[Amps] Why does power output increase?

[MU 4CX250B]

Victor, do you notice any change in the resting plate current? I don’t
recall the circuitry of the TL922, but it’s possible the operating
bias point has changed if there is a zener diode in the cathode bias
circuit. Another possibility is heating of a fixed capacitor in the L2
part of the tank circuit, causing the loading to change. I’ve seen
that happen on the lower frequency bands in one of my homebrew
amplifiers which uses fixed caps in parallel with the variable load
cap. THese are switched into the circuit on 80 and 160m. I understand
that it’s not a problem, but it’s definitely unusual (and therefore
interesting) behavior.
73,
Jim W8ZR

Sent from my iPad

> On Jun 4, 2022, at 3:35 AM, Victor Rosenthal 4X6GP <k2vco.vic at gmail.com> wrote:
>
> This isn't a problem, just a phenomenon that I would like to understand.
> I have a TL922. Mods include directly grounded grids. I also measured the filament voltage with my Fluke true RMS meter and put a small resistor in the primary of the filament transformer to provide exactly 5.0v in standby.
>
> It has been running with the same genuine Eimac 3-500Zs in it for about 6 years.
>
> So here is what happens: after a longish CW transmission, the power output INCREASES. On 20m, for example, it may go from 1200 to 1400 watts. When it cools, the power drops back.
>
> I've noticed this with several wattmeters. I have observed no change in SWR when this happens (it's 1:1 thanks to a Johnson KW Matchbox).
>
> It happens when the amp is driven with an Elecraft K3 and also a TS890s.
>
> I'm interested in hearing theories about why this might occur.
> --
> 73,
> Victor, 4X6GP
> Rehovot, Israel
> CWops #5
> Formerly K2VCO
> https://www.qsl.net/k2vco/
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