[microgpt]

At least you already have some working tubes to start from. Some guy did it with Nixie tubes, of course those are a lot smaller.

[TylerE]

Exactly - and look what those cost despite being produced in relative quantity. Now scale up to something that's 1000 times the size with 1/1000th the production volume. Wouldn't surprise me if the per-tube cost was in the millions.

Nixies are also cold cathode, low current devices. Radio broadcast tubes can be handling tens or even hundreds of thousands of watts.