White Verticle Line on my '58 Admiral Screen?

[Magnavox300]

I've been enjoying my newly restored Admiral portable all summer,
thanks to everyone's help here at the forum.
There is a new vertical white line that showed up, just to the left of the center of the screen.
I don't know what's causing it, does anybody know?

I have a strong feeling, since every tube in this set is an original 54 year old Admiral tube,
it's probably a weak or bad one, but not sure which it might be?
Thanks for any help!

Magnavox300