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.

[Chad Hauris]

I think it depends on the set....I've got many old sets with original or at least old replacement vertical tubes that have plenty of available vertical height and good linearity, however some sets seem to put more stress on the tubes and seem to get to where I can't get good linearity with enough height on the old tube. 6GF7 is an old standby that seems to work well in the circuits it is used in...also 17JZ8 always seems to work.