That sounds like what happened when I tested my gone-to-air 21AXP22 on my Sencore CR70. The filament had continuity but the tester made unhappy noises and I got no hint of emission.
I was told to look at the little peek-a-boo slots on the neck. There, the getter looked white -- a sign that the tube had leaked.
I was able to substitute a newer 21FJP22, which you can read about at
http://antiqueradio.org/RCACTC-4ColorTelevision.htm .
Phil Nelson
