How Come Early CRTs contained more Ions?

[zenith2134]

...thus requiring an ion trap on the tube neck? It's my understanding that later crt's contained few or no ions. I don't understand why this is.

[Reece]

Ions are atoms of material off the cathode that ride along the electron beam. Electrons are deflected by magnetism, but ions aren't. Older tubes had their electron guns "bent" so they didn't shoot straight. The ion trap had magnet(s) that bent the beam back straight during operation of the tube. Since ions aren't bent by magnetism, they went off at the preset angle and impacted the side of the tube neck, doing no damage. Newer tubes have the electroluminescent phosphor coated on the back side with a thin coating of aluminum, which deflects the ions. Their electron guns are aimed straight at the screen, so no ion trap is required. Someone else may give a more technical explanation.

[jr_tech]

My 2cents (or less) worth

I believe the biggest change was aluminizing the screen, as the aluminum layer reduced the ion impact on the screen. This may not have been 100% effective, but it helped a lot. Ion trap use started to diminish when aluminized screens became prominent.

Other improvements to reduce the number of ions include:
1. Getter improvement from SAES and other suppliers.
2. Funnel flashed (rather then neck flashed) getters to provide more surface area of the getter flash.
3. Pump cycle improvements, improved diffusion pump "oils".
4. Improved RF processing (Bombing) while the crts are being pumped.
5. Improved raw materials/cleaning methods.

jr

[jeyurkon]

I think jr pretty much covered it.

But, not resisting the urge to be pedantic ; the ions are not from material off the surface of the cathode. They are produced from the residual gas in the CRT. It's impossible to achieve a perfect vacuum. The majority of the ions are produced when the electron beam hits a gas molecule. Some may be produced by the heat of the cathode.

Ions are bent in a magnetic field. Just not much. The force acting on them is the same as for an electron of the same velocity, but they have many thousand times the mass of an electron and are thus deflected less.

Electroluminescent has a different meaning than would be used in a CRT, even though the same material may be used. I don't completely agree with the wikipedia definition either. Most sources define it as a phosphor being excited by an alternating electric field or current.

Pumping speed can only buy you so much. The RF induction heating that was mentioned is an important element. Even in a very high vacuum the materials used in a CRT will only outgas at a certain rate unless they are heating to help drive off the gasses. The heating cycle of the envelope is important also.

Which translate to time and money. CRT manufacturers don't want a product to die so quickly that they get a bad name, but they could probably care less whether it dies in 5 years or 50 years of use.

The getter flash helps them to minimize how well they have to outgas the CRT.

This guy avoids having to due a getter flash for a vacuum tube by seriously degassing the tube before sealing it.

http://paillard.claude.free.fr/

O.K. I'll be quiet...

John

[zenith2134]

Wow, great replies. Thanks. So it is due to the gas inside the crt...I had a gut feeling it might be related to that, being that vacuum pumps probably got better as time went by...My oldest b/w sets have the ion traps in the neck but I never questioned why they did. No wonder why they can make or break the picture. On my Admiral, even the slightest adjustment hurts the contrast ratio.

[andy]

...

[cwmoser]

Originally Posted by Reece

... Older tubes had their electron guns "bent" so they didn't shoot straight. The ion trap had magnet(s) that bent the beam back straight during operation of the tube. Since ions aren't bent by magnetism, they went off at the preset angle and impacted the side of the tube neck, doing no damage. ...

I've understand that the ION Magnet should be adjusted to the brightest image. I backed mine off slightly from maximum brightness to eliminate shadows on the edge of the picture tube. I assume this is OK considering that the ions still will not hit the phosphorous because the electron gun shoots out deflected?

Then I have heard that if you don't adjust the ION magnet just right, it will mark the center of the picture tube.

Comments?

[andy]

...

[jr_tech]

Originally Posted by cwmoser

I've understand that the ION Magnet should be adjusted to the brightest image. I backed mine off slightly from maximum brightness to eliminate shadows on the edge of the picture tube. I assume this is OK considering that the ions still will not hit the phosphorous because the electron gun shoots out deflected?

Then I have heard that if you don't adjust the ION magnet just right, it will mark the center of the picture tube.

Comments?

Possibly you could re-position the yoke slightly to obtain reduce the "neck shadow" and then obtain a brighter picture by re-adjusting the ion trap to its optimum position, but I don't think that a slight mis-adjustment of the trap magnet will allow ions to strike the screen. A VERY MUCH stronger magnet would be required to bend the ion beam enough to travel straight down the gun.
BTW The screen is coated with "phosphor*" not the element phosphorous, many confuse the two terms.

*For example, "white" P4 Phosphor is actually a mix of "blue" Zinc Sulfide (silver and zinc doped) and "Yellow" Cadmium Sulfide (silver doped).

jr

[jeyurkon]

Originally Posted by andy

That's a common misconception. The ions will hit the inside of the gun, no matter how the magnet is adjusted. The only danger is that if the electron beam hits the inside of the gun, it can overheat, or even melt a hole in one of the grids.

It's true that the ions won't hit, but perhaps he's referring to the effect in the attached snippet of an article on ion-traps.

The electron beam can sputter material from the gun electrode which will damage the screen.

My 10MP4 has a small nickel sized area that appears to have been damaged in this way. It's barely noticeable, but there.

John

[jeyurkon]

Originally Posted by jr_tech

Possibly you could re-position the yoke slightly to obtain reduce the "neck shadow" and then obtain a brighter picture by re-adjusting the ion trap to its optimum position, but I don't think that a slight mis-adjustment of the trap magnet will allow ions to strike the screen. A VERY MUCH stronger magnet would be required to bend the ion beam enough to travel straight down the gun.
BTW The screen is coated with "phosphor*" not the element phosphorous, many confuse the two terms.

*For example, "white" P4 Phosphor is actually a mix of "blue" Zinc Sulfide (silver and zinc doped) and "Yellow" Cadmium Sulfide (silver doped).

jr

Thanks. I keep having people with degrees as high as PhD's ask me to coat viewers with Red Phosphorus when they mean Red Phosphor. They think I'm strange for expecting them to know that much chemistry. I'm the one who should be confused, chemistry was my worst subject.

[old_coot88]

Originally Posted by andy

I've always thought "ion trap" was a poor name for the magnet on the neck of the CRT because it has nothing to do with trapping ions. The bend, or angled grid, in the neck is the ion trap. The magnet is there to straighten out the electron beam so it will pass through the gun.

Yeah, it's like "shock absorber" which in reality is the rebound damper. The spring is what absorbs shock. Bill(oc)

[Charlie]

Originally Posted by andy

I've always thought "ion trap" was a poor name for the magnet on the neck of the CRT because it has nothing to do with trapping ions. The bend, or angled grid, in the neck is the ion trap. The magnet is there to straighten out the electron beam so it will pass through the gun.

I noticed in the Zenith Service Manual for my porthole, it's indeed listed as beam bender instead of ion trap.

[jeyurkon]

Originally Posted by Charlie

I noticed in the Zenith Service Manual for my porthole, it's indeed listed as beam bender instead of ion trap.

Same for my Sylvania. They list it as a beam bender, though in a few places call it an ion-trap.

John

[zenith2134]

The "phosphorous" vs "phosphor" thing reminds of of the old George Carlin skit: "it's not a HOT water heater; it's just a water heater!!!!" I shouldn't laugh though: I'm no chemist. Phosphor compositions & associated jargon confuse me pretty quickly.

So has anyone seen a tube where the beam-bend was so far off that it melted the grids?