I'm sure there are other valves that do the same thing, but I came across the 13EM7 twin triode, and thought it is pretty cool (6EM7 is the same thing with 6V heaters). It was originally intended for use as a vertical deflection oscillator and amplifier in TVs. The first section is a high-mu amplifier (μ=65), and the second low mu (5.5) but with 10W max plate dissipation. This makes it ideal for use as a one-valve amp - see the Miniblok and 6EM7 SET Vertical Amplifier. Circuit diagrams below. Has anyone used this valve? And would this be suitable for a headphone amp, not needing an OPT?