The 6SN7 vacuum tube is a dual-triode preamp tube with medium gain. It was a commonly used tube from the early 1940's into the late 1950's. It was then generally replaced with smaller preamp tubes but was rediscovered in the 1990's where it became popular in the hifi audio world. Other western equivalent or substitutions are 6SN7GT, 6SN7GTA, 6SN7GTB, CV181 and VT231. In Russia and China these are known as 6N8C or 6H8C...More Details