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There are at least 3 flavors of 21AXP22 that I'm aware of:
The earliest ones had the same phosphors as a 15GP22 giving a greenish look to the screen and had normal grey dag on the inside, the chassis they were used with always had series anode resistance to protect the flyback in case of an arc in the CRT. Part number would be 21AXP22.
Next is the same tube, just with improved brightness from different phosphors. This one was used in most of the CTC-4's you see out there unless a replacement was installed, and it's easy to tell because the screen appears white instead of greenish. Part number is still 21AXP22, still all grey dag.
Last version to come out before the glass tubes was the 21AXP22A. This is the one with a white screen and red resistive dag on the inside, which eliminated the need for series anode resistance for arc protection from the previous model. All CTC-5's used this tube and it's compatible with earlier chassis.
The same is NOT true of putting a 21AXP22 (non-A) into a set having a CTC-5 chassis, which has no built in series anode resistance. While it's true it will 'work', the caution there is that without any series anode resistance you run the risk of shorting all HV anode current to ground in the event of a CRT failure. The whole point of series anode resistance is the protect the chassis, without it you blow up more parts when the CRT goes. So if you have a CTC-5 and your CRT has all grey dag, either install some resistance in the anode lead or prepare to replace the flyback if the tube dies.
The red dag doesn't prevent arcing, it merely saves other parts from death should an arc occur.
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Evolution...
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