The Family DuMont

[Eric H]

When was that NOS 19AP4 made?

I know there are differences between tubes made in the 40's-50's, the phosphor is a uniform color, B&W tubes from the 60's-70's seem to have a different formula, if you look at them up close while on you can see yellow and bluish colors in the mix. I would assume it was done to make the image look brighter, like adding Bluing to wash makes it look whiter.