The ECC82 (also known as the 12AU7) is a lesser used preamp valve. They're actually, conveniently, two small valves in one glass shell, with two symmetrical sides. They have a maximum amplification factor of 20 (contrast to the typical ECC83/12AX7 factor of 100), making them ideal for more hi-fi applications where distortion isn't desired. They can also conveniently replace 12AX7s in preamps to lower the gain and cause the distortion to occur deeper in the preamp circuit, causing a different gain texture.
ECC82s are most commonly used as output valves for very low output circuits (for example headphone outputs) and phase inverters to prevent excessive, messy distortion.
JJ preamp valves are one of the industry's favourites due to the high amplification factor, the low microphony and noise to signal ratio, as well as being both durable and economical.
It is advised against placing a JJ ECC82 in a cathode follower valve position due to the vulnerability of the filament to high cathode voltages.