If a picture tube loses its vacuum...

[yagosaga]

Originally Posted by Tom_Ryan

The Early Television site lists the set you pictured as the SE-IV from 1935/38 as 180 lines. I'm not sure who's correct.

SE-IV is wrong spelling. It has to be FE-IV.

Originally Posted by Tom_Ryan

There is one good thing to be said about a "gassy" tube. The ionization properties of a residual gas inside the CRT can be used in your favor to provide a source of electrons to help emission - but there are some side effects to consider. Historically, the Telefunken had 180 lines of resolution in their early TV sets. It was probably less than that using a gassy tube. A gassy tube would cause the divergence aperature of the electron beam from the gun to be very large.

I cite the original source for the FE-IV:

"Mit so einem Strahlenbüschel kann man nur wenig beginnen, zumal dann, wenn als Ziel ein einziger Fleck erreicht werden muß. Wenn man in die vorhandene Glasröhre ein Edelgas, zum Beispiel Helium oder Argon füllt, so kann man eine scharfe Bündelung des Strahlenbüschels zu einem Strahl erzielen, wenn man den Heizfaden mit einem Metallzylinder umgibt, der mit einer negativen Vorspannung verbunden sein muß." (Das Fernsehheft, 1935, p. 27-28.)

Translation:

One can do with such a bunch of rays only a little, especially if one wants to get a sharp limited single spot on the screen. If one fills in the existing glass tube with a noble gas, for example, helium or argon, one can achieve a sharp focussed spot of the rays if one surrounds the filament with a metal cylinder which must be connected with a negative voltage.

The problem in these days was the focus of the cathode beam. They did not have proper electronic focus lenses. With the adjustment of the negative voltage for the metal cylinder, the beam was focussed on the screen.

In the later tv sets of the end of the 1930s (see for example the E1), they used electromagnetic coils for focussing the beam.

- Eckhard