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The 6SN7 tube is a medium-mu (think of mu as a "gain" in tube's term) twin triode in an octal based package, usually glass, although some metal envelope types were made. In normal use it operates as a class A amplifier. It was widely used in the early days of television as the vertical amplifier, and it's use was so common in most TV chassis designs that vintage 6SN7 tubes are still fairly easy to find today. Audio designers soon found it made a great audio preamp with it's large plates, ample power reserve, and low microphonic octal package. The GTA and GTB types, having been fine tuned for demands in television chassis vertical sweep circuits, can handle up to 7 watts maximum plate power!

The demands for these tubes today are primarily in vintage and recent design audio amplifiers and preamