Emission and Cut Off
 

I've often read on these forums threads mentioning CRT emission and cut off when referencing CRT testing. Can some one explain a little more in depth as to what these terms refer?

Hi All;
VTS1134, Emission is the ammount of electrons that are boiled off of the filament/Cathode of the tube, Whether it be a regular tube or a CRT.. We see it as light as in a light bulb or an orange glow in a normal tube/CRT.. Without emissions there is no transfer of electrons and the tube would be what we call dead.. In a CRT the combination of the various grids after that, aim and control the ammount of electron getting to the Phospher screen.. When no electrons the screen is blank and some electrons show a small dot and many electrons show a larger or brighter dot.. The larger or brighter dot is also controlled by one of the gride and also by the focusing coils.. When you have it focused correctly the dot is small and round.. Otherwise it is large and may or may not be round.. One fo the grids is the control grid, which as its name implies, controls the ammount of electrons getting to the screen.. So even if the filament/cathode is emitting electrons, if the control grid is at cut-off then no electrons will reach the screen, and if turned on a little some will get to the screen.. Like turning on the water faucet, as long as there is pressure from the main water pumping station, when the faucet is turned off we get no water, there is plently of water pressure, but it is cut -off.. As we open the faucet, we get some to alot of water depending on how much we have opened it.. Does this clarify your question ?? If, not please ask, what you are confused about..
THANK YOU Marty

Another way of thinking about it is that good emissions = good brightness and good cutoff = good contrast.

I think as a CRT wears out, that the control grid loses it's ability to effectively control whatever emissions the cathode is still capable of producing. So the picture becomes more and more washed out. Ability to focus goes out the window too. You ultimately end up with a leaky valve and a low water supply, or so it seems. :D

In some cases of extremely weak cathode emission in a B&W CRT, the pic is so dim it can only be viewed in a darkened room. The pic will have a 'silvery' cast, and high-luminance parts of the scene will actually be photonegative. That's because the control grid is commanding high beam current which the cathode can't deliver. And the grid is actually pulling available electrons out of the beam.

Yea, that photonegative appearance I'm familiar with. My old TG&Y will do this sometimes when turned on, then disappear.

Is it possible to have a CRT with good emissions and bad cutoff? What would the result be?

The B&K CRT testers I have used at home and work have a "set cutoff-add one meter division" function if I remember right. Near-dead CRTs at work would never move that pointer at all, until I rejuvenated them, then maybe it worked properly.

Good emission but bad cutoff would give you an overly bright picture, lacking in contrast, as the grid cannot effectively "pinch off" the beam current to produce the dark parts of the image.

Some rejuvenators can actually cause this problem, by eroding the control grid aperture when drawing the heavy current used to restore the cathode.

Could that be what I'm experiencing here? I am adjusting my brightness control up and then down.
http://www.youtube.com/watch?v=LsUmkal9L9s
The lower I set the picture brightness the better the picture. The best picture is actually dimmer than my camera can pick up and must be viewed in a pitch black room. I thought that the picture was loosing focus, but perhaps it is loosing contrast and that makes it look like it is loosing focus.

Might oughta clarify that the diagonally running bars are a camera artifact and do not exist.

Should have mentioned that.
I found that cutoff was not the problem I was experiencing with my set.

Hi All;
I would like to know more about that, Is that like becoming one with your set ?? Or a new religion ?? Or just something I wasn't taught in Basic Television ??
THANK YOU Marty

This explains pretty well what some texts referred to as a "gassy" CRT or one not under full vacuum..
so maybe gassy is incorrect :scratch2:, otherwise you may see some violet glow in, but not so much around, the electron gun.

Is this the same reason for the color smear (when increasing brightness/screen/bias) when a corresponding gun is weak in a color CRT?

All the cases of extreme weak emission i've ever seen did not exhibit signs of vacuum loss or purple glow. Weak emission and 'gassy' are quite different critters.
You'll see this sometimes in a solid state set with a weak gun when someone, trying to compensate, has cranked up the driver transistor for that gun. That can cause the transistor to saturate on peaks, resulting in the trailing smear. I don't recall ever seeing it on a tube type set.

It's the ability of the cathode to supply electrons that deteriorates, not a change in the ability of the grid to control them. The control ability is determined by the spacing and size of the electrodes, which doesn't change as the tube gets older. (There can be problems if cathode material migrates to the grid and the grid itself starts to emit electrons)

In a good tube, electrons form a cloud in front of the physical cathode, and the grids control flow from this cloud. When the emission gets too weak, this cloud can be depleted by trying to pass too much current in the bright parts of the picture.