Elektronenröhre
Vacuum tube or "valve" projects are common in audio DIY. While, for mass market audio components, the vacuum tube has been replaced in modern times with the transistor and IC, the vacuum tube remains prominent in specialty high end audio equipment. Thus, interest exists in building components using vacuum tubes, and the vacuum tube is still widely available. There is a wide variety of tubes manufactured nowadays, and many tubes on the market are advertised as "NOS" ("new, old stock", meaning unused stock of old manufacture); not all of the latter being genuinely NOS. Circuits utilizing tubes often are far less complicated than those utilizing transistors or op-amps. Tube enthusiasts often use transformers, sometimes custom-made ones, or even hand-wind their own transformers using cores and wire of their own choice. Note that vacuum tube projects almost always use dangerously high voltages and should be undertaken with due care.