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. After that time it 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 it's 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.