[Moon-net] ALL, Input impedance of a tube

[Chris Bartram]

Kim

> I naturally ment, the the tubes input / grid  will get more capacitive as F
> rises.
>
> But in general below at least 500 MHz, the input L/C part will be "eat "
> / parallel to the surrounding components.
> SO a 1 GHz tube does not to be taken into account vy much at such a rather
> low freq.

The GI7B is usually operated as a grounded grid tube, (I don't know about the 
GS35, as I gave-up using tube PAs many years ago - except on 3cm!) so 
classically the input impedance is the reciprocal of the transconductance 
shunted by whatever the reactive part is at the frequency of interest - it 
doesn't have to be capacitive, particularly at higher frequencies...

Common cathode amplifiers aren't so easy, either. There's the little matter of 
the Miller effect altering the input impedance, and that depends on the 
reverse transfer characteristic of the tube. While many of the multigridded 
tubes in use by amateurs do have quite small reverse characteristics, it's 
very wrong to assume that the grid circuit of even a well neutralised tube 
can appear as a purely capacitive load.

There are good text books which will explain this much more fully than I could 
here.

Vy 73

Chris
GW4DGU