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Discussion Starter #1
Hi,


what happens to the tube if you omit the focus magnet?


Say I take a P19 tube and strip off the focus magnet/coil and power on the PJ.

Would I just get a megabig unsharp spot on the phosphor?

Or would the electron beam spread up until it hits the neck glass and kills the tube?

If that would be the case, would that also happen if I would not deflect the beam?

Does anyone know the angle at which the electron beam diverges?


I am thinking about experimenting with a tube while it is completely stripped, only with the neckboard and the anode voltage connected. No deflection yoke, no focus magnet, no astig rings...


Roland
 

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I thought that this would spot burn the center of the tube.......


My reason for this is that with out the deflection coil (like a scan fail) the beam would go to the center.


Or maybe the deflection coil controls it so it would be a wide off tube face pattern.......


try it on a dead tube.........


THIS IS JUST A NON EDUCATED GUESS,
 

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Discussion Starter #3
Thanks for the warning.

Of course I will try this only with a dead tube (spotburned).


I think if I omit the focus magnet, the spot would be qite large and if I keep the beam current low, it should not burn in.

My only real concern is, if the beam widens so fast that it gets wider than the inner neck and hits the glass...

As there is vacuum involved and no yokes/coils around the tube I would not even want a spotburned tube to implode...


Roland
 

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It's not uncommon to run a CRT without current through a non-permanent type focus magnet (no magnetic field). The beam is just very large and unfocused.
 

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I was thinking that without the magnets in place you might get some unusual xray emissions happening. Might want to be careful handling that tube under power
 

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Discussion Starter #6
Quote:
Originally Posted by tse
It's not uncommon to run a CRT without current through a non-permanent type focus magnet (no magnetic field). The beam is just very large and unfocused.
Do you know how large the beam would be at the tube face?

I have just measured the way from the gate electrode to where the neck starts to widen into the funnel. Its 23.5 centimeters. Tha Neck has an outer diameter of 36.5mm and I think maybe 3mm glass thickness (please correct me anyone who knows exactly as I wold be interested to know), so free opening would be around 30mm.

As the way from the gate electrode to the phosphor is around 37cm, the unfocussed spot on the phosphor must be smaller than 30mm*37/23.5, makes 47mm, or some part of the beam will hit the glass, right?


Roland

P.S.:

As long as the beam does not hit the glass there is no X-ray issue.
 

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Why do you want to do this? This sounds dangerous to me. If you look at the way an 8500 is constructed, it has lead shielding around certain parts of the tube. That lead shielding isn't there for looks...


If you just want to know how CRTs work, the equations are very well established. Deflection is easy to understand... focusing and astigmatism are a little harder, but not by that much.


I don't much like the idea of "experimenting" on a CRT tube. If there is something specific you want to know, someone can probably tell you. Playing around with multiple KeV electrons isn't wise if you don't know what you are doing.


The emission angle will depend on the physical design of the control grid.


See
http://www.tpub.com/content/neets/14...s/14178_88.htm
 

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Discussion Starter #8
I want to experiment with the electron gun (C/V characteristics, dynamic behaviour...). It is much easier to do that without all the deflection and focus circuits running.


Also for the tube I want to use for the experiments I have no focus magnet here. Could take one from another PJ but do not want to ruin the perfect setups by slaughtering the focus coil out of it...


Roland
 

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I think you would be better off asking if a forum member has the data sheet for a tube you are interested in knowing about. You will learn more from a technical data sheet than you ever will from experimentation, and you will be much safer.
 

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Discussion Starter #10
I already have the datasheet but I am not satisfied with it :)


Has noone here ever done this?


tse,

do you know if it is also not uncommon to do this with a projection tube?


Roland
 

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Can you post the PDF of the datasheet? I'd like to see it out of curiosity myself.


BTW, if you just want to play with the control grid, why do you want to remove the focus elements anyway? Why not just leave them on?
 

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Forgive my ignorance, but shouldn't C-V be flat? Given the geometry involved, I wouldn't think it wouldn't change a whit -- the distances are huge compared to the width of the grid. Capacitance is a function of geometry only, and it's not like this is a semiconductor with changing conduction band depths...


If you know the size of the anode and the distance from the control grid, you've got a pretty good idea of the angle of emission btw... That might be on the datasheet.
 

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First off, If your going to run your CRT without deflection then you need a pulse generator to feed into the neck board, this simulates scanning. Otherwsie, depending on the drive voltage even with a large spot the current density maybe such that you will damage the phosphor.


The beam diameter without any focus will be large but it will not spread to encompass your entire faceplate. If you want to determine the divergence angle of the electron beam without focus then you need to know the size of the aperature of the last electrode in the gun. I would guess it's about 0.020". From there it is just trigonometry to calculate the divergence angle.


