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...

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)

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.

Incredible. Thank you both. Does G2 voltage affect the speed of emission ?

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

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

Wow!!!

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.

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

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)

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."

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

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

Ahhh Yes! nothing like a little MgO to fix up the old droopy secondary emission curve.:yes:

jr

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.

Thanks, that is an interesting reference.

JR Tech brings up an important point about the second crossover. I'm starting to doubt the accuracy of the statement I quoted. It was written by the Dean of an Engineering College so I'm reluctant to say it's wrong.

Sticking certainly is a real phenomenom, but does it occur in non-aluminized production CRTs that are operated within their specified normal range. I've not been able to find much about the second crossover values for phosphors. Just that it can have a huge range for dielectrics.

John

I can't find a good reference that shows the secondary yield curves for various phosphors, but as I recall, most exhibit second crossovers in the range of 8 to 15 kV. I suspect that most early non-aluminized crts were operated below the point that "sticking" could occur, so the description of the landing energy of the electrons being controlled by the internal conductive coating of the tube would be valid. Much above about 10kv all bets are off...aluminized screens were likely necessary to achieve full landing energy (minus a kv or 2 lost in penetrating the aluminum layer).

jr

Speaking of aluminum, do you know how thick the coating is? This site seems to indicate 500 angstroms.

http://www.thevalvepage.com/teletech/crt_manu/crt_manu.htm
That would be thick enough to be opaque to light, but I'm not sure it would protect the phosphor from ions.

I assume the main ion species are singly charged hydrogen from gettering water and nitrogen which is more difficult to getter.

I've attached two simulations for hydrogen and nitrogen at 16KV on aluminum.

The left is for hydrogen and the right for nitrogen.

You can see that even 2,000 angstroms wouldn't protect agains hydrogen and 500 angstroms would barely protect against nitrogen.

I probably should have made some damage plots, but I think this shows the problem. My 40" rear projection set seems to be showing some ion damage on the blue CRT after about 20 years of use. It's a slightly dim blob in the center.

John

Nice plots!:thmbsp:

I think 500 angstroms would be really thin for crt use. If I remember correctly 800-1500 was used for lower voltage crts (10kv or so-early monochrome) and 3000-4000 for higher voltage color types. It would not surprise me to find 5000 or so in those 30-40kV projection tubes. Argon would be another likely residual gas, as it is not pumped by barium getters.

jr

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.

Heard it said that that is fast enough for Einstein's theory of relativity to make the electrons gain a little extra mass. This theory says something to the effect that as something made of regular matter tries to go near as fast as the speed of light, it gets more massive.

So there's some science happening in the boob tube... :D

Heard it said that that is fast enough for Einstein's theory of relativity to make the electrons gain a little extra mass. This theory says something to the effect that as something made of regular matter tries to go near as fast as the speed of light, it gets more massive.

So there's some science happening in the boob tube... :D

I think you must be correct...Because back in my college days (way long ago) I can remember watching the boob tube with friends and someone saying "Man was that ever heavy" Little did I know that he was referring to the mass of the stream of electrons drawing the picture on the phosphorus!:yikes::):)

It is amazing the amount of knowledge that resides here at VK:thmbsp: