Transistors that are used in Fuzz Faces. They're known for being unreliable. Well, back in the 60s at least. They sound smoother and have less gain than Silicon transistors, which are more reliable than germaniums. Jimi Hendrix used to use a Germanium Fuzz Face modded by Roger Mayer. He started using a Silicon on Axis: Bold as Love, until the end of his career.