"Brian Gaff" wrote

In some later television mags, when valves were being phased out, some
enterprising folk actually made valve replacements circuits using
semiconductors on a valve base!

Plug-in semiconductor equivalents for valves were produced by the major
manufacturers for many years and widely used in professional and industrial
equipment.

As I recall, one flashover and they were history.

Only if they had been incompetently designed.

David.