[Amps] Thoriated tungsten lifetime when idling tspa

[John T. M. Lyles]

Larry and all,

Eimac typically says 150% of normal current, I believe, for TT 
filaments. Most big toobes use either a ramped up variac or power 
supply, or a current limited transformer that has a gap in the core, 
that will limit the current to 150% or less on turn on, when the 
filament resistance is low (and cold).

My RCA 7835 triodes run 6800 amps on the filaments, at 4.6 VDC. Tube 
has 96 radially aligned 'unit' electron gun assemblies which each run 
70.8 amps through a single TT bar. The filament power cables to the 
socket are the size of fire hoses and have water cooling through the 
center of the cables. We have >150 GPM of plate cooling water 
flowing, hoses are again large. Yes, we like big, um, toobes.

>For an extreme case take a look at the RCA 6949. The MAXIMUM ALLOWABLE
>inrush surge current is spec'ed at only 165% of the nominal operating
>current!
>
>You guys that like big jugs (toobes, not boobs) and water cooling might
>like this 6949. It requires a minimum of 80 GALLONS PER MINUTE of
>DISTILLED water through the anode cooler, 7 GPM  through the beam
>forming cylinder, and 1 GPM through the socket grid connection!

I have never heard that story that running filaments alone without HV 
will shorten emission life. However, I might believe that you could 
contaminate the grid with thorium compounds from the filament running 
if you never run any heat and no beam. In which case the grid might 
become an electron emitter and have some funky V/I characteristics, 
until it is burnt off. However, I don't have much to back that 
assumption up either, except that we have some glass tubes (triodes 
made by machelett, no longer made) which get to high leakage (anode 
current when it should be cutoff) after running in our systems for 
years. They are extremely overrated so that the duty factor of the 
pulsed anode current is low, not a lot of dissipation among the 
elements. Wwhat we do is bombard the grid by drawing higher grid 
current in a fixture, heating it up, and driving off the coating on 
the grid. The tube then functions normally again. We have managed to 
keep these tubes running for at least 15 years, with about 8 
months/year of 24/7 operation on them. This seems to be a plausable 
explanation for tubes which don't produce after idling for so long. 
Emission loss though, I would love to hear the theory behind that and 
some more examples besides the RF heating app you quoted. Unless they 
had the filament set too high. If the tubes are not drawing current, 
(anode) then they should be backed off on filament to get even longer 
life during this time. Thales and other tube companies call this 
"dark heat" standby mode.

73
John
K5PRO

>Back to filament life, when I was still working there was a RF heating
>unit buried deep in a classified area. I only knew about it because a
>pair of 3CX1000's would show up on my buddies desk about once a year. He
>said they would go bad because the filaments were left running all the
>time and anode power was only applied when it was needed. I got a pair
>and checked them and sure enough, NO EMISSION! Seems I've seen this
>written up by Eimac or some other manufacturer. Running thoriated
>tungsten filaments without anode current shortens the life time faster
>than on-off cycling. Any comments on this failure mode?
>
>73,
>
>Larry - W7IUV
>DN07dg
>http://w7iuv.com
>