[Amps] DC On tube filaments

[wl fuqua]

Filaments generally loose emission long before they thin out and burn out.
Also, in comparison to light bulbs they run a good deal cooler.
Semiconductors, to function, require a crystal lattice structure. So it 
would not take much
shifting of atoms in the lattice to make them nonfunctional. Also, your 
dimensions seem
to be incorrect.Current density (amps per square units)??

73
Bill wa4lav

At 08:31 AM 10/14/02 +0100, Peter Chadwick wrote:
>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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