CRT Emissions vs Cutoff

[MuzzcoVW]

Been a long time guys. I have a GE 21FDP4 out of an old Philco here. It tests with great emissions but tests pretty much no cutoff. As far as I know it was never rejuvenated. Refresh my memory, what causes no cutoff but really good emissions? I do have a replacement CRT for the set when restoration is completed and am curious what i might see if I try using this one first. Thanks, Craig.