[Amps] Filament voltages

[Tom W8JI]

> Lastly, Eimac publishes a technical bulletin for 
> maximizing tube life
> through the measurement and control of filament voltage 
> whereby
> emission is used as an indicator, with the tube heater set 
> at just the
> point where the "knee" in emission increases, and then 
> just under,
> maximizing the tube life, and further, Eimac has a plan 
> whereby this
> adjustment can be brought to bear in the last hours of 
> service, both
> indicating
> a need to start the purchase process for replacement and 
> also adding
> many hours of useful life.

Hal,

I think you are mixing models or applications.

I think you are considering things that work for thoriated 
tungsten tubes operating at ONE constant power level (like 
an FM broadcast transmitter) with another totally different 
application.

1.) A metal oxide cathode tube will be poisoned if operated 
where peak emission demand reaches available cathode 
emission. We NEVER want to run a metal oxide cathode tube 
anywhere near peak emission limits, and that's exactly what 
we would be doing by adjusting for a power drop and then 
adjusting voltage a small amount higher. That is a very bad 
idea and very bad advice.

2.) In amateur service peak cathode current varies from band 
to band and with drive power and other variables. You would 
have to set peak emission under the worse case conditions 
for all bands, modes, voltages, and tuning conditions. If 
you set it too low with a thoriated tungsten cathode tube 
you just lose IM performance and efficiency. If you do it 
with an indirectly heated cathode like the 8877, you 
splatter and eventually will lose the whole tube by 
poisoning the cathode.

I've replaced several low emission 3CX1500A7's and 
3CX800A7's, and every single one was in amplifiers that had 
life-extending reduced filament voltage! It's a dumb idea to 
reduce voltage on an indirectly heated cathode power tube to 
just above the point when output power drops. It's just a 
useless waste of time, but not harmful, to monitor voltage 
to 50mV when the failure mode in amateur service is 
virtually never emission life.

73 Tom