6SN7 / CV181 / VT231 / 6H8C Tubes
The 6SN7 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
The 6SN7 tube was common in the 1940's for a decade or so but was replaced by "modern" tubes such as 12AU7 and 6CG7 designs that were physically smaller and less prone to noise and microphonics due to vibration. By the late '90's the 6SN7 tube made a resurgence in high end hifi audio largely because of its rich, warm, "classic" tube sound. It is still fairly popular today and found in headphone amplifiers, preamplifiers and input stages of some power tube amplifiers.