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Steve is right, that tube would have to be a replacement since it's the 'A' version. See the red dag on the inside? It has a resistance to it, so it can be used with chassis not having the series resistance in the anode lead. The 21-CT-55 had an extra anode lead with the resistors in it, the CTC-4 had them built into the HV tower connection on the chassis. These served as arc-over protection for the flyback, but when the 'A' version of the tube came out they were no longer necessary. Hence chassis using the 'A' version had no series resistors, as they were in effect built into the CRT. It would be a very bad idea to use a non-A 21AXP22 in a chassis without series resistors, there would be no protection at all from arcing.
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Evolution...
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