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#1
05-05-2011, 10:51 PM
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So the greenish (early version) would be the 21AXP22 and the later version (whitish) would be 21AXP22A? This has been eluded to before but it would be good to clear that up.
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Bryan 
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#2
05-05-2011, 11:33 PM
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Quote:
I've seen 21AXP22 and 21AXP22A tubes that are grey faced, the only difference noted in the data sheets being the addition of a resistive inner coating for arcing protection for the 'A' version. As pointed out by yoda, the early 21AXP22 tubes with greenish phosphor colors on their face are likely related to the 15GP22. It's thought to be the result of RCA wanting the 21" version of set (21-CT-55) to be just like the earlier 15" one (CT-100), though no definitive proof exists to my knowledge. It's all conjecture by the collecting community at this point. Furthermore, the change of phosphor was likely related to light output. As I understand it, customers complained of dim tubes so RCA set out to make some improvement. Perhaps this is why early AXP's are green like the 15GP22, while later ones are grey? Changes like this continued throughout the roudie's life, eventually leading to the very bright FJP/FBJ line of picture tubes. While they did not produce colors as accurately as the early ones, obvisouly customers were willing to trade off some loss in color fidelity for a bump in brightness. This also explains the changes in circuitry, since there's no point in decoding the NTSC signal exactly if you're not going to reproduce the picture on a tube that gives 'true' colors. Cost was also a big factor in those times, so it's a real chicken vs egg scenario. In any event, some day I'd like to get my hands on the instrument used to make such measurements. That way I can test all versions of tube, in order to figure out what phosphors were used in which version of tube. Then we can make definitive statements about the tubes, instead of wondering 'why?'
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Evolution... 
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