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Old 05-05-2011, 09:57 PM
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old_tv_nut old_tv_nut is offline
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Join Date: Jul 2004
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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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