[Milsurplus] Smart People: Gassy tubes , again

[hwhall at compuserve.com]

I thought Ralph's reference to the 10 percent boost applied to tubes in general. I did know that strongly heating a filament was reputed to drive off accumulated contamination in some cases. But what that had to do with helping the getter, I didn't 'get'. Or was the point perhaps that by releasing it from the cathode the getter was able to 'get' at it & was helped in that way. (Sorry for the puns, couldn't help myself.) 

One reads a lot of reports about how to rejuvenate tube cathodes, but the ones that seem most credible are the ones that take the different cathode chemistries into account. Though cathodes get poisoned by gas, perhaps I ought not stray so far from the original issue of gas removal into a new (yet old) cathode rejuvenation topic. 
 

 Wayne



-----Original Message-----
From: J. Forster <jfor at quikus.com>
To: hwhall <hwhall at compuserve.com>
Cc: ramcam <ramcam at magma.ca>; milsurplus <milsurplus at mailman.qth.net>
Sent: Tue, Dec 3, 2013 3:12 pm
Subject: Re: [Milsurplus] Smart People: Gassy tubes , again

>     I remember those, but as I remember them, the rejuvenators were 
> installed and left in place for the remaining life of the picture 
> tube. It's easy to see how the higher temperature can comepensate for 
> a cathode's waning emission ability. 
 
Yes. 
 
> I don't understand how running the filament at boosted voltage for only 
> 20 mins affects the getter function. Is it because the getter flash 
> material gets hotter? If so, would longer times also make a greater 
> difference for tubes with more gas? 
 
No. A picture tube has a large surface area. The amount of gas that 
diffuses in is proportional to surface area, all other things being equal. 
 
Now, when a gas atom is struck by an electron, it acquires a (+)positive 
charge, so it flees the plate and moves toward the cathode. When it 
impacts the cathode, it 'poison's' the cathode. An increased voltage for a 
short time, 'boils' off this poisoning to some extent, cleaning the 
cathode and improving emission. 
 
This is why I suggested running a tube at 10% to 20% of rated current. If 
you degas at higher Ip, the cathode gets poisoned and Gm goes down. 
 
BTW, I -think- that the TV rejuvenators are used with the electron gun 
biased off, but am not cert6ain. 
 
FWIW, 
 
-JOhn 
 
=================. 
 
 
 
> 
>    Wayne 
> WB4OGM 
> 
> 
> 
>  -----Original Message----- 
>  From: Ralph Cameron <ramcam at magma.ca> 
>  To: milsurplus <milsurplus at mailman.qth.net> 
>  Sent: Tue, Dec 3, 2013 9:18 am 
>  Subject: [Milsurplus] Smart People: Gassy tubes , again 
> 
> 
> 
> 
>  One may remember the trick of restoring CRTs that had lost emission due 
>  to filament contamination. Some of the so called "CRT" rejuvenators 
>  boosted the filament voltage with directly heated cathodes to burn off 
>  some of the accumulated "junk" in the vacuum. Most of this junk was 
>  metal contaminants. A 10% boost in filament voltage for 15-20 mins may 
>  help the getter. 
> 
>  Ralph 
>  VE3BBM