[Milsurplus] How Much Vacuum in "Vacuum Tubes?"

[J Forster]

From:    "David Stinson" <arc5 at ix.netcom.com>

  "Vacuum" is a relative term.
  Just how much "vacuum" does one need to make a vacuum tube work?
  Our valves can't be at perfect vacuum; no way that thin glass
  would hold up.

Not really so. The pressure the glass is required to withstand is only 1
atmosphere....  roughly 14.7 psiA That's all the pressure on the tube,
even if there were zero gas molecules inside the tube. The issues are
seal leakage and diffusion through the glass.

Seal leakage where glass meets metal can be an issue if tubes were not
designed or made properly. Many tubes with seal leakage are likely to
fail pretty quickly...  say within a few months after manufacture
whether in storage or use.

Glass is porous at some level, especially to Helium and there is He in
the atmosphere. I've seen tubes destroyed in days in a He atmosphere.
Most other gasses take very much longer.

  So is it possible for an amateur to create a
  vacuum pump and chamber good enough for tubes?

Using surplus stuff, yes. You'd need a roughing/backing pump and a
diffusion, turbo, or cryo pump. Also glassblowing equipment and skill
and a way to bake the tubes out and replace the getter. Doable, yes,
easy NOT! It's only worth it for very big, expensive tubes.

  Kinda thinking about how to recover rare or expensive
  tubes that have gone gassy or low-em.

If the getters are good and the tube gassy, run them with normal
filament power and about 20% of full load plate current for a day or
so.  I've done it hundreds of times and it works about 80% of the time.
Low emission may not be fixable.

-John


  I have engraved-base, big-globe types 80 and 50
  that would look soooo good in a classic '20s homebrew,
  if only.....