From jlow–(at)–iwaay.net Thu Apr 18 12:11:04 CDT 1996
Article: 8663 of rec.audio.tubes
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From: jlow–(at)–iwaay.net (Joe Lowe)
Newsgroups: rec.audio.tubes
Subject: Re: Restoring power audio vacuum tubes
Date: 18 Apr 1996 15:52:27 GMT
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:Do someone know how to regenerate (weak, used) power audio vacuum
: tubes?
: Some time ago I read something about rejuvenating the barium
: oxide cathode coating but not remembering the specifics, tried 8V heater
: voltage between pins 2 and 7 (draws about 1 A. current) during 60-80
: seconds and left all other pins open. Got a nice orange glowing cathode,
: but did not seem to help much.

As far as I know the ONLY type of cathode that can be “regenerated” is
the directly heated thoriated tungsten _filament_ types. The theory is this:
The active thorium is dispersed throughout the filament wire and is
diffused to the surface at a rate that will keep up with the demand for
electrons. If the filament is operated at too low a voltage or overloaded
the surface layer of thorium will be depleted faster than it diffuses to the
surface.

The reactivation process starts by operating the filament at about 3X
normal voltage for a couple of min. this causes the thorium which is
bound with carbon in the filament to become an active emitter of
electrons, and cleans the surface of the filament. Next the filament is
operated at a slightly elevated voltage for a period of hours to allow
the thorium to diffuse to the surface. I have tried this and it does
work, however on extremely old tubes it will not last long, the
thorium eventually becomes depleted and the tube is worn out. This
process will however fix a tube damaged by a heavy overload, or that has
been operated at the wrong filament voltage.

Trying this with a coated cathode WILL NOT WORK, in fact it will damage
the cathode further.

My source for this information is _A Radio Physics Course_ by Gerabaldi,
and various Radio texts from the 30’s, 40’s, and 50’s

Joe