Or if the tube hasn't been used much, you should expect an incredibly small spot in the center.

My hunch right now is that the ions are scattering off some element of a particular gun type, and it's creating the characteristic X pattern shown in that manual. I'm not necessarily talking about 'static vs magnetic focus here, but rather some difference in physical construction. I've seen 10BP4s with very different looking guns that functioned identically. This gun type probably became more popular around the advent of the rectangular tube. Note that the page from the book doesn't exclude the possibility of a round tube developing the X pattern burn, it only says it's much more likely to be seen on a rectangular tube.

I'm not buying into a mis-adjusted ion trap hypothesis. A mis-adjusted trap would only allow the electron beam to overheat an element of a gun, perhaps burning a hole in it. I doubt very much that any trap magnet can produce the field strength necessary to redirect the ions toward the screen.

I did occur to me that the ions would be hitting the wall in a bent gun tube and since the ion trap magnet isn't strong enough to effect the ions much, they would never reach the screen. Even if the trap was mis-adjusted. I remember seeing a picture. maybe in that same book, of a gun element with a notch in the side of the aperture, produced by electron bombardment from a mis-adjusted trap.

Other than a picture (or artist conception) in one or several tv repair books that many of us have seen, has anybody here actually seen an "X" shaped ion burn in real life? Under what conditions?

jr

Is it possible that the phosphor damage in an X shape is really NOT an ion burn? I can't think of another reasonable explanation, but it wouldn't be the first time one of those repair books misattributed a symptom to the incorrect cause.

I have a CRT that had worked for a long time with a misaligned ion trap magnet. The result is a sickle-shaped ion burn:

http://i042.radikal.ru/1704/51/5a923caeaaa6.jpg

Although, it isn't very noticeable on a real picture:

http://s001.radikal.ru/i193/1704/5e/a08b00f80d0e.jpg

The bad thing is that the misdirected electron beam may bombard and overheat an element of the gun, causing release of gases from its surface and hurting the vacuum, thus even more ions are produced

Nope I can't see any ion burn in that picture. :D

This may answer why 7JP4 tubes were not rebuilt in the past (from what I have learned). If the phosphor is dead, it may be too costly to recoat on what, at the time was becoming an obsolete tube.
I guess I'm lucky with my 10 and 12 inch tubes. I see no ion burn on any, even the really high hour.
Would a double ion trap vs a single make a burn difference?

Yes you are, because the only thing responsible for ions is the quality and depth of the vacuum inside a CRT. That's why some straight-gun CRTs work for decades with no ion burn, while some others catch a horrible one in a year or two.
A fellow CRT rebuilder says that their shop has a very positive experience of rebuilding tubes with neither ion trap nor aluminizing. They use some modern, "very advanced" vacuum pumps, and ovenize CRTs very hard before evacuating.

CRT rebuilder?? Wow! Do they do any rebuilds for customers outside of the country?

jr

I would be very leary of a Russian rebuilder. Look at the "quality" of Russian small signal tubes...

I think they could do, but safe shipping would be the main problem...

And what's wrong with them? :scratch2: I've never bumped into any problems with Russian tubes, at least with vintage ones.
Anyway, they do provide a nice warranty, as well as use modern brand-new cathodes with 10000-hour lifetime.

P.S. If you'd like to discuss the quality, let's get started with a random example: 2A3 RCA vs 2S4S Svetlana

Hence the reason why the traps used to be called beam benders by techs or engineers. This is a good thread, but I'm still left with a question that's been in my mind for many years, but never really needed answering as I never burned a tube. The procedure I have always used (I forget now where I learned it) is to set brightness to a mid level with the magnet in approximately the correct position (over the split/gap in the gun) and rotate for max brightness. Then move slightly back and forth again for max brightness. Sometimes corner shadows require retouching of the magnet. I assume that maladjustments that produce burn must be if the magnet is adjusted for anything but max brightness. I never have taken a chance on that and always work fast when setting one.

Here's another interesting topic that I don't believe is covered here. What about the double ion trap magnets. A crt such as a 10BP4 calls for one. I have an RCA 8T 243 (that I am trying to get rid of) that I got with a 10BP4 and it had a single trap. It did produce a raster, but reading data on the set and the tube told me that it needed this double magnet, so I found one thanks to the Internet and possibly this site. Before the WWW, it would be almost impossible to find such an item. Anyway, it's still there and I never finished the set's slated restoration. I think I will snag the trap when I find a home for it. I will of course inform the new owner who will still be getting a heck of a deal.:thmbsp: Point being, I never knew and still don't know what the double magnet does that a single doesn't and why some tubes call for one and not the other specifically. I bet there are a bunch of BP4 owners here that are running on a single trap with no problem.

This is an interesting thread, I hadn't thought much about it.
Whether the gun needs a double magnet probably just depends on the gun design. Its also possible the if the tube is rebuilt it might not have the original gun design. Seems like there were many types of gun designs. The people that designed the guns must have understood how the ion traps worked, some of that info must have been lost.
Probably a lot of experimentation. 'Beam bender' does seem a more accurate term.
Whether the 'burn' on a crt is made by ions and just were they come from to do the damage is a good question.

I've had issues with Russian tubes, and they have a very low reputation among most audiophiles and antique radio restorers. Even the terrible Chinese tubes seem to be better. I'll wait for rebuilding to get off the ground at the ETF...

Yes, it depends on the gun design. The two basic types and bent gun and slant cut. As near as I can make out, the bent gun uses a single magnet and the slant cut uses a dual trap.

I agree with what notimetolooz posted earlier. You're not going to cause ion burn by a misaligned trap but you can damage the gun elements with the electron beam.

Here's a diagram of how the double magnet type works.
https://c1.staticflickr.com/3/2808/3...69046747_o.png