New Old Stock (NOS) tubes were built for quality, durability, and reliability. Not only used in audio equipment but in numerous industrial applications which required a zero-tolerance for failure. For example, both the US and Russian space ventures at the time required the use of tubes for computers, communications, navigation, and rocketry. The first computer-guided missile systems were all tube-based circuitry. The entire nation, civilian and military, relied on tubes that far surpassed the quality of modern production tubes. That legacy is the stuff that NOS tubes provide. An affordable modern equivalent of the same build quality is probably only possible in Asia with their lower production costs and still the retail consumer price would be equal to or greater than those expensive NOS tubes we crave. We are indeed fortunate to have a substantial quantity of NOS tubes to rely on for our audio equipment.