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dc vs ac vacuum tube filament winding for reliability/lifetime/durability?
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coppercone2:
I read the traditional debate of DC vs AC in vacuum tubes, but I have not seen any talk about the steady state field present as a result of DC through a heating coil.

Can it cause something with the offgassing/metal vaporization depositing in a peculiar way? I know that they need to be inrush controlled for the sake of strain in the metal or whatever it is exactly.
Gyro:
If you're talking about AC vs DC on the heater, then DC is heating is mostly audiophool - it is only relevant when very low levels of hum pick up are required - eg. Mic amps. Even then, tubes like the EF86 were constructed with bifilar-wound heaters so that AC could still be used.
coppercone2:
no I mean the dynamics of the gas and vaporizing of the electrodes in the presence of the DC field. Does contamination do anything funny.

 AC at 60Hz is pretty slow though, but it should still randomize it. Higher frequencies should effect it even less.

Like can the filament migrate towards one side (polarity of the DC) and short some shit out or increase contamination in a certain spot?
Circlotron:
Have a read up on vacuum tubes and design techniques for old timey undersea analog telephone repeaters. They had to run a squillion hours without quitting. Might be something relevant there.
LaserSteve:
For high current gas tubes, ie Ion Lasers, we use AC or equal duty cycle "spread spectrum" flipping DC  to distribute the 10 to 35 amp arc over the cathode. Otherwise the arc will etch the 75 to 125 watt (depending on model) directly heated cathode at one end weld. Some rare apps (mostly semiconductor inspection)  use the slowly flipped DC to avoid having a strong sixty Hertz  spectral line in the laser's amplitude spectrum.

Steve
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