This is an odd tube. It’s marked Amperex and has Mica & getter structure o Amperex, but at the same time it’s produced on Blackburn factory in UK in 1958. It’s looks like they were outsourcing tube production to various factories even running under the same brand. Sound wise, it’s not Amperex, but a replica of Mullard 12au7 Long Plates. A beautiful sounding tube. Highly Recommended.