The 12AU7 (ECC82) is a Twin Triode vacuum tube, it is very popular in the audio world because it is rather rugged and can be operated at lower voltages. You will find these tubes in vintage amplifiers and organs. They are even used in older vacuum tube volt meters and their life span can reach into decades. The 12AU7 has an amplification factor (µ) of about 17, this is moderate as compared to its cousin the 12AX7 that comes in at 100. For purpose of the headphone amplifier the 12AU7 tube will be used in a common cathode configuration, and the incoming signal will be amplified by approximately 10dB. The 12AU7 is usually operated at plate voltages of over 120 volts, but fortunately it can be operated at lower voltages with decent results.