Ct-100

[dtvmcdonald]

Originally Posted by Tom Albrecht

Anyone have any idea why I'm seeing the screen voltage behavior I mention above (red is stronger at low screen voltage, not higher)? I wonder if the higher screen voltage is throwing off the purity? Wouldn't really seem to make sense that it is actually lowering the emission...

I have a theory. I derived this as my red gun was slowly dying before it arced
so bad it destroyed itself (and then arcing became almost continuous).

The theory is that the center of the cathode is completely dead and the emission from the outer ring is, with the screen at normal voltage,
hitting an electrode before it gets out of the gun. When th screen is lower,
the focus changes and the electrons get through. This is of course a very far-out idea. It could be tested using an electron gun field simulation program, if we knew the geometry, or by examining a gun removed
during a rebuild.

[benman94]

Originally Posted by dtvmcdonald

I have a theory. I derived this as my red gun was slowly dying before it arced
so bad it destroyed itself (and then arcing became almost continuous).

The theory is that the center of the cathode is completely dead and the emission from the outer ring is, with the screen at normal voltage,
hitting an electrode before it gets out of the gun. When th screen is lower,
the focus changes and the electrons get through. This is of course a very far-out idea. It could be tested using an electron gun field simulation program, if we knew the geometry, or by examining a gun removed
during a rebuild.

I believe you're onto something here. A weak B/W tube will often give blackened highlights when driven too hard, and show poor, non-uniform focus. Dial the brightness and contrast back and some tubes will behave more normally.

I've seen this effect blamed on a dead cathode center region in period papers. It's possible the same effect that causes this behavior in B/W tubes is causing the strange behavior of Tom's weak red gun.