[Amps] Two questioins about tetrode amplifiers.

[Victor Rosenthal 4X6GP]

To do the test with the resistor and the VNA, you should leave the tube 
in place, since the output capacitance is important. Of course you must 
disconnect plate and screen supplies.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco

On 31/10/2024 8:37, flynth at gmail.com wrote:
> Hi,
> 
> Could someone please confirm the output impedance of a classic tetrode
> linear amplifier does'n change as plate voltage sags or swings?
> 
> I imagine it must be the same because no tetrode amp design I saw
> stabilised the plate voltage. Then we also have a tuning switch that drops
> plate voltage by half in a tetrode military amp many hams restore around
> here . If output impedance changed by a lot it would make no sense to tune
> with half the plate voltage
> 
> The second question is, has anyone figured out how to tune a tube amp
> matching network with a nanoVNA? My tube is supposed to have an output
> impedance of 2500ohms  so I tried removing it from the socket and replacing
> it with a 2500ohm non inductive resistor then I tried to probe the output
> port. I think this didn't show me the correct values.
> 
> 
> Many thanks,
> F.
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