> Since I had some time here Sunday morning I did some
> looking into the matter
> of "filament voltage" concerning these 2 tubes. This was
> an issue that
> someone had brought up. Here is what I found, a major
> University built a plasma
> pulsing generator using 6 parallel 3CPX1500A7 tubes. The
> filament voltage is
> rated at 5.5 volts on this tube. A regular 3CX1500A7
> tube is rated at 5
> volts. The University project used 5 voltsof filament
> voltage for "longer tube
> life". This leads me to believe that if a 3CPX1500A7 is
> used in place of a
> 3CX1500A7 with filament voltage at 5 volts, that the
> filament may in fact have
> an expected extended life when run at 5 volts vs. 5.5
> volts. Could there
> then be an advantage of running the 3CPX1500A7 tube in
> this regard? The
> University apparently thinks so. We might be splitting
> hairs, I do not know, but
> if a 3CX1500A7 is run at 4.5 volts Half a volt less than
> rated, is there any
> reduction in emission?
Lou,
I'm really pretty sure, but not absolutely positive, the
heater and cathode of the 3CPX1500 is the same as the
3CX1500. Eimac jacks the filament voltage up to the upper
limit to boil more electrons off.
A cathode tube like this demands the electron cloud near the
cathode NEVER be brought to zero. That cloud shields the
cathode from contamination. If you draw enough current from
the other elements to strip the electron cloud away the
cathode will deteriorate. It is a much worse condition than
running excessive voltage.
Many amateurs encourage needless concern about filament
voltage. They generally point to the correct fact that large
thoriated tungsten tubes used in broadcast service have
shorter emission life as filament voltage is increased, and
somehow tie that into amateur service.
The truth is a tube run very conservatively for almost 9000
hours a year with little or no thermal cycling is an
entirely different situation than amateur service. In
amateur service nearly all tube failures are totally
unrelated to emission loss. In amateur service worrying
about a tube's emission lasting 5,000 hours is almost
meaningless, let alone trying to milk one out to 30,000
hours. If someone operated 20 hours a week with an amplifier
on it would only be 1000 hours a year!
The fact is inside that 1000 hours would be hundreds of
filament off-on cycles, periods of overload in tuning, tube
temperature cycles dozens of times each day, and many other
factors. Couple that with poorly manufactured and mishandled
and abused tubes and we have the cold hard fact that nearly
every failure has nothing to do with filament operating
voltage.
We are much safer operating the filaments of indirectly
heated tubes within the published values than we are
reducing voltage. If we run a metal oxide cathode at the
point just above where it shows power loss at maximum peak
power we almost certainly INCREASE the risk of damage.
In all the years I've been doing this the single most common
loss of emission I have seen in 3CX800A7's and 3CX1500A7's
has been where the user reduced filament voltage or drew
current from the tube before the cathode was fully up.
Reducing filament voltage near to or below minimum rated
values is at best useless and at worse stupid....and we
should stay away from it in amateur service.
73 Tom
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