With the revival of tube electronics for audio, the 6SN7GT became a major player again, due to its availability, and, most likely, due to its inherently good audio qualities. Its high power dissipation compared to miniature tubes made it reliable and able to be run hard. The large internal structure compared to miniature tubes contributes to its low distortion. The existence of many variants and many manufacturers of the 6SN7GT over the years gives lots of types to play with—each with its own sonic fingerprint.
Audiophiles may have their own favorite dual triodes, ranging from exotic European types like the ECC40 to the rugged industrial types like the 5687. But the 6SN7GT is arguably the best general-purpose audio triode. Its large octal format is more rugged and less prone to intermittent pin connections than miniature type. Its mu of 20 is a nice compromise between the old low-mu types and low-current high mu types. The plate resistance is low enough to drive choke or transformer loads. The 0.6A heater current, while not as low as the 12AU7, is reasonable, and assures the cathode has plenty of emission reserve. The fact that it is still manufactured and that there are still lots of N.O.S. tubes available make it possible to design the 6SN7GT into modern equipment without fear of obsolescence.
To put the 6SN7GT into perspective, it does have some flaws. There is no shielding between sections, so unwanted interactions can occur. It is more microphonic that the better miniature types, so it can’t be used in very low-level stages. Most types don’t reject heater hum as well as some specific audio types, such as the EF86, 6J7, 12AT7, good 12AX7s, etc. It is rather large, compared to miniatures. The current-production Russian and Chinese types don’t sound as good as American or European ones from the 1940s, 50s and 60s. But, no other tube combines so many good characteristics.
A Note on Nomenclature: The tube type described here is often called the “6SN7″. Technically, this is incorrect, since all glass American octal tubes released before about 1955 were followed by “G” or “GT” (plus optionally some other descriptors). An octal type without a G or GT was a metal type, and there were never any metal 6SN7s. In the 1950s, the “GTA” and “GTB” types replaced the regular 6SN7GT. Unless the special characteristics of the GTA or GTB types are important to the discussion, the name “6SN7GT” will be used throughout. Interestingly, some European companies made American equivalents, and often named them simply “6SN7″, both for convenience, and more likely, since they didn’t care about the subtleties of American tube naming conventions.