I ran into this issue with my Sonora portable:
http://www.videokarma.org/showthread.php?t=260202
It should have a 8.4vac 450ma CRT, but someone stuck a 6.3vac 600ma CRT in it. It does work, but it takes quite a while for an image to appear because it's being starved of heater current. I'd go with a transformer as Eric suggested if possible.