[Amps] Why does power output increase?

[MU 4CX250B]

Victor, I believe low filament voltage is much worse than high
filament voltage, at least for tubes with indirectly heated cathodes.
I formed this belief after losing the tube in one of my HB amps which,
unfortunately, was running a nearly new Eimac 8877. I remember talking
to Bob Sutherland at Eimac  and he confirmed my belief.

My guess is that this is not as big an issue for tubes without a
separate cathode, like the 3-500Zs in your TL922, and I would be
pleased to be corrected by anyone who has experienced otherwise.
73,
Jim w8zr
Sent from my iPhone

> On Jun 4, 2022, at 7:51 AM, Victor Rosenthal 4X6GP <k2vco.vic at gmail.com> wrote:
>
> 1. I have often wondered whether the filament voltage should be measured under full power or in standby. I operate CW only, so even during a transmission, 50% of the time the voltage is at the key up value. And probably more than 90% of the time the amp is on, I am not transmitting. That means that the filament voltage would be high most of the time.
>
> I guess the question is whether the high filament voltage condition during standby would reduce tube life -- or if slightly low voltage in key-down would be worse. Another interesting question!
>
> 2. See my previous message.
>
> 73,
> Victor, 4X6GP
> Rehovot, Israel
> CWops #5
> Formerly K2VCO
> https://www.qsl.net/k2vco/
>> On 04/06/2022 15:23, Shane Youhouse wrote:
>> Vic,
>> 1.  You want 5.0 volts under full bore TX, not at receive.
>> 2.  I'd suspect the filament primary resistor.  If it's heating up and dropping resistance, more current will be allowed through.  This will cause the voltage on the filaments to rise su sequently increasing emissions from the tubes.
>> Is the resistor in the airflow to keep it cool?
>> --WP2ASS / ex KD6VXI
>> On Saturday, June 4, 2022, Victor Rosenthal 4X6GP <k2vco.vic at gmail.com <mailto: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/ <https://www.qsl.net/k2vco/>
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