The 5690 is a dual rectifier similar to a 5U4, but not exactly like any commercial vacuum tube. Part of the special “Red Base” series, it came out after the 5691, 5692, and 5693 so not many people have heard about it. The 5690 is so unique it even has a special larger base. It is a dual rectifier, literally - two completely separate diodes with completely separate heaters. With triple micas, 4 “D” foil getters and built like a tank, this is a special tube for that special project where you want absolutely the best rectifier. These are in original military boxes and all have date codes from 1966.