I actually have played with a few old VFDs. I didn't make any real
measurements, but I pushed the plate to the full 400v of my power
supply with no ill effects. The only limiting factor was power
dissipation. When the tube got hot enough, the glass cracked. With
enough negative grid bias it was happy to sit there all day with 400v
on the plate. I was surprised it didn't arc with that much of an
overvoltage.
Andy Cuffe

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