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According to GE's "Essential Characteristics", there are minor differences in the three types. Apparently 25GP22 is the oldest of the three, has the older phosphors (not rare-earth), a 1.35A heater, and was not tested for X-ray emissions. 25AP22 seems to be next, with a .9A heater and non rare-earth phosphors. 25GP22 and 25AP22 both have a "A" version with rare-earth phosphors, and the 25AP22A was X-ray tested. Lastly, the 25XP22 has rare-earth phosphors, a .9A heater, and was X-ray tested. There are slight differences in light transmittance (darkness of tint in the glass?); 25AP22 and 25XP22 are given as 41% while 25GP22 is given as 42%.
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Erich Loepke
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