[old_tv_nut]

This was mentioned again during last night's on-line ETF meeting. It was revealed that this tube uses a wire grid rather than a cylindrical one, apparently to get enough sensitivity to dispense with a video amplifier stage.

Is it possible to post a drawing or picture of the gun structure?
From all that has been mentioned (including the unusual grid structure) I would guess that electrode misalignment is the problem, as has been suggested. It could be that sensitivity to alignment is much greater than usual in this gun design.