Electrostatically focused CRT G4 Voltage Range?

[Kevin Kuehn]

A typical G4 spec seems to be -50 to 300v. Is there an actual sweet spot in that range for a specific tube, or is G4 voltage so tolerant that anything in the ball park works pretty good? I recently tried varying the voltage between 0-300 on a 17BJP4, and to my eyes, I could not detect a focus change. I guess I don't understand why such a wide range of voltage is called out?

[miniman82]

Depends on the tube, they are all slightly different. Generally they have focus voltages near B+, but larger tubes naturally require more.

[Kevin Kuehn]

I was just curious why in the tube manual specs they call out such a broad range on the focus grid voltage. My assumption was that I would see a dramatic change in focus when I adjusted the voltage from 0-300.

[dieseljeep]

You piqued my interest on this. I have an RCA CTC53, that has a low focus voltage CRT. I know it's a color set, but, focus is set up in the same way. I'm not thrilled about the focus quality. The scan lines aren't clearly defined. I has only two focus settings. B+ and ground. I might try a pot to varie it, between 0 and 300 volts.
I'll report my findings.