Quote:
Originally posted by Unimatic1140
. . . I found a repair slip inside and apparently the picture tube was replaced on 4/22/6??? with a 21FBP22 (I wish I could see that last and most important of all digits). I know this tube doesn't have the deep reds like the original 21CYP22  but looking on the bright side this tube might have 5 to 10 years less life on it.  . . .
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It was the 21CYP22 that had the orange looking red phosphor. This phosphor was inefficient, which required that the red gun be driven harder. Often when a 21CYP22 goes bad, it is the red gun that fails. This was fixed in 1961 with the 21FBP22, which used a new red phosphor that was really red, and with beam currents approximately equal in all 3 guns.
Look at the type number on your tube. If it is a 21FBP22-A, it was made sometime after 1964 and has a rare earth red phosphor, which is even better than the 1961 version.