Horizontal Linearity - CTC38

[maxhifi]

I've noticed that horizontal linearity isn't perfect across the screen. An object moving across the screen, will appear slightly wider on either side, than it does in the middle. This is really obvious with something like on demand movies, where it shows a matrix of movie covers on the screen, with those in the middle appearing narrower than those on either side. With normal program material, it isn't extremely obvious.

I did some reading about it, and I'm thinking to either change the horizontal output tube and damper, or try and adjust the horizontal linearity (efficiency) control. The thing I don't understand, is why this control is adjusted for minimum cathode current, and not best linearity - how are the two related?

I won't be terribly bothered if this issue can't be corrected, because it is fairly minor - but given there is a control marked horizontal linearity, I really would like to see just how linear things can be!