If a picture tube loses its vacuum...

[yagosaga]

Originally Posted by tubesrule

While this worked fine for oscilloscopes and such test equipment, gassy tubes are not very suitable for use in television. A gassy tube can only be focused over a narrow band of beam currents. Since the beam current needs to be modulated for television, this resulted in poor focus.

Hi Darryl, these early German gassy picture tubes were used up to 1936. From 1937 and later the CRTs were high vacuum tubes.

In a gassy tube the negative voltage at the metal cylinder was adjusted for a focussed beam with maximum brightness. When the beam current was lower or low, the focus became poor, and as a result, the point on the screen became darker. In this way, one effect of the gassy tube was an improvement of contrast, but a loss of gradation.

The high vacuum tubes generated a more natural b/w picture with an improvement in gray tones and gradation.

- Eckhard