Nice work!

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.

Is the spot size from the red gun larger under these conditions? I'm guessing that with lower g2, much of the beam current is being generated from a larger area of the cathode surrounding the very center, which is likely fairly well depleted by now.

just a WAG,
jr

I think you're on the right track. Now that I've modified the screen control circuitry so that the adjustment range of the screen controls is at a little lower voltage, as I go from the lowest screen voltage to about 1/3 the way up, red brightness increases. Beyond about 1/3 up, the red brigthness decreases, and near the top of the range, I think the entire picture (all guns) is defocusing a little. This might be consistent with additional current dragging down the voltage on either the first or second anode, without additional beam current actually making it to the screen (it's hitting elements within the gun).

I don't think I'm seeing a bigger spot size on the red. Picture is pretty sharp overall. Don't know if that's consistent with the idea that the center of the cathode may be worn out, but I'll simply report what I see.

Tom, I know that I am late to the party. I did see the pictures that you posted a few days ago. Just wanted to say how wonderful it looks. I must admit that I only recently became aware of the 15GP22 CRT thanks to this forum and the Antique Radio Forum. I will admit to having never seen a real 15" color set (vintage that is).

Very Nice!

I just had an idea. I improved my 7JP4 with the grid-cathode short remover
of my Sencore. Maybe my 15GP22 arcs are not caused by gas ... I never saw
a purple glow. I'm going to try that short remover on every single pair of
electrodes in the gun, including ones between different guns (excluding
heater-cathode). It can't hurt. I'll first check it with a 2kV supply set to
current limit at say 50uA. That should catch gas.

Sure sounds like it's worth trying. If we can stave off the death of one more 15GP22, that would be very nice. Let us know how it goes!