Re: Filament Issue

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >
 > Hi Jim,
 >
 > On 1 Dec 2003, at 8:03, Tesla list wrote:
 >
 >  > Original poster: "Jim Mitchell" <electrontube-at-sbcglobal-dot-net>
 >  >
 >  > Hi, I was testing the voltage to the filament on my 813 tube in my
 >  > VTTC, and I noticed that it was like 4v, when the 813 has a 10v
 >  > filament!  I am wondering, is this a bad thing?  The filament does get
 >  > hot, and the VTTC works, but it doesn't work optimum, could low heater
 >  > voltage cause this?
 >
 > Most certainly. Running a thoriated filament tube with an
 > underpowered heater is an absolute no-no as it will destroy the
 > emissive properties in short order. I'm not sure about a pure
 > tungsten heater but nonetheless, the emission will be *way* down.
 >
 > Malcolm

	Pure tungsten filaments are not damaged by running at low voltage and
in spite of their hellish power requirements it's not common to use them
where tubes are operate space-charge limited.  As already mentioned,
thoriated filaments are a totally different story.  If the filament
voltage was really as low as you say you probably couldn't get enough
plate current to hurt.

Ed