NOS = New Old Stock
Even though these tubes come from the heyday of tube technology and have been around for several decades, they are actually brand new, as tubes do not age when stored properly. NOS tubes are often considered to be of high quality, both in terms of sound and durability. There are several reasons for this, most of which are related to production techniques and materials used. For example, modern production tubes cannot contain lead in the glass in order to be classified as "RoHS-compliant." Additionally, the expertise may no longer be available, as the technicians from that time have since retired or passed away, taking their knowledge with them to the grave.
It is fascinating to see (and hear) how these "historic" tubes can still impress with their unparalleled quality and characteristic sound to this day.
