Width & Focus in DuMont RA-103

[jr_tech]

Originally Posted by Phil Nelson

Looking back at a couple of these photos, now I am wondering whether this CRT was rebuilt.

The sticker on the CRT bell is hand-lettered 12JP4, which made me think "rebuild." But the stamped name looks like it could be Hytron. Would a rebuilding shop use a factory sticker? Or something more like "Joe's Rebuilt Picture Tubes?"

The picture of the gun inside the neck doesn't show any welds or seams in the glass. When you rebuild a tube, wouldn't you leave some kind of visible seam in that area?

It's a very strong tube either way. Just curious.

Phil

But the gun does not appear to be a 12JP4...remember a 12JP4 does NOT require an ion trap, your gun appears to be a tilted offset/ slashed field type which requires a single ring magnet. Why would the factory put the wrong gun in a tube? I suspect that rebuilders often made substitutions when the proper gun was unavailable.

The re-neck seal could possibly be hiding under the yoke or focus coil.

Also, the first letter of the logo on the label does not look like an "H", but instead has a closed top... so I don't think it is "HyTron" but perhaps something designed to look similar.

jr

[jeyurkon]

Originally Posted by jr_tech

But the gun does not appear to be a 12JP4...remember a 12JP4 does NOT require an ion trap, your gun appears to be a tilted offset/ slashed field type which requires a single ring magnet. Why would the factory put the wrong gun in a tube? I suspect that rebuilders often made substitutions when the proper gun was unavailable.

The re-neck seal could possibly be hiding under the yoke or focus coil.

Also, the first letter of the logo on the label does not look like an "H", but instead has a closed top... so I don't think it is "HyTron" but perhaps something designed to look similar.

jr

Any idea of which type it could be? I think this is a diagram of the tilted offset/slashed field that you're describing. It's from the 1969 RCA tube design book.

John

[jr_tech]

Perhaps 12QP4, RP4, UP4, ZP4 ...? I don't know,but all of these 12" types are shown in my old (don't see a date...early 60s?) GE tube manual, as requiring only a single magnet.

jr

[Eric H]

I know a 12JP4 is shorter than a 12LP4 (maybe because it doesn't need a trap?), also a genuine Dumont 12JP4 is a lightweight tube with a thin somewhat rounded face, possibly Pyrex glass, and the envelope is shaped somewhat differently, also the anode connector is different having a small ball inside for the connector, the 12LP4 is heavy and has the usual thick flat face and just looks like a bigger 10BP4.

That said I have an unmarked tube that is supposed to be a 12JP4 but it looks like a 12LP4, it doesn't use a trap though and it has an Ion burn just like most of the non-trapped 12JP4's

If you Google the numbers you will find some PDF's with the data for both.

The ETF has a picture of a real 12JP4, if you look closely you can see it's more rounded and there's a bump where the neck meets the bell.

http://www.earlytelevision.org/postwar_crts.html

[Phil Nelson]

This tube has a little ball inside for the anode connector, and the face is slightly rounded. Without pulling the CRT forward, I can't see clearly whether there's a bump where the neck meets the bell. When you push the deflection yoke forward, the bell (or bump) actually touches the coil inside before it meets the rubber pads. There's a gap of maybe 1/4 inch between the bell and the pads.

The gun looks like the diagram. The ion trap doesn't work if placed right over the gap as in the diagram, however. The only spot where it's bright is pushed back against the tube base. Don't ask me what's going on here, but the CRT is as bright as any B/W tube I've seen.

The bulbed face drives me nuts when I'm trying to take a screen shot. The image looks sharp to my eye, but not as sharp in the photo. Maybe the camera's auto-focus can't decide what to focus on . . . .

Phil

[bandersen]

Man! I've learned more about CRT/gun design, ion traps and rebuilding this week than in all my collecting years combined.

Good luck getting it humming along properly.