[Heath] Reactivation procedure for thoriated-tungsten filamented tubes.

[Kenneth G. Gordon]

FYI, here is the procedure I have used with success with properly-built 
thoriated-tungsten filamented tubes:

1) Apply 2.5 times normal filament voltage to the tube, with no other element 
voltages applied, for 1 minute. (For a 5 volt filament tube, this means 
applying 12.5 V)

2) Reduce filament voltage to 1.5 times normal filament voltage and hold for 
one hour. (Again, for a tube with a 5 volt filament, this means 7.5 volts)

3) Test. If the tube is not up to original emission capability. Repeat the above 
procedure.

4) Re-test. If the tube is still not up to original emission capability, discard.

What this procedure is supposed to do is to "boil" the thorium that is 
embedded "deep" within the filament wires to the surface, where it is 
deposited, then "baked" into the surface.

The thorium is normally only about 1 molecule thick anyway.

I have never had to repeat the procedure for any tube on which I have 
"operated".

If the tube was properly made to begin with, the first time through this 
procedure always brought the emission of the tube back up to normal.

As to how long this effect lasts, I have no idea, since all those tubes I have 
ever used it with lasted longer than I ever needed them.

Attempting this procedure with weak or flat Chinese-made "572Bs" resulted 
in complete disappearance of any remaining emission whatever. In other 
words, it simply completely burnt them out. Although the filament still lit up, 
very prettily, the emission was totally gone. I suspect from this that there is 
very little thorium in their filaments, and when it has been used up in normal 
use, all you have left is a complex light-bulb. 

YMMV. ;-)

vy 73,

Ken W7EKB