[Milsurplus] Lamp 991 re-fire

[Bruce Gentry]

John Hutchins wrote:
> Good evening group -
>
> Picked up a BC-348-O made by RCA.  Condition very dirty, just covered in 
> dirt.  Cleaned it up.....   And yes brought the power up to 227V over 2 
> days, initially set B+ at 10V for 12 hours.   Any how the 991 
> "regulator" lamp did not fire.  I measured the voltage at the OSC line 
> at 110V-DC.  Cleaned the contacts, inspected the bulb, looked in real 
> good physical shape.  The 991 still would not fire.  Then I remembered 
> that some one talkig about school where there electronics professor 
> would put Neon - Lamps into a plasma state by allowing high voltage and 
> current  to flow through them making them glow a deep purple blue.     
> Who ever that was a darn good Idea!  I got the Lamp to fire at 130V
> It started off red then made a sound like a light bulb makes when the 
> filament blows, a light metallic rattle.  The 991 works just fine.  So I 
> thank the person who related that old school time story,  It saved me 
> from ditching a perfectly good "regulator" lamp.
>
> I wonder why it was so hard to start the 991 after sitting so long?  Its 
> like the Neon had gone to a solid, had to be gassified.
>
> -Later-
> Hutch
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>   
 The neon will not condense or solidify unless cooled hundreds of 
degrees below zero. There are two likely reasons for the lamp behaving 
this way, and the remedy is to get it lit until it heats up for a while. 
First, the neon may enter the crystalline structure of the electrodes. 
It does not combine chemically because it is an inert gas.  Heat will 
drive it out and restore the gas pressure to the correct level. Second, 
the opposite effect will happen with other stray gasses interfering 
with  the operation of the lamp.  These gasses may enter through tiny 
imperfections in the glass and foul the neon. Once the lamp lights and 
heats up, these will combine chemically with the electrodes and be held 
there.  Neon signs undergo these effects over time if they are idle or 
leaky. Stray gasses eventually combine with the electrodes and interfere 
with contact to the inert gas. Because the sign tubing is too expensive 
to discard, the ends are cut off and ones with new electrodes are 
welded  to the tube, which is then  evacuated  and refilled. Does anyone 
remember the now rare "flicker flame" bulbs for decorative lighting?  
They used chosen electrode materials and gas pressure to give the effect. 

      Bruce Gentry