tube heating : was Re: Spring Line Hum

[danial stocks]

>I read somewhere that DC heating of tube causes filament reliability
>problems (migration?). Since you used that I guess that this statement is
>not true? You have certainly a lot of experience to share...
>

Tube DC heater...
The heater side that connects to the cathode must be the + side.. the heater 
supply is thus a -ve vtg..[ if the heater supp is totally isolated fron the 
rest of the cct then it probably wont make much diff..] if you do it the 
other way around electron bombardment of the heater destroys the insulation 
eventually.. Klystrons and other radar tubes which run DC heaters to keep 
the noise down emphasize this quite heavily.. of course, if the tube is a 
real cheapie like a 12ax7 etc, maybe the convenience of not having to make 
elaborate heater supplies is worth accepting the reduced life.. I dont know 
how much it is reduced.. it certainly doesn't blow the tube overnignt..
Cheers,
Dan
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