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Kevin
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stromberg6 |
#2
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The only 21AX's that have this greenish color phosphor have been mostly found in CT55's. I also have a very early CTC4 which has a 21AX with a greenish looking face. Another intresting discovery is that, at least on the crt from my 55, which was rebuilt by Hawkeye, RCA was using up the 20 pin stems left over from the production of 15GP22's, rather than using a 14pin stem as would normally be expected. Take a close look at the stem on your tube. See if you can count the number of wires by looking into the end of the neck through the neck glass. The old gun removed from my 21AX was mounted on a 20 pin 15G stem. All this leads us to believe that the initial production run of 21AX tubes, which were used in the CT55 sets, and which have this greenish color phosphor, may be using the full gamut phosphors like were used in the 15GP22 tubes. Some day we will need to test one of these green faced 21AX tubes with a colorimiter to see if they are indeed full gamut or not. CT55's are known for having a brilliant color picture. Perhaps full gamut phosphors are part of the reason for this. AFter all a CT55 is actually a CT100 with a 21" crt and a modified HV section. Perhaps the phosphors are also an extension of the CT100 series. Time and further investigation will reveal the answer. Bob
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Vacuum tubes are used in Wisconsin to help heat your house. New Web Site under developement ME http://AntiqueTvGuy.com |
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Sounds like another interesting project for a failed tube, obvisouly we would not want to destroy a rebuildable one. What is involved in the testing? Are samples taken of the actual phosphor, or just the light given off by it when excited? The observation makes sense, the screen has the same kind of look that a 15GP22 does with a little bit of sparkle I can't quite put my finger on. Quote:
That's what I've always thought, but now the expert has confirmed!
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Evolution... |
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The gamut has to be measured by the light from an operating tube. Chemical analysis would require smashing the tube - not recommended!
Up until I saw this comment, I thought all 21AX's used the same phosphors as the 15G. There are two defining characteristics of the 15G phosphors: the blue is true NTSC blue (more cyan and less violet than modern tubes), and most important, the green is P1, less yellow than the later sulfide or cadmium sulfide green. (And, of course, the red is not very efficient.) The question is which phosphor or phosphors were changed in the 21AX's that changed the face color. If it was the red phosphor, it might be hard to determine, as most of the red phosphors used over the years fluoresced in about the same color, until the cadmium sulfide red in the all-sulfide tube. This phosphor was strongly colored under ambient light, resulting in a fairly strong green face color, and also its emission turned orangy under high beam current. The all-sulfide tube (including sulfide blue) could not be made until techniques to prevent contamination were developed. Contamination of the blue sulfide by copper would turn it into a green phosphor. The uncontaminated blue is more deep blue than than NTSC specs, and is what we have today. |
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What I'm really shooting for is to get a light sample from all types of tube, so we have a reference. Currently I have the 2 AXP's, a green FBP, a green FJP (sans lens), and a pair of grey faced CYP tubes for comparison. One of the CYP's is an 'A' version, not sure if that matters or not. That's the reason for the investigation.
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Audiokarma |
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Actually there is a colirimiter that attaches to a laptop. A couple of the collectors have one. I think it was about $200. I would bet Cliff Benham owns one. I would email him and see what he has to say about it. I believe that Cliff and John Folsom tested a tube at John's house some time back using the device. Bob
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Vacuum tubes are used in Wisconsin to help heat your house. New Web Site under developement ME http://AntiqueTvGuy.com |
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John |
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charts and graphs: The first is the spectrum response of the 21AXP22 taken with the EYE-ONE Pro Spectroradiometer. This is a gray faced tube as I remember. The second and third are both CIE charts comparing the color coordinates, some measured and some published, of various color sources and defined colors. Disregard "RX43" in the title of the third one. Hope this helps answer some of the questions. Cliff |
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It would be interesting to see the different blends of phosphor used over the years ploted on the color spectrum horse shoe(I can't recall the right name) for comparison.
Tom C. |
Audiokarma |
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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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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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By the way, when you measure a tube, you get the operational color, not the pure phosphor color, because any imperfections in the color purity settings, light scattering, electron scattering, etc. are all included.
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For example, I took measurements at screen locations that exhibited the best purity. All readings were made with a dark cloth over the set. Results did support the 15GP22's reputation for gamut. Only the green fell a bit short of the 0.21, 0.71 1953 NTSC standard. But not by much. At the time, John Folsom platted the results on a graphic that included the 1951 CBS and 1953 NTSC, and I believe the sRGB standards. As I am in SC visiting relatives after the ETF convention, I unfortunately do not have access to the graphic at this time. For me, this was one step in the quest to reproduce what one would have seen in the early days of color television. Wayne Bretl has demonstrated mathematically that the TK-41 was capable of generating a 1953 NTSC compliant image. Now if we can only get Mrs. Astaire to allow us a DVD of her husband's restored October '58 special, we could demo the real thing at the next ETF convention... Pete |
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For example, I took measurements at screen locations that exhibited the best purity. All readings were made with a large dark cloth over the set. Results did support the 15GP22's reputation for gamut. nly the green fell a bit short of the 0.21, 0.71 1953 NTSC standard. But not by much. At the time, John Folsom plotted the results on a graphic that included the 1951 CBS and 1953 NTSC and I believe the sRGB standards. As I am in SC visiting relatives after the ETF convention, I unfortunately do not have access to the graphic at this time. For me, this was one step in the quest to reproduce what one would have seen in the early days of color television. Wayne Bretl has demonstrated mathematically that the TK-41 was capable of generating a 1953 NTSC compliant image. Now if we can only get Mrs. Astaire to allow us a DVD of her husband's restored October '58 special, we could demo the real thing at the next ETF convention... Pete |
Audiokarma |
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