A quick introduction
Apparently T.J. Watson sent a company directive - starting the slogan "Solid State in '58".
The 1401 was all "solid State", except for one vacuum tube in an alignment aid circuit in the 1403 Card Reader/Punch - hidden away in a little aluminum box.
Coincident current magnetic core memory had been used in other computers, but because of "interesting" drive requirements, for the X and Y drives, had been driven by vacuum tubes.
From Rick Dill (8/12/09)
... On old TUBE computers (yes I cut my teeth on them) the usual discussion is about filament burn-out (which was usually only a small portion of the tube failures). When we went to transistors, we counted the number of diodes used, but in most of the tube computers, diodes were used to do the logic and the tubes were just there to bring the output signals back up to the levels needed for the next logic stage (diodes again).
In the 1950's, most silicon transistors were grown junction type. These were totally uncompetitive. There is no reason to think that they were good for computers. The saving grace was being able to operate at higher temperatures. In the 1960's all that changed with the use of photolithogaphy along with diffused junctions for both emitter and base regions, along with epitaxial growth to allow higher voltage collectors (low doping in the epitaxial collector ) along with the low contact resistance of a highly doped wafer.
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