https://youtube.com/watch?v=acGXBJv6AT4
Vintage technology training film: 1943 Vacuum Tubes, very good quality, produced by Army Signal Corps and Air Corps, “The TRIODE and Multi-Purpose Tubes.” Narrator gives clear explanations of how the various tube components work. Run time: 13 minutes.
Uploaded by Computer History Archives Project (CHAP)
See also:
Early Vacuum Tube Computers:
The Cathode Ray Tube
Television, How it Works
https://www.youtube.com/watch?v=ntVqsysgaJs&t=34s