Russian power pentodes GU-50 (ГУ-50). These tubes are commonly used in Hi-Fi audio amps, both in single-ended and push-pull modes. The GU-50 is a direct replacement for the Telefunken LS50 tube. Pairs were selected out of a bulk of tubes, which were measured with the precise RoeTest V10 tube tester (plate current matched tubes). New in box. Years of production: 1987-1989.