What are the differences between pre-transistor and transistor era jet engines? What are the differences between pre-transistor and transistor era jet engines? Towards the end of the second world war, most of the air powers had started development on jet engines. From what I understand, all of this predated any practical application of transistors. The transistor era obviously gave way to integrated circuits and microprocessors. On any piston engine I can usually spot the evolution of components because I'm well versed in the operation of piston engines. For jet engines it is less obvious to me, there are for example differences between turbofan and turbojet engines. I superficially understand the difference between them but I've never even operated one, much less been hands on with it.

What are the significant changes from pre-transistor to transistor-era jet engines? I've never seen vacuum tubes used on jet engines so it doesn't seem like it is just a matter of "they replaced the tubes".
The vibration levels on jets preclude the use of vacuum tubes. They may work if everything is fine and all rotating parts are well balanced, but once you have a defect and an out-of-balance condition, the electronics would fail quickly, just when you need their help the most. Especially the afterburner adds a lot of vibration when it works as designed already.

Earlier jet engines used hydraulics for adjusting things like compressor vane angles (litte stationary "wings" positioned between the rotating discs of the compressor, which help to convert rotational flow energy into compression) and the nozzle size. Like on piston engines, the amount of adjustment was much lower back then, and all adjustments were done mechanically.