I remember reading the same test about the ac on filaments. I recall the
test was run because the lamp manufacturer specified a long life bulb and an
aircraft manufacturer installed them and found substantially less life in
the aircraft than specified. The lamp manufacturer ran the test, and
changed the long life specifications to ac.
There are some examples of dc on filaments, however. Almost all of the WW
II aircraft radio equipment ran on dc - and many are still running (often on
ac, however). However, the war was relatively short while amateurs expect
their tubes to last 30 or 40 years. Radios from the 20s all ran on
batteries, and many of those tubes are still running.
Some say that if you run the tube on dc, you should alternate the polarity
of the filament every so often, to offset the migration problem.
There are other things that may apply to the equation, however. DC is
easier to regulate, and may be easier to ramp up for a controlled start.
Also, the more you otherwise abuse the tube, the less important the filament
is. For example, if you overdrive it, under-cool it or overload it, the
filament source becomes less important - because the tube is less likely to
fail because of filament failure and more likely to fail for other causes.
73, Colin K7FM
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