'Vacuum tubes could allow systems to be reprogrammed; they could be used repeatedly and flexibly. The tubes made complex computations possible, but they were unwieldy, both prone to breakage and laborious to repair. ENIAC, the US army’s world-leading computer, introduced in 1946, used 18,000 vacuum tubes to calculate artillery trajectories faster and more accurately than any human. That made it revolutionary, but its utility was limited by the fact that it was the size of a room, and that whenever a single tube failed, which happened on average every two days, the whole machine broke down.'

https://www.lrb.co.uk/the-paper/v45/n06/john-lanchester/putting-the-silicon-in-silicon-valley