Tubes are fully tested on L3-3 precise tube tester & matched in pairs. Tubes were originally designed for mil. applications. OTK stamp. Manufactured in Voskhod - Kaluga. Years of production: 1976 - 1977. Long life -E, high reliability -B, tubes. Gold - platinum grid. Pure cooper leads to the pins. Not less than 5000 operating hours. Upgrade for your Little Dot, Little Bear and other amps.