[AMRadio] 813 filament voltage


Donald Chester k4kyv at hotmail.com
Mon Nov 6 17:52:55 EST 2006


Running a thoriated tungsten filament tube at too low filament voltage wil 
shorten the life of the tube more than running it at too high voltage.  The 
manufacturers say keep it within +/- 5% for optimum tube life.

It can safely be run at slightly reduced voltage as long as there is NO 
reduction in the peak emission during normal operating conditions.  If the 
filament voltage is reduced enough to starve the electron stream, this will 
destroy the molecules-thick coating of thorium on the surface of the 
tungsten filament and the tube will  go flat.  In an effort to prolong tube 
life, some broadcast engineers reduce voltage until a reduction in peak 
output is just barely noticed, then the voltage is slightly increased beyond 
the point that restores full peak emission.  But this requires very careful 
monitoring of both filament voltage and peak output.

A weak tube may be brought brought back up to full output for a limited 
period by increasing filament voltage above normal.  This is not recommended 
for a good tube.

RCA says it's ok to reduce the filament voltage to 80% normal during 
STAND-BY periods (zero cathode current).

Don k4kyv

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