[AMPS] Cathode Impedance Measurements

[km1h@juno.com]

On Mon, 29 Mar 1999 09:28:16 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>
>In Class A, a g-g amp has an input impedance of 1/gm. What happens in 
>Class
>AB or B is another matter. Since the gain is approximately the ratio 
>of
>cathode volts to plate volts (because plate current is common to both
>circuits) I surmise the input impedance can be determined from   
>Pin/(Ek
>squared). That means settling down with the tube curves and working 
>out the
>volts and amps over the cycle. 
>
>Using the resistors is fine if you know the value to shoot for - as 
>Jon did.
>If you don't, then the whole thing is probably more miss than hit. 
>It's also
>complicated because you're relying on the tank circuit Q to keep it 
>free
>wheeling, while for best linearity, you really want to make the feed
>impedance very high - see Pappenfus et al.

Another problem area is that the MFJ-259B manual recommends a resistor "
equaling the approximate driving impedance of each individual tube is
installed between the cathode of each tube and the chassis".

This can be confusing to many since it does not give the means to
determine the value. And the subject is not very visible in ham books
either.


73  Carl  KM1H


>
>73
>
>Peter G3RZP
>
>
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