A fresh NOS tube usually needed for good vertical linearity

[wa2ise]

In addition to replacing wax caps in the vertical deflection circuits, replacing the output tube is usually needed to restore decent vertical linearity in the TV picture.
The TV I was working on uses a 6CZ5 for the vertical output, but all I had on hand is a 5CZ5. It's a transformer powered set. Well, a 2.2 ohm power resistor for the heater took care of that. But the linearity problems, after fiddling a bit with the vheight and vlin trimpots, were taken care of.

I've heard it said that vertical deflection is rather sensitive to underperforming tubes, even ones that seem to test good but not as "hot" as a new (NOS) tube would register.