[Amps] Why does power output increase?

[Jim W7RY]

No fixed loading caps in a TL-922 OUTPUT on 20 meters... The tuned input 
is another matter though..

73, Jim W7RY


On 6/4/2022 7:03 AM, MU 4CX250B wrote:
> 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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