[Amps] Filament voltages

[Tom W8JI]

> 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