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Old 09-26-2009, 10:16 PM
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zenith2134 zenith2134 is offline
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Is it possible to know the SPEED of electron emission

in a Cathode ray tube? Must be extremely fast. If given in miles per hour or kilometers per hour, I can't even fathom what it would be.

Surely someone knows...
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Old 09-26-2009, 10:35 PM
old_coot88 old_coot88 is offline
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Cool

Actually the beam is under constant acceleration from the time it leaves the gun till it hits the screen. The higher the HV, the higher the acceleration and the higher the final velocity. I dunno what the final velocity is, but it's really honkin' on.
Bill(oc)
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Old 09-26-2009, 10:45 PM
JBL GUY JBL GUY is offline
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The speed of light is ~3*10^8 m/s and

In general....

The speed of electrons through free space is affected by their energy, therefore their voltage.

A CRT where the high voltage is about 30 kv will have the
electrons traveling something like about 0.30 (About 1/3) the speed of light.

The greater the high voltage the faster the electrons will travel.
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Old 09-27-2009, 02:13 PM
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Incredible. Thank you both. Does G2 voltage affect the speed of emission ?
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Old 09-27-2009, 03:08 PM
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Quote:
Originally Posted by zenith2134 View Post
Incredible. Thank you both. Does G2 voltage affect the speed of emission ?
The final velocity depends only on the difference in potential from the cathode to the screen. G2 will change it's velocity along a small portion of the path, but that doesn't affect the final velocity.

John
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Old 09-27-2009, 03:18 PM
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Quote:
Originally Posted by old_coot88 View Post
Actually the beam is under constant acceleration from the time it leaves the gun till it hits the screen. The higher the HV, the higher the acceleration and the higher the final velocity. I dunno what the final velocity is, but it's really honkin' on.
Bill(oc)
That's partially true. If you look at the construction of CRT you'll notice that the aquadag, which is connected to the HV, extends back into the neck of the CRT. There are additional electrodes that we don't always think about. These are acceleration electrodes. The aquadag connects to the last electrode and the electrons reach their final velocity as they pass this electrode which happens before the beam passes the yoke.

While it may seem odd, the region past this last electrode inside the bell region is basically field free because it is all at the same potential.

John
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Old 09-27-2009, 03:48 PM
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Dan Starnes Dan Starnes is offline
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Wow!!!
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Old 09-27-2009, 04:07 PM
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Wow is right. I keep asking these " obscure " type questions because they are the type which interest me the most in this hobby. The stuff you rarely hear about.
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Old 09-27-2009, 07:48 PM
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A little followup.

The attached photo is of a gun from a 12LP4.

The first cylinder on the left is G1. The heater and cathode are located inside of G1 and are not visible.

Next is G2.

The third cylinder is G3 or the accelerating grid or anode. Whatever you prefer to call it. You'll notice that it has no leads going to it. On the right hand end it has spring contacts that press against the aquadag to make contact with the HV. At the end of G3 is a small hole where the electrons exit. It's at this point that they attain their maximum energy/velocity.

John
Attached Images
File Type: jpg 12LP4_gun.jpg (106.2 KB, 53 views)
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Old 09-27-2009, 09:12 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by jeyurkon View Post
That's partially true. If you look at the construction of CRT you'll notice that the aquadag, which is connected to the HV, extends back into the neck of the CRT. There are additional electrodes that we don't always think about. These are acceleration electrodes. The aquadag connects to the last electrode and the electrons reach their final velocity as they pass this electrode which happens before the beam passes the yoke.

While it may seem odd, the region past this last electrode inside the bell region is basically field free because it is all at the same potential.

John
Y'know it IS embarrassin' to have been in the trade for 30 years and now long retired, and never given thought to that accelerating electrode being connected to HV via the internal coating of the jug. So the beam is in fact not accelerating once it leaves that cylinder. Funny how such innocuous misconceptions can be carried for a lifetime unless someone corrects 'em. Thankya, John
Bill(oc)
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Old 10-07-2009, 10:11 PM
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A Reference

From "Electronic Fundamentals and Applications", John D. Ryder, 1964.

"Since the electron beam striking the screen is carrying negative charge and the screen is insulated by the glass, the removal of this charge is necessary; otherwise the potential of the screen would fall to such a negative value as to repel the beam. Fortunately, as the electrons strike the screen they not only cause the screen to give off light but also to emit other electrons. This effect, known as secondary emission (see Chapter 3), may result in an average of more than one secondary electron being emitted per electron in the beam. The secondary electrons are attracted to a graphite coating placed over the interior bulb walls and connected to the second anode. As a result, instead of the screen acquiring a negative potential, it may be a few more volts positive with respect to the second anode."
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Old 10-07-2009, 10:35 PM
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But this is true only below second crossover, the voltage at which the secondary emission ratio of the insulator again becomes unity. Aluminized crts overcome this problem and allow landing energies at the screen at almost full anode potential rather than the "sticking potential" determined by the secondary emission properties of the phosphor.

jr
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Old 10-07-2009, 11:10 PM
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It's true that aluminization overcomes this problem.

Potassium silicate is normaly used in the deposition of the phosphor.

Strong silicate solutions, or evaporated magnesium oxide, allows them to adjust the second crossover point to closely follow the applied anode voltage.

A B McFarlane 1957 Br. J. Appl. Phys. 8 248-252

John
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Old 10-07-2009, 11:48 PM
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Ahhh Yes! nothing like a little MgO to fix up the old droopy secondary emission curve.

jr
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Old 10-08-2009, 08:44 AM
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Some interesting material can be found in the first few pages of this book:

http://www.pmillett.com/Books/hoag.pdf

Especially the fact that relativistic corrections (to account for the increase
of mass at high velocity) are required above 7000 volts.
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