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wrote:
Doug, a suggestion about "Longevity" for 4-400's (Actually, any
thoriated-tungsten filament power tubes):

These tubes are very sensitive to filament voltage. Make sure the tube
pins and socket are nice and clean and shiny (don't use any
abrasives!).. Measure filament voltage at the socket. This is all to
make sure the tubes REALLY get the correct filament voltage to the
filament... It's 28 Amps for two tubes!

Actually in amateur service filament voltage is not that big a worry. I
think the excessive worry comes as a carryover from BC service where
tubes are operated far below ratings and can be operated for almost
9000 hours per year.

In BC service, excessive voltage shows up in a noticable period of time
since the tubes are operated 24/7. The voltages and currents of other
elements are a much lower percentage of maximum compared to amateur
service, and filaments are not cycled. As such, emission failures are a
common failure mechanism.

In amateur service, it takes many years and thousands of fialment
cycles to accumalate the hours a BC tube sees in one year. Tubes in
amateur service much more often fail from seal leakage, element
structure defects, and occasional abuse.

Out of hundreds of tubes I've seen fail in amateur service over the
past 30 years, I can count the number of emission failures on my two
hands.

I'm not saying filaments should be run extra hot, but I am trying to
point out what worries BC stations is often not a concern in amateur
use.

By the way, two of the emission failures I saw were in 8877 tubes that
were operated too cold. The owners had reduced filament voltage below
4.75 volts. NEVER run a MOX cathode tube at reduced voltage unless you
also plan to limit peak emission current.

My advice is to run all tubes in amateur service at RATED filament
voltage or in the allowable tolerance range for voltage.

As for cooling, seal cooling is critical in glass tubes. So is periodic
operation to keep the tubes gettered.

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm

73 Tom

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