by John Atwood
Born just before America’s entry into World-War II, the 6SN7GT dual triode had a precocious childhood, becoming one of the most common tubes by the end of the war. Despite competition from younger, smaller upstarts, the 6SN7GT was widely designed into consumer, industrial, and military equipment throughout the 1950s, and as a replacement tube was extensively manufactured in America until the end of its tube industry in the mid-1980s. Equivalents of the 6SN7GT were widely used in the USSR and China, and are still being manufactured there.
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.
According to Ludwell Sibley’s Tube Lore, the 6SN7GT was registered with the RMA on March 3rd, 1941, with data from RCA and Sylvania. RCA was likely the developer, since it had assigned a development number of “A4273B”. The immediate predecessor of the 6SN7GT was the 6F8G, developed by RCA in 1937. The 6F8G was a dual version of the 6J5, which was a slightly “hotter” version of the 6C5 of 1935. The 6F8 had the larger “G” (or ST-12) bulb with the troublesome grid cap for one of the triodes. The compact straight-sided GT “Bantam” types, introduced by Hytron in 1938, had become the rage by the early 1940s, and the 6SN7GT was the natural evolution of the unwieldy 6F8G. Another predecessor, at least in nomenclature, was the 6N7, a dual triode intended for push-pull class-B operation. It really wasn’t an electrical equivalent to the 6SN7GT, though, because of its high mu (35) characteristics, high plate resistance, and common cathode connection.
What’s with the “smoked” glass? Most 6SN7GTs made through the early 1950s had a graphite coating on the inside surface of the glass bulb. This served to drain excess accumulations of electrons that could affect the operation of the tube. Why this was dropped isn’t clear, but probably was due both cost-cutting and the use of a different type of glass that was not prone to electron accumulation.
Sylvania and Raytheon came out with a loctal version, called the 7N7, which, interestingly, was registered in 1940, well before the 6SN7GT. Primarily due to the non-acceptance by RCA (the Microsoft of its time), the loctal line, though technically superior to the octal types, withered away in the late 1940s.
The 6SN7GT came just in time for America’s involvement in World-War II. The heavy usage of the 6SN7GT wasn’t in conventional radio equipment, but rather in radar. Radar had many pulse-handling circuits where the low plate resistance and compact footprint of the 6SN7GT was ideal. Most of the wartime American radar sets used dozens of 6SN7GTs. By the way, many 6SN7GTs made during the early years of the war were marked only “VT-231″, the Army Signal Corps designation.
The glut of war-surplus parts after the war included incredible numbers of 6SN7GTs. Just as the war wound-down, two brand-new industries became heavy users of dual triodes: electronic computers and television. 6,550 out of the 18,800 tubes in the ENIAC computer were 6SN7GTs. Early televisions had up to six 6SN7GTs each. And, of course in audio applications, from movie theater amplifiers to mixing boards to P.A. amplifiers to the early HiFi amplifiers, the 6SN7GT was widely used.
By the end of the war, it became clear that the regular receiving tube construction and manufacturing procedures were inadequate for military use. The military wanted both ruggedness and high reliability, as well as tightly-controlled electrical characteristics. One of the first military types was Sylvania’s 6SN7W with its unique metal shell base. (Note the lack of “GT”!) This had a stiffening rod added to the otherwise conventional structure. By the mid-1950s, most tube manufacturers made a 6SN7WGT type with the extra stiffening and often thicker micas, as well as a low-loss micanol base. The last versions were basically receiving tube construction that were tested and sorted to tighter specifications.
In 1948, RCA came out with the “Special Red” line of ultra-reliable tubes. Rather than just a slightly modified receiving tube, the Special Red types were designed from the ground-up with high purity materials, extra rugged construction, premium processing, and thorough testing. The 5692 was the Special Red version of the 6SN7GT, and was made by RCA and CBS-Hytron. Only the RCA tubes used the red base. Although the construction is impressive, the audio quality is nothing special. Don’t forget – ruggedness does not correlate to good sonics!
Several variations of the 6SN7GT were developed by RCA. A 12 volt heater version, the 12SN7GT came out in 1941 and permitted use in 12 volt vehicles and in 0.3A series-string sets. This was later upgraded to the 12SN7GTA, to match the power dissipation increase in the 6SN7GTA. Some mobile and aircraft radios during the war were designed to be run with a plate supply of 28 volts, eliminating the need for a dynamotor or vibrator high-voltage supply. However, conventional tubes behaved erratically at this low voltage. In 1946, the 12SX7GT was introduced as part of a series designed for 28 volt supplies. These were the same design as the conventional types, but were specially processed and tested for low-voltage operation.
The main competitor of the 6SN7GT after the war was the 12AU7 and its variants. A dual version of the 6C4 triode, the 12AU7 was developed by RCA in late 1946. It used half as much heater current and had a smaller footprint than the 6SN7GT. RCA and GE (having bought Ken-Rad in 1945) heavily pushed miniature tubes after the war, and many miniatures were designed-in where octals or loctals would have been the natural choice. The higher cathode emission and plate dissipation of the 6SN7GT kept it popular, though. The combined maximum plate dissipation was upped to 7.5 watts in 1950 in the 6SN7GTA. In 1954, the 6SN7GTB added controlled heater warm-up time to make it more reliable in series heater-string TV sets. In 1954, RCA came out with the 6CG7, which was pitched as a direct equivalent of the 6SN7GT. Although audiophiles found that the 6SN7GT typically sounded better, this was the beginning of the end for widespread usage of the 6SN7GT. The 6CG7 was unique in having a shield between the two sections. This was removed as a cost-cutting measure in the 6FQ7.
The 6SN7GTB continued in high production in America as a replacement tube. As the tube companies started dropping out one-by-one, production was concentrated in fewer factories. The last known American production was the 6SN7WGTA by Philips ECG (formerly Sylvania) in 1986. Production of 6SN7 types in Japan and Western Europe also wound down by this time. The 1980s were the “dark ages” for tube technology – it was considered hopelessly obsolete, and the production machinery and tooling was simply scrapped. A replacement market still existed, and some 6SN7GTs came out of the USSR, marked as “Made in Holland” or “Made in England” to get around cold-war import restrictions.
Although the 1980s were the dark ages for tube production, new interest was growing in tubes for high-end audio amplifiers. Enthusiasts started searching for N.O.S. tubes at ham radio swap meets and at garage sales. Then China opened up to the west and the Berlin wall fell. Both Russian and Chinese tubes, still in production for military purposes, started to come into the Western markets. As the demand for audio tubes rose, the former cold-war factories concentrated on the popular types, and the 6SN7GT was one of them. Most new 6SN7GTs are from Russia or China, although some are made in Eastern Europe as well.
Barbour, Eric, “6SN7: Driver of Choice”, Vacuum Tube Valley, Issue 11, Spring 1999. Extensive description of the history and different variants of the 6SN7.
www.tubedata.info – Frank Philipse’s huge collection of tube data sheets online.