Home Tubes Asylum

Questions about tubes and gear that glows. FAQ

Re: You know, that glass . . .

Regarding the glass, I'm no expert on this, but clearly there were lots of variations and factors. There's a whole chapter on glass as used for tubes in the 1962 RCA Electron Tube Design volume. Most receiving tubes used relatively softer glasses, defined as working temperatures during manufacture <1000 deg C. Important aspects include sufficient thermal expansion coefficient to work with the chosen lead out wires (rather than expand differentially and leak when the tube gets hot), also electrical resistivity. We think of glass as an insulator, but some are more so than others. When electrical conduction occurs in glass, as it can between lead outs/pins connected to plates and other elements, for example, it changes the actual structure of the glass, reducing the mechanical integrity. Such electrolysis is also highly temperature dependent. I believe electrolyisis and the migration of sodium ions in particular in interaction with lead compounds in the glass is one reason the bases of glass miniatures can gradually turn dark grey with extensive use, like some people look out for in e-bay "NOS" 12AX7s.

Bendix touted that the glass envelopes of some of their missle-proof Red Bank tubes were routinely tested by surviving 3 mins in boiling water immediately followed by three minutes dunking in ice water!

Jeff W.


This post is made possible by the generous support of people like you and our sponsors:
  Parts Connexion  


Follow Ups Full Thread
Follow Ups


You can not post to an archived thread.