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  #16  
Old 10-08-2009, 02:29 PM
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Thanks, that is an interesting reference.

JR Tech brings up an important point about the second crossover. I'm starting to doubt the accuracy of the statement I quoted. It was written by the Dean of an Engineering College so I'm reluctant to say it's wrong.

Sticking certainly is a real phenomenom, but does it occur in non-aluminized production CRTs that are operated within their specified normal range. I've not been able to find much about the second crossover values for phosphors. Just that it can have a huge range for dielectrics.

John
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  #17  
Old 10-08-2009, 05:02 PM
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I can't find a good reference that shows the secondary yield curves for various phosphors, but as I recall, most exhibit second crossovers in the range of 8 to 15 kV. I suspect that most early non-aluminized crts were operated below the point that "sticking" could occur, so the description of the landing energy of the electrons being controlled by the internal conductive coating of the tube would be valid. Much above about 10kv all bets are off...aluminized screens were likely necessary to achieve full landing energy (minus a kv or 2 lost in penetrating the aluminum layer).

jr
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  #18  
Old 10-08-2009, 09:09 PM
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Speaking of aluminum, do you know how thick the coating is? This site seems to indicate 500 angstroms.

http://www.thevalvepage.com/teletech...u/crt_manu.htm
That would be thick enough to be opaque to light, but I'm not sure it would protect the phosphor from ions.

I assume the main ion species are singly charged hydrogen from gettering water and nitrogen which is more difficult to getter.

I've attached two simulations for hydrogen and nitrogen at 16KV on aluminum.

The left is for hydrogen and the right for nitrogen.

You can see that even 2,000 angstroms wouldn't protect agains hydrogen and 500 angstroms would barely protect against nitrogen.

I probably should have made some damage plots, but I think this shows the problem. My 40" rear projection set seems to be showing some ion damage on the blue CRT after about 20 years of use. It's a slightly dim blob in the center.

John
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File Type: jpg ion_aluminum.jpg (79.5 KB, 10 views)
File Type: jpg nitrogen_aluminum.jpg (138.3 KB, 7 views)
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  #19  
Old 10-08-2009, 10:53 PM
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Nice plots!

I think 500 angstroms would be really thin for crt use. If I remember correctly 800-1500 was used for lower voltage crts (10kv or so-early monochrome) and 3000-4000 for higher voltage color types. It would not surprise me to find 5000 or so in those 30-40kV projection tubes. Argon would be another likely residual gas, as it is not pumped by barium getters.

jr

Last edited by jr_tech; 10-09-2009 at 12:09 AM. Reason: add info
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  #20  
Old 10-09-2009, 02:13 AM
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wa2ise wa2ise is offline
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Quote:
Originally Posted by JBL GUY View Post
electrons traveling something like about 0.30 (About 1/3) the speed of light.

The greater the high voltage the faster the electrons will travel.
Heard it said that that is fast enough for Einstein's theory of relativity to make the electrons gain a little extra mass. This theory says something to the effect that as something made of regular matter tries to go near as fast as the speed of light, it gets more massive.

So there's some science happening in the boob tube...
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  #21  
Old 10-09-2009, 02:26 AM
JBL GUY JBL GUY is offline
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Quote:
Originally Posted by wa2ise View Post
Heard it said that that is fast enough for Einstein's theory of relativity to make the electrons gain a little extra mass. This theory says something to the effect that as something made of regular matter tries to go near as fast as the speed of light, it gets more massive.

So there's some science happening in the boob tube...
I think you must be correct...Because back in my college days (way long ago) I can remember watching the boob tube with friends and someone saying "Man was that ever heavy" Little did I know that he was referring to the mass of the stream of electrons drawing the picture on the phosphorus!

It is amazing the amount of knowledge that resides here at VK
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