You can also speak with Joe Fratus of Art Audio. He's a great guy and excellent source of info about all things SET and the tubes for them. he's at www.artaudio.com or tel # 401-826-8286. It is my understanding that due to the operating parameters of circuit design, some manufactures choose to run the tubes at "full tilt boogie" resulting in a shorter usable life span. Considering the cost of these tubes if you want N.O.S., this is an important factor. Happy Tunes!
