I remember reading (probably only on Wikipedia, for what that's worth) that the Germans poineered the use of magnetic amplifiers for these machines as valves couldn't be counted on to work propperly under the heavy acceleration and vibration G-forces of a rocket. Mag amps are fascinating things in their own right - I'd like to do some more tinkering with them one day.
On the other hand, I believe that the radar proximity fuses manufactured in America did have valves in (ableit specialy designed ones) - and they would get fired out of guns, so it must have been possible to make them sufficiently robust.
It's tempting to think that you should be able to recreate this kind of stuff yourself in the shed 70 years later, but I don't know... The engineering these pople came up with just blows my mind every time!