While reading up on tungsten filament life, I came across this interesting
article. This was written for ordinary lamps with tungsten filaments but I
imagine the recommendations may be applicable for power tubes using thoriated
tungsten filaments also? The interesting part was shortened filament life using
DC
current. I had thought about using a regulated DC power supply voltage for
power tube filaments like the 3-500Z but after reading this it may not be such
a
good idea? Also an interesting recommendation to run a small voltage to keep
the filaments barely warm when switched off. I think some other people here
have recommended against using DC on tungsten-based power tube filaments? 73
Todd
WD4NGG
Miniature and subminiature incandescent lamps are widely used as status
indicators and to illuminate tuning dials and meters. They operate in a
self-destructive mode that causes failure over time. Furthermore, their
filaments can be
prematurely damaged by vibration and by cold filament shock when they are
switched on. However, there are several ways to extend their life. For example,
running them even slightly below their rated voltage significantly extends
their
life expectancy and makes them much less susceptible to vibration damage.
Applying a very small voltage (less than one-percent of rated voltage) while
they
are switched-off keeps filaments slightly warm and greatly reduces cold
filament shock when they are switched on. Incandescent lamps last approximately
twice as long running on AC, rather than DC, because DC accelerates grain
growth
in tungsten filaments that causes earlier development of hot spots.
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