3-500's

[Bob And Bettina Groh]

I've been watching the exchange regarding filament voltage on RF power 
tubes and thought I might chime in with a few thoughts.  But first I 
would like to share some web sites and information that everyone might 
find interesting (all dug up by googling on 'vacuum tube filament life'):

<http://www.burle.com/cgi-bin/byteserver.pl/pdf/tp202.pdf>
TECHNIQUES TO EXTEND THE SERVICE LIFE OF HIGH POWER VACUUM TUBES
Walter Johnson, Voice of America, Technical Material Support Division.
Very nicely written paper covering filament inrush, turning filaments 
on/off, minimum filament voltage, etc.

<http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm>
Covers a wide variety of topics on tubes including filament voltage 
considerations

<http://www.transfixr.com/strafe_tube.htm>
Another interesting bit of information on tubes.

My take on a very quick reading of all of this:
1.    Running the filaments continuously is not a problem although you 
may have to watch the seal temperatures on the tubes.
2.    Decreasing the filament voltage will net large increases in 
filament lifetime. But there are negatives to this that need to be 
watched carefully.
3.    Increasing filament voltage above the manufacturer's rated levels 
is very bad.
4.    Current inrush on turn on is bad.

Makes me think I might look at this now that I am finally retired from 
electrical engineering!  A regulated filament supply with inrush current 
protection and 'orange' filaments during standby.  Interesting.

73
Bob Groh, WA2CKY

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