Cork - Might as well thrash the crt a little more; I'd like to get it right:

I agree that the oldest tubes weren't aluminzed and that their dag didn't extend to the screen. I suppose the screen potential became just a little less positive than the dag, to help shoo the secondaries back to the dag, closing the return circuit. The ZnS was probably a great insulator, so maybe the potential wasn't too uniform on the rear of the screen. Vol. 18 of the Rad Lab Series, "CathodeRay Tube Displays" shows in figure 18.13 that the difference between screen and A2 potentials is about 5V for A2 potentials up to 6kV. Above 6kV, the "sticking potential", the screen begins to go negative with a slope of unity. "A thin layer of barium "getter' can increase the sticking potential."

But then came the aluminized boob crts where the Al reflected backward directed photons and increased the brightness (and ion traps were no longer needed). The thin Al layer must have at least guaranteed that every part of the screen had exactly the same potential.

Maybe color CRTs do have an actual connection between the aluminized screen and the dag.

In that case, when a resistor boob is turned on. The dag and screen comes up to A2 voltage. (Lots of field in front of the face plate until negative charge makes its way to the front of the glass). But, when the beam gets going, the screen potential gets lower by only a few volts.

_

CORWIN MALOTT wrote:

> Hi Jon,

>

> I don't want to thrash this thing too much, but aobut the CRT, you mention that the screen is conductive enough to maintain the A2 potential in the big end of the tube. I don't have any idea how conductive the phosphor is, but there is no connection from the screen as a whole to the second anode. Remember there is always a band of clean glass between the screen edge and the A2 dag inside the bulb. The only way I can imagine for it to get to A2 potential is by secondary emission fron the phospor. Somehow the original start up induced charge on the phospor is not zero volts, which makes sense since the screen can "see" infinity" and that's where the zero volts is. I remember some strange behavior with some of the really low A2 voltage CRTs that I messed with in the distant past. On start up it seems like there were forbidden reagons on the screen where the beam would not go for the first few seconds.

Here are a couple of web things that claim a conductive aluminum coating on the back of the phosphorus. But I do remember the clear glass between the dag and the screen. If it is secondary emission, what is the return path for the primary beam? And what would keep the screen from building up to a potential much higher than A2?

http://www.arcadecontrols.com/files/Miscellaneous/crtfaq.htm#crtcac

Then, an aluminum overcoat is deposited over the phosphor/black matrix. This has several functions:

* Provide the return path for the electron beam - connected to the EHT 2nd anode.

* Reduces backscattering or secondary emission. Electrons that bounce back from either the shadow mask or the screen may hit a phosphor elsewhere and thus cause unwanted white light. That reduces contrast and colour purity.

* A side benefit is that it blocks negative ions from residual air molecules from hitting the phosphors. These might result in an unsightly blemish in the center of the screen since they are much heavier (many thousands of times the mass) than electrons and are not deflected very much. (This was a problem in the early days of CRT production but apparently not with present high vacuums and getters to clean up whatever is left.)

__________________

The inside of the CRT faceplate is coated with a thin layer of conductive material (often aluminum-coated) which is connected to the high voltage power supply. This high voltage is ultimately responsible for attracting the electron beam to the faceplate. This inside coating of the faceplate is coated with yet an additional layer of chemicals called phosphors that emit light when struck (excited) by an electron as shown in Figure 5.

CORWIN MALOTT wrote:

> Hi Jon,

>

> I don't want to thrash this thing too much, but aobut the CRT, you mention that the screen is conductive enough to maintain the A2 potential in the big end of the tube. I don't have any idea how conductive the phosphor is, but there is no connection from the screen as a whole to the second anode. Remember there is always a band of clean glass between the screen edge and the A2 dag inside the bulb. The only way I can imagine for it to get to A2 potential is by secondary emission fron the phospor. Somehow the original start up induced charge on the phospor is not zero volts, which makes sense since the screen can "see" infinity" and that's where the zero volts is. I remember some strange behavior with some of the really low A2 voltage CRTs that I messed with in the distant past. On start up it seems like there were forbidden reagons on the screen where the beam would not go for the first few seconds.

>

> I see your point about the mixers. Who cares about the 2f stuff since it is far away from the IF. That's one reason they run the L O above the incoming f.

>

> I am kind of committed on the car deal too. I have an oil=fired p/u, and if things get too bad I can go back to my oil rabbit P/U which gets about 40 miles/gal. What do you think aboutBushy's fiasco in Iraq? It's finally coming out how determined he was to attack, no matter what any of his advisors said would happen, that is winning the military battle was easy; taming a decapitated tribal country was another thing, certainly not our forte. I am not sure Bushy realizes the problem even now. The only thing that is for sure is that things couldn't be going better for ben ladin's gang; this is exactly what he wanted to have happen when he destroyed the WTC. It's probably going to be difficult to get the U N into there with total anarchy about to prevail. The scary thing is that I don't think Carry is up to it either. He seems like a mindless demo liberal with no new ideas on any subject, just scathing critisism in accordance with old-time democratic dogma.

>

> The digi cam I bout a few years ago was a Cannon S50. It is a metal bodied chunky little rock that will fit in your pocket. That's why I bought it. A camera that's not with you is worthless when your traveling, hence the SLR size things are alway back in the hotel. With these peewee size cCD cells, digis don't need a very big lens. (I think aperature-limited-resolution is not a factor here.) And most all of them have optical zoom plus what would have passed for super macro capability back in the SLR days. I havn't had any squacks with this thing so for, but it is not easy to learn or remember, and its little LCD screen is the only way to configure it thru its many menu pages, and you can't hardly see the LCD in the daylight.

>

> 73,

>

> Cork