I believe the problem is metal migration - a well known phenomena in
semiconductors. There, in order to maintain life at 150 deg C chip temperature,
the maximum current density is about 2 to 4mA per square micron, depending on
the exact metal process. A 3-500Z filament is, from memory, around 0.75mm in
diameter, so the current density is way above 2mA per micron!
Using AC prevents metal migration. As someone else said, the military aren't
generally looking for extended life in tubes - maybe in big TWTs in radars, but
they'll be run on AC. Howver, an automatic polarity reversal circuit with a
latching relay isn't hard to do....
73
Peter G3RZP
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