These are the super military low noise equivalents of the 6SL7 hi mu octal preamp tube found in many vintage Ampeg guitar amps as well as OEM tubes in the new Ampeg Heritage series B15N reissue. The octal equivalent of a 12AX7, these are low noise, low in microphonics with sweetness in the entire audio range. Many modern hifi and guitar designs are using these tubes now. Two sealed cases available. New old stock for the price of current production.