What is the theory behind low voltage focus CRTs?

[LukeSimon]

Originally Posted by zeno

As far as I know they work the same as Hi focus.
I would guess its just easier = cheaper that way.
On the US side most MFGS sold a 14" probably all with the RCA
jug. RCA also had a 19" low focus, Zenith 14" & 16". Most used taps & a few
a control to adjust. After a few years you usually didnt see a
difference adjusting them unless the CRT was VERY strong.
From Japan TONS of them were sold through Sears. Mostly
Toshibas & Sanyos. They looked even worse but on the test
jig not so bad at all after you put a hand full of low
level tubes in them.

73 Zeno
LFOD !

As a CRT's cathodes age, the cathode ray emitting layer at the center tip of the cathode wears away, causing the center tip region of the cathode to emit fewer electrons. The image becomes dimmer because of this, and so people increase contrast and maybe brightness to make the picture less dim. These adjustments cause electrons to be pulled from a larger spot at the tip of the cathode, which makes the cathode ray slightly thicker and the picture less sharp.

I have a theory as to why high voltage focus became more popular than low voltage focus. The high voltage focus has both a lensing effect to focus the cathode ray into a tighter beam, AND it also has an acceleration effect due to a high increase in voltage in the direction leading towards the face of the CRT. The faster electrons move towards the face of the CRT, the less time they have to spread outwards from the center of the cathode ray. This, btw, is one reason why a low ultor voltage causes the picture to be blurry compared to a high ultor voltage. Less acceleration from the ultor anode gives the electrons more time to spread out.

The low voltage focus designs, assuming they are a true Einzel lens, only focus the cathode ray, they do not change its acceleration along the axis perpendicular to the face of the CRT like a high voltage focus design does.

This theory would help explain why low voltage focus CRTs had a reputation of being both less sharp than high voltage focus CRTs AND a reputation for not aging as well when it comes to losing ability to focus a sharp picture.

Side note: vintage television is fun because it gives you an excuse to play around with a particle accelerator. Yes, a CRT meets the scientific definition of a particle accelerator.