17LP4 CRT ( Need one... )

[Eric H]

Magnetic focus uses an external magnet assembly, either electric or mechanical to focus the tube, electrostatic focus uses voltage to focus it internally inside the gun, that's probably what the missing pin is for.

You might get lucky and have it work without the magnet assembly but probably not.

[jr_tech]

Originally Posted by Eric H

You might get lucky and have it work without the magnet assembly but probably not.

But it might be possible to add a magnetic focus coil or permanent magnet assembly salvaged from another set to adjust focus on this tube. Years ago, I used an aftermarket add-on permanent magnet focus assembly called a Quam "focalizer" (or something close to that) to focus a crt in a set where the focus coil had failed... something like that might work.
jr

[RitchieMars]

Right, I was thinking that. I wonder if this worked like it does on an electrostatic speaker where the magnetic field is created by current flowing through a coil, which would means the coil would have to be tapped into a circuit somewhere. Obviously, a permanent magnet would be much easier. I'll look into this a bit...

I'm still unsure about this type of tube and I wish I could find some specifics on it. I'd hate to pass up a NOS tube. In my opinion, changing to a magnetic focus would be an improvement over the electrostatic if it were possible. With the electrostatic type, I understand that the focus degrades over the life of the tube.

[jr_tech]

Originally Posted by RitchieMars

Right, I was thinking that. I wonder if this worked like it does on an electrostatic speaker where the magnetic field is created by current flowing through a coil, which would means the coil would have to be tapped into a circuit somewhere. Obviously, a permanent magnet would be much easier. I'll look into this a bit...

Here are a couple of pix of a permanent magnet focus assembly (mounted between the yoke and the ion trap) in my 1953 Philco... it would take a fair amount of mechanical modification to support such an assembly in your set, but it might be worth a try if you can't find the correct CRT. The focus adjust is the little brass knob on the flex shaft, shown on the second picture. The little lever, with the wing-nut lock at the top of the assembly is the position adjuster.
The Philco uses a 17YP4, which also has a cylindrical face.
jr