Looking for an old film/vid w/servo regulated variacs in BIG tube filaments

[Opcom]

There is an old back and white? video that has a scene showing the automatic up and down movement of servo controlled linear autotransformers as filament voltage regulators, controlled by the power demand of the huge transmitter.

I think the brief scene showed a row of 4 quite large glass type TX tubes, and one of the transformers was under each socket. Anyone remember this and know where it is?