> 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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|