Steve, I am sure you realise that I am being factitious regarding the use of valves (I prefer that term when referring to diodes, triodes, pentodes, etc. because that's what I grew up with). I in fact love them and encourage their use. I certainly would be using them if I had access to them (at a reasonable price). After all Baird's mechanical systems were not really mechanical, more of a hybrid. His systems would not have worked without them.
In fact transistors and ipso facto, the rest, are actually easier to build than valves (tubes) and do come into the sphere of "homebrew". Obviously a P4 is getting a bit much but a CPU built of homebrew transistors is certainly possible (even if it would make Albert's LED matrix seem a doddle).
AC7ZL's book
Instruments Of Amplification
Fun With Homemade Tubes, Transistors, And More
Is a good example of what can be done - that and The Voice of the Crystal are good reads - and I shall certainly be obtaining a copy of Marvelous Magnetic Machines when he releases it.
Note that he shows how to make valves (tubes) too, disappointingly all of those experiments require a fairly expensive vacuum pump.
However I am more serious with CRTs (which I know you love) and it is really those that I come at for various reasons not least, I can't make one.
No different to anything that has been out of production for a while, say a Jaguar XK140 or a Piper Cub.
Er, yes that was my point all right...
Next time I swing by your place I'd like a look in the garage please...