It's not done very often. Most tubes are available at reasonable prices. Transistors require different biasing and operate on lower voltages, higher current.
There are some cases, such as 1L6 in Zenith T/O's, where tubes are scarce. Transistor modules are being made as replacement for this tube.
Norm
:since most of yall are into restuffing caps for an original look. do people ever put a transistor a tube socket bottom soldered to the pins but leave the rest of the tube there with the light working for looks also. thank you.
T.
:I don't see why someone doesn't just manufacture the tube. I suppose it is easier to work with common transistors.
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:T.