🚀 go-pugleaf

	Here's a question for you net.analog.gurus out there, which
has us normally sane EE types freaked out:

	Consider your normal, low technology 40-wat two tube (four
pins per tube) fluorescent light.  This is the kind which says "Rapid
Start - Preheat" on the envelope.  We have such a beast which appears
to eat tubes; the tubes turn very black at one end.

	Replacing the ballast and checking the line voltage (which is
OK) has no effect; the fixture lunches a tube in about two days of
residential use.

	I was always under the impression that the ballast was some
kind of funky saturable reactor which, when all that nice magnetic
flux wasn't being used to sustain ionization in the tubes, allowed 
the filament current to increase to the level which causes ionization
(and once the tube ionizes, the heavy core saturation, distribution
of the windings, whatever caused filament voltage to fall back so
they last a long time.)

	I was wondering about the one in my plant light as well, which
uses a "starter" (which appears to be something like an Amperite time
delay relay...no, I'm not that old :-) :-) ) but whose filament seems
to burn only at **one** end when starting. (The rapid start lights 
start so quickly, you can't see the filaments heat up...). In the plant
light case, the tube turns black at one end as well, but it is very
finicky about the combination of lamp and starter (usually, several
packs of starters have to be tried every couple of years to find one
which makes the lamp happy...).

	Tubes with burned filaments at one end don't work or glow very
dimly (obviously)....

	Anyway, back to the subject.  The "Rapid Start-Preheat" 40 watt
tubes above my head have been in there about 6 years, operate 8 hr/day,
no problem.  The victim, who claims to have wired the ballast correctly
(and how could you screw this up?) says his "identical" fixture eats tubes
in a few days. What gives???

	And briefly, what is the ultimate limit on the tube life (I assume
lack of filament emission is fatal, but what about the tubes which have
only **two** pins??)  

	Is the structure in the tube just a heater at each end, or a heater
with a cathode tied to one pin (of some sort)....And, is the saturation
of the starter due to self-rectification (cold cathode a la 0Z3 tube) or
AC saturation.

	They didn't teach about these things when I went to college, so
please bear with my ignorance :-) :-) :-)

York David Anthony
DataSpan, INc