Something that came to mind of late why cold cathode vacuum tubes never replaced the filament valves ? Farnsworth's image dissector was cold cathode late 20s and later (THE FARNSWORTH MULTIPACTOR TUBE) so the idea was there even before on early CRT's
https://borderlandsciences.org/journal/ ... _Tube.html
Dobischek's idea was to pretty much replace the the filament vacuum tube to with cold cathode type .