In nearly every gridded tube we use, we practice filament voltage
cutback to improve lifetime. I have to contradict the advice that
only nonlinear class C FM amplifiers with overkill tubes should do
this. In all but one case, I am talking about Thoriated Tungsten
tubes rated from 500 W to 250 kW dissipation, but they are also used
in linear class AB 1 or 2 service. BUT I am not dealing with
communications, where 2 tone IMD and such is a problem. I am talking
about single frequency driven systems, with pulse modulation. We have
increased lifetimes typically 50% or more. In some cases the
manufacturer will give us the recommended filament voltage for the
loadline we are running (i.e., THALES/Thomson doesn't always spec
typical filament voltage on big tetrode datasheets). What this does
is give us more emission reserve in the tube to use when it drops off
after time, and we can dial up the voltage a tad and squeeze more out
of it until the replacement is installed.
In the BURLE 4616 tetrode with a painted oxide heater (on small
bars), we only cutback a small amount to prevent fatigue failure due
to crystal growth in the Hastalloy B material. Its not a great
situation, but sometimes there are compromises in tube designs to get
extreme power or gain or frequency. We still observe better life with
reduced voltage, but it is not an emission issue. In the 4616 the
manuf only spec a maximum voltage that you should not exceed, but
nearly every user cuts them back for longer life.
john
K5PRO
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