Oldest operational computer was Re: cctalk Digest, Vol 54, Issue

Jules Richardson wrote: Tubes are really not great as digital devices. Cut-off is pretty good (very high impedance) but saturation is soft (curved) (see audiophiles) and still leaves a relatively high impedance at saturation which varies with age and between tubes. The output voltage at the plate is a far cry from the needed input voltage at the grids, so voltage dividers are needed (more variability). Then you need to sum the multiple inputs of a gate into the grid of the tube: If you don't have nice non-linear devices like 'crystal diodes' (or multiple grids, or neons (not to start that discussion again)) that becomes another region in the system behaving in a very analog fashion. Start adding up all these variabilities and you end up requiring some pretty tight tolerances: in power supplies, tubes, and passive devices). In a radio receiver you can compensate for the disjoint variabilities in a string of a dozen tubes with a single twist of the volume control. That's the miracle of analog systems. No such luck in a digital system.