[Amps] Why does power output increase?

[Jim W7RY]

W8JI has some filament voltage measurements for the AL80B on his web 
site and how he measured it under load.

73. Jim W7RY


On 6/4/2022 8:51 AM, Victor Rosenthal 4X6GP 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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