Emission and Cut Off

[old_tv_nut]

Originally Posted by Kevin Kuehn

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.

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.