[Amps] Two questioins about tetrode amplifiers.

[Steve Thompson]

Output impedance is something that's rarely quantified for high power RF
amps. That said, for a tetrode running well clear of saturation, plate
voltage supply changes have a small effect on output power.

However, that's not how amps are typically used by hams. The plate load
is adjusted to use all of the voltage swing the PSU can support together
with all the current the PSU or tube can handle. In that situation,
changing the plate voltage can make a noticeable difference to output power.

In the amp you mention, I'd guess that the low voltage tuning position
is to protect the tube and matching network from excessive voltages and
dissipation during the tuning process.

Steve G8GSQ

On 31/10/2024 06:37, 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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