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[AMPS] filament cutback

To: <amps@contesting.com>
Subject: [AMPS] filament cutback
From: jtml@lanl.gov (John Lyles)
Date: Thu, 21 Jun 2001 05:28:02 +0100
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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