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.
Sent from my iPhone
> On Jun 4, 2022, at 7:51 AM, Victor Rosenthal 4X6GP <firstname.lastname@example.org>
> 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.
> Victor, 4X6GP
> Rehovot, Israel
> CWops #5
> Formerly K2VCO
>> On 04/06/2022 15:23, Shane Youhouse wrote:
>> 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 <email@example.com
>> <mailto:firstname.lastname@example.org>> 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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