Sub-Miniature version of the 12AU7 valve (vacuum tube)
There is no significant difference between the ECC82 and the 12AU7 - it's just different nomenclature for the different markets. As for the heater voltage, the two heaters can be connected in series (12.6 V @ 150 mA) or parallel (6.3V @ 300 mA). Parallel is more common since most mains transformers for valve designs have a 6.3V secondary winding. I have no idea whether a sub-miniature version is available.