|[AMRadio] 813 filament voltage|
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).
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