Common items you passed up that turned rare when you wanted them - test-drb@ccmp.vtda.org

31 Jan 2006

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 From: ard at p850ug1.demon.co.uk

 Hi
  No, even the tube may be revived some. You can
 often run the heater at a higher voltage for some
 time and check the emission periodically. Usually,
 you can safely run the filament at 1.5 to 2 times
 the rated voltage for 5 or 6 hours. It is usually 
The method generally used on TV tubes of the period (and I don't see why 
monitor tubes would be any different) was something like : 

1) Over-run the heater. Typically a 6.3V heater would be run at 8V or 10V

2) Apply a fairly high (200V-ish) +ve voltage the control grid (wrt the 
cathode), all other electrodes floating (no EHT applied either).  
Hi
 I've had better luck with 0 volts. If the tube has not been used
for a long time, it will be a little gassy. This tends to boil
off the cathode along with the other stuff. When you have the voltage,
the electrons will hit the gass atoms and ionize them. These are
then slammed back into the cathode. One article I read stated
that the first stage should be done with 0 volts and then switch
to a + volts on the grid(s)+anode for the last part.
 I've not tried this myself but the method makes sense. In the
first part, the getter has time to catch the outgassing. The
last part helps to freshen the surface.
 I know that there are many articles that state to put the voltage
on from the beginning. I've also read in a only a couple of
articles that the outgassing from the initial stages can poison
the cathode enough to make the process useless. I recover
many old vacuum tubes for my old battery radios. I've had
good enough experience with the 0 volts and enough bad
experience with the voltage on the grid/plate that I use
the 0 volt.
Dwight

...

This, combined with the overheated cathode, stripped the top layer of 
emissive stuff off the cathode, hopefully exposing a fresh surface. It 
worked will on some CRTs, on others, it totally removed the emissive 
material and you ended up with a totally useless CRT. But you've nothing 
to lose by trying it on an otherwise useless tube

There was also a simple way to do an emission test. Again the only 
electrodes you used were the heater, cathode, and control grid. I think 
you ran the heater at the normal voltage and used the cathode/grid as a 
diode and measured the current flow with a fairly low (if any) +ve 
voltage on the grid. The higher the current, the better the cathode was 
emitting.

Many designs for such devices have been published in the magazines. The 
lethal ones get the 300V or so by rectifying the mains (or in your case 
using a voltage doubler on the mains). More friendly ones used an 
isolating transformer or a oscillator/step up transformer from a battery.

-tony