Inject your composite source to pin one of the video amp tube (make sure to disconnect existing connections going to pin 1 first!). Make sure to add a .1uf say 630v cap in series to isolate from any dc feedback. Then of course connect the shield of the wire to ground!
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Admiral C322C2 Regent (Restoring)
RCA CTC-7 Pensbury (Restored)
RCA CTC-5 Westcott (Restored)
CRA CTC--4 Director 21 (Restoring)