[Amps] Thoriated tungsten lifetime when idling tspa

[Tom W8JI]

> Eimac typically says 150% of normal current, I believe, 
> for TT
> filaments

They say twice for most tubes we deal with. The exact text 
says limiting current to two times the operating value helps 
reduce damage (movement) from repeated filament thermal 
cycles.

> I have never heard that story that running filaments alone 
> without HV
> will shorten emission life.

I've never seen a warning like that either.

As a side note, when I was working closely with Eimac Salt 
Lake in the 80's, I specifically asked the engineers about 
problems like inrush.

Buzz Miklos, WA4GPM, was head engineer for the power grid 
group at Salt Lake. Buzz said they clamped a 3-500Z directly 
to the terminals of a 200 amp transformer and despite 
hundreds of cycles they could never measure any damage.

The thing  everyone has to remember is reducing long term 
inrush current stops uneven heating where one area of the 
filament heats much hotter than normal and then settles back 
to a normal temperature. The hot vs. cold stresses are still 
there and are a matter of how often the temperature cycles 
between extremes.

There are TWO ways to extend the life.

1.) You don't turn the thing off and on needlessly.
 2.) You do a filament circuit that limits extended warm-up 
current to twice the rated current or less.

Neither one excuses or eliminates the other problem, and the 
guideline is two times for most Eimac power grid tubes 
likely to be found in a Ham shack...even 3 by 3's.

73 Tom