6S31B (6C31Б) is a high quality russian subminiature triode of improved reliability, designed for amplification of lf voltage. This tube is a part of legendary microphone LOMO 19A19 and also OKTAVA MK-13. Service life guarantee: 2000 hours. Tubes were produced in the 60's and 70's. New old stock. Manufacturer: Nevz, Novosibirsk.