Vacuum tubes were replaced by transistors in:

Computer generations are classified by the core electronic switching technology used for logic and memory, not just by decade: first-generation systems were built on vacuum tubes (thermionic valves), and each later generation is marked by a shift to a smaller, faster, more reliable switching technology than the one before it.

Vacuum tubes were bulky, fragile, and power-hungry, so replacing them with solid-state transistors -- which were smaller, ran cooler, and were far more reliable -- is exactly the change that marks the boundary between the first and second generations (roughly the 1950s-1960s). So the generation in which vacuum tubes were replaced by transistors is the second generation.

• first generation computers -- this generation is itself built on vacuum tubes; it is the technology being replaced, not the replacement.

• third generation computers -- this generation moved from individual transistors to integrated circuits (many transistors on a single chip); transistors were already the established technology by then.

• fourth generation computers -- this generation is defined by microprocessors and VLSI circuitry, several technology shifts after transistors were introduced.