in solid-state amps? 

A matching system in a GG class B or AB amplifier serves three 
important functions:

1.) It matches impedances

2.) It provides a low impedance path to the chassis for tube 
cathode harmonics

3.) It prevents cathode harmonics from reaching the exciter and 
causing problems

You can't do that with a broadband transformer. It requires a low 
pass network with a low-reactance capacitor (compared to the 
tube's driving impedance) shunting the tube cathode to ground.



73, Tom W8JI
W8JI@contesting.com 

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