Back in the 1940s to the very early 1960s computers used vacuum tubes as logical components. They were replaced by discrete transistors, which were replaced by integrated circuits.
There are still companies making vacuum tubes, something I’m assuming individuals effectively cannot do if only because of the costs involved. Assuming a bunch of hobbyists came upon complete and correct plans for a vacuum tube computer and decided to save their pennies to buy all of the hardware and build it, could they? Is anyone making the correct types of tubes anymore?