(Don't react until you read this first part through:) You can of course never build a real classic computer, by definition its not a real classic unless its really old and was built by people who were, at the time, working with state of the art components and techniques. Part of the attraction of the old machines is that they subtly document the skills, preconceptions and ignorance of their designers and the prevailing conventional wisdom. Another part of the attraction is to have a piece of equipment that has been in existence for so long and still works. So the best you could ever do is come close. Any attempt will be a compromise of some sort, so an interesting question is how close is close enough? The closest I think is to attempt to exactly recreate a particular early machine. Chris Burton is doing this with the Manchester SSEM and Tony Sale with the Colossus, both with significant help from other folks in the Computer Conservation Society. I've seen them in progress, they are both fabulously, meticulously accurate (as far as I can tell) and very nearly as much fun as if the original machines still existed. And despite all the help they are largely one-man projects, or could have been, which shows that its actually possible to consider doing it "right" by yourself if you have the time. Sort of like building a boat or an airplane. People can do that. All this having been said, I'm sure you feel as I do that it would be fun to build a machine in the classic style but an exact recreation is too much. So how authentic do you have to be? My point is this - *** Its up to you ***. Since any such project is necessarily a compromise, the exact tradeoffs are not really important. Your project should reflect what you want from it, other people's opinion doesn't matter unless its a group effort. Here are a some projects I personnally find interesting: 1. A small Williams-tube memory. Designed and built from scratch using 6SN7's and a common 5" oscilloscope CRT like maybe a 5GP1. (Chris has beaten me to this but it doesn't matter - for that matter, Williams beat us both; the fun part is doing it yourself.) 2. A complete 32-bit CPU and memory in the classic style, an accumulator machine, built in just 4 parts: a FPGA, DRAM, EPROM and clock. This is easily possible with existing technology. It would also have a 2.5" IDE disk drive for mass storage and a small printer and keyboard for user I/O. It would be fun to do and would demonstrate the incredible miniaturization of electronics when compared with my room sized IBM 7094. 3. Emulators for all my old machines, and machines I wish I had. This is the only project I've actually made any progress on. So do what you find most compelling. -------------------------------------------------------------------- Here are some thoughts on the current proposal (building a machine in the classic style of a tube machine but with discrete transistors to be more practical), some of these reinforce comments others have already made: Your main problem is not the logic, it is the memory. There is no old memory that is anywhere near as easy to use as, say, SRAM. Your major choices are core, magnetic drum/disk, or delay line. Core is going to be a lot of work. Drum or disk will require expertise in mechanical fabrication but is pretty attractive otherwise. Acoustic delay line (the only kind with reasonable capacity) may be best but will require some research and experimentation. I would recommend magnetostrictive wire acoustic delay line memory if you can figure out how to build it and don't mind having a very small memory. About the logic, while very early tube circuits were strange using multiple grids, in most tube computer circuits the actual logic was done with diodes. The tubes then invert and drive the next stage. Flip-flops were used for temporary state storage and sometimes for registers. A very common computer tube in the US was the 5965 dual triode. To see some IBM 705 circuit drawings (all but the inverter, which apparently wasn't used much in this machine but was used in others such as the 709) look at http://www.teleport.com/~prp/collect/705dwg/ The diode/tube logic translates very nicely into common early transistor circuits such as RTL, RCTL and DTL. For a project like this I would recommend building a serial accumulator machine, one which works on one bit at a time and has a single architecturally visible register, like the PDP-8. It will have much less logic than a parallel or multiple register machine and will be very classic. A serial CPU will be a good match with serial memory such as drum or delay line memory. A good, pretty clean example is the Royal McBee LGP-30. Better is the SWAC which had a very good clean minimal architecture but was parallel, a serial version would be pretty easy. Note that small tube machines like the LGP-30 and Bendix G-15 have only around 300 tubes. To get an idea of how big it would be, the Packard Bell PB250 (see http://www.teleport.com/~prp/collect/mini.html ) is just what I've been talking about - serial, transistors (RCTL) and delay line memory. It takes up half a 19" rack, same as a PDP-8 but I think the logic is physically less dense. About a quarter of the volume is taken up by the memory. Paul