In November 1953 the University of Manchester's experimental Transistor Computer became operational for the first time. This appears to be the first stored-program computer to use mainly transistors as switches rather than vacuum tubes. The transition from vacuum tubes to transistors in computer design was generally delayed because of reliablility problems in early transistor manufacturing.
"There were two versions of the Transistor Computer, the prototype, operational in 1953, and the full-size version, commissioned in April 1955. The 1953 machine had 92 point-contact transistors and 550 diodes, manufactured by STC. It had a 48-bit machine word. The 1955 machine had a total of 200 point-contact transistors and 1300 point diodes, which resulted in a power consumption of 150 watts. There were considerable reliability problems with the early batches of transistors and the average error free run in 1955 was only 1.5 hours. The Computer also used a small number of tubes in its clock generator, so it was not the first fully transistorized machine" (Wikipedia article on Transistor Computer, accessed 09-19-2013).