One of the things that I've learned from talking to Tony and my readings on the topic is that most, if not all, vacuum or electron tubes have been designed as general purpose tubes without any specific function (ex. audio, transmission, etc.) in mind. As a matter of fact, a lot of the highly-regarded JAN (Joint Army-Navy) tubes are nothing more than general purpose tubes that were tested to meet military specs. A popular preamp tube, the RCA 12sx7, was widely used in U.S. military avionics before it found contemporary use as an audio tube. Provided that a tube does not possess characteristics that would make it unsuitable for audio use (ex. microphonic, etc.), any tube (including those used for radar, transmission, TV, radio, etc) can theoretically be used for tube audio if someone clever enough can design a circuit that can best take advantage of its characteristics. The deciding factor for which tube to use would be a matter of economics rather than engineering since Japanese demand for so-called audio tubes have caused the prices of these tubes to skyrocket from mere cents until the late 1990's to more than a thousand dollars for the highly-sought-after ones. The trick then is to find a tube that is still well-below the radar and to design and implement an outstanding tube audio circuit for it. This is what Tony has been doing for the growing DIY community.
