[Amps] Interesting Article On Tungsten Filament Life - AC is Best?

[ToddRoberts2001@aol.com]

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.