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Integrated circuits inside thermionic valves exist. The 6502 has four thousand transistors, i.e. equivalent to a cube of fifteen components on a side. The restriction of two dimensions which applies to semiconductor circuits isn't a problem for valve construction, so the third dimension can be used.
The component count can also be reduced by other techniques, including reducing the word length to six bits, processing data in series rather than parallel and making the CPU stack-based.
So this is what i imagine. Design a CPU with a word length of six bits, processing data in series and construct it using the kind of components valves are made of. It's asynchronous. Make it cube-shaped. Provide storage using a "fuse"-based PROM and ferrite core memory. Enclose the entire computer in a single evacuated glass chamber. This is shaped somewhat like an hourglass. One side of the hourglass contains the computer hardware itself, including RAM, ROM and CPU, interfaced to the outside by radio. A vector-scan display CRT is situated from the waist of the hourglass onwards with the computer generating the display itself. In order to improve the reliability of the system, it stays on permanently and is enclosed in a larger container of argon. Refrigerate the whole system from within, radiating heat out at the opposite end to the display through a heat sink. Power is supplied by nuclear batteries.
One problem with this is that as soon as anything goes wrong, the entire system is ruined, so it's more like contemporary hardware than might at first be apparent.