I frequently turn off deflection to focus an electron beam to a high level of percision by putting an AC voltage on the focus coil. This will only work with an electromagnetic coil.
 

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Quote:
Originally Posted by RoBro
Do you know how large the beam would be at the tube face?

I have just measured the way from the gate electrode to where the neck starts to widen into the funnel. Its 23.5 centimeters. Tha Neck has an outer diameter of 36.5mm and I think maybe 3mm glass thickness (please correct me anyone who knows exactly as I wold be interested to know), so free opening would be around 30mm.

As the way from the gate electrode to the phosphor is around 37cm, the unfocussed spot on the phosphor must be smaller than 30mm*37/23.5, makes 47mm, or some part of the beam will hit the glass, right?


Roland

P.S.:

As long as the beam does not hit the glass there is no X-ray issue.
The undeflected, unfocused spot will not be very big. Only about a mm or two. This is with a 9" EM focus projection CRT. It will burn the phosphor as well as the glass faceplate in short order with even very low current.
 

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Quote:
Originally Posted by mdtiberi
I frequently turn off deflection to focus an electron beam to a high level of percision by putting an AC voltage on the focus coil. This will only work with an electromagnetic coil.
Just curious, are you doing this with projection CRT or something else? I didn't think there was too much of this kind of stuff still happening.
 

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Discussion Starter #16
Quote:
Originally Posted by tse
The undeflected, unfocused spot will not be very big. Only about a mm or two. This is with a 9" EM focus projection CRT. It will burn the phosphor as well as the glass faceplate in short order with even very low current.
So I will be safe not hitting the neck with the beam, right?

Quote:
Originally Posted by mdtiberi
First off, If your going to run your CRT without deflection then you need a pulse generator to feed into the neck board, this simulates scanning. Otherwsie, depending on the drive voltage even with a large spot the current density maybe such that you will damage the phosphor.
Many thanks for that advice!

I thought about that when I had the idea of doing these experiments, but had forgotten it meanwhile.

Quote:
Originally Posted by mdtiberi
I frequently turn off deflection to focus an electron beam to a high level of percision by putting an AC voltage on the focus coil. This will only work with an electromagnetic coil.
Can you explain what waveform and frequency you use and how you adjust for best focus?


Roland
 

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Quote:
Originally Posted by RoBro
P.S.: As long as the beam does not hit the glass there is no X-ray issue.
There seems to be a little confusion about the production of x-rays. In a CRT, x-rays are generated by the breaking kinetics otherwise known as Bremsstrahlung Radiation. The wavelength of the x-rays is a function of accelerating voltage, the density of the material electrons are slamming into (usually metal such as the aluminium layer on the faceplate) and the incident angle. If electrons are hitting the glass with the DAG or conductive coating then x-rays will be generated but not as significant as the from the faceplate. If you keep the duty cycle low, which you can do with the pusle generator, then very little x-rays will be generated.
 

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Discussion Starter #18
Hi mdtiberi,


with the quoted sentence I meant that if I would not hit the glass of the neck, the beam would land on the faceplate as it usually does and so the x-rays would be no worse than in normal operation.


Would you like to share your AC-voltage focus adjustment procedure with us?


Roland
 

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This procedure will only work for an EM focus coil and can be dangerous if you are not really, really careful. You must be able to adjust the pitch, yaw and X-Y translation of the coil with respect to the tube. You will need a pulse generator so as not to burn the phosphor, set it to a 10 ns pulse duration and 20ms pulse spacing. It is best to do this in a darkened room so you can see the spot with the current as low as possible because you will be looking directly into the tube preferably with magnification. By keeping a very low duty cycle x-rays are not a worry but if you have a meter I always check to make sure. The pulse generator is connected to the video input on the neck board. You will also need a variable 60Hz AC power supply which is connected to the focus coil.


With the deflection yoke off make sure the beam is centered on the faceplate. When racking through the AC power the beam will begin to focus. You will see two spots in a horn pattern begin to merge. The ideal situation is you will see a larger area that is luminescing, the penumbra (or halo) and the brighter center spot of the two merged beams, kinda looks like a bulls-eye. The direction of the horn will tell you if the center of the focus is high or low with respect to the beam. If it looks like a partial moon then the pitch is off.


There is no better way to focus an electron beam than with this method and is usually done only in high precision CRTs where the spot size is 10 microns or less. It is not an approach for the novice, it took me many years and a lot of burned CRTs to get this right.
 

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Discussion Starter #20
Hi,

sounds very interesting!

But what means focus high or low and what means pitch is off?

Do you mean rotation of the coil and displacement sideways or up/down?


I think I have to sleep over it, I am just too tired and will go to bed now. Maybe you can describe pitch, yaw and X-Y translation with other words?


Roland

P.S.:

If this works out how it sounds you may get as famous as Guy :)
 
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