The VT-37 is a 5-pin vacuum tube triode quite common in pre-war radio sets. Four years ago, I built a very simple stereo preamp around the VT-37 that I listen to every day (pretty much all the music I listen to at home passes through this preamp.) I noticed that this tube makes a hell of a racket (quiet, but dense) while warming up. The glass and filament expanding from the rising temperature make a cacaphony of clinks and klangs.