How much does the beam of electrons disperse in a CRT?

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Offline Ron Hughes

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Say we had a CRT that is 5 meters long, what would the dot on the phosphor look like?
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Offline Soul Surfer

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How much does the beam of electrons disperse in a CRT?
« Reply #1 on: 04/05/2010 00:10:54 »
To answer your headline question not very much if the design is correct.  Remember this is essentially the way electron microscopes work.

The question in the question cannot be answered without more information being specified and this information is of a highly technical nature relating to the size of the electron source the electron optics the intensity of the beam and the accelerating fields.  Again if correctly designed the spot can be made very small
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Offline Ron Hughes

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How much does the beam of electrons disperse in a CRT?
« Reply #2 on: 04/05/2010 00:54:42 »
Lets take an old o scope crt. It was probably 10 to 12 inches long. Stretch the neck out so that the total length is say 3 meters. Do I still get the same size dot?
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Offline Geezer

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How much does the beam of electrons disperse in a CRT?
« Reply #3 on: 04/05/2010 04:14:56 »
Lets take an old o scope crt. It was probably 10 to 12 inches long. Stretch the neck out so that the total length is say 3 meters. Do I still get the same size dot?

I suspect you would not be able to focus the beam without making some changes to the drive electronics.
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Offline LeeE

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How much does the beam of electrons disperse in a CRT?
« Reply #4 on: 04/05/2010 16:39:38 »
Lets take an old o scope crt. It was probably 10 to 12 inches long. Stretch the neck out so that the total length is say 3 meters. Do I still get the same size dot?

If you're just stretching the neck of the tube then I don't think there'd be much difference because the focusing and deflection plates/coils are usually located at the throat of the neck, so the distance between the focusing and deflection plates/coils and the phosphor screen will be about the same.  If you scale the entire tube up though, then you'll be increasing this distance so I'd expect that you'd either need better focusing and deflection to maintain the same dot size/sharpness or settle for a larger, more blurry and dimmer dot.
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