Drive line Question

[ohohyodafarted]

I am completing the recap of the ctc4B chassis for my Cheltenham.

I was plagued with a strong vertical bar in the center of the screen which I believe is referred to as a "drive line" Adjusting the horizontal drive control to minimum would not make the drive line go away. As it turned out the 6CB5 HOT (which tested very good) was the cause. I had to try 3 NOS tubes before I found a tube that cured the drive line issue. Just by luck one of the NOS tubes made the drive line disappear.

I am not terribly well versed from an electronic theory standpoint. I know that there are many of you who understand electronic theory way better than I do. Can someone explain what causes the drive line, and what the reason was that I had to try several known good NOS tubes to find just the right one to make the drive line go away?

Any enlightenment would be gratefully appreciated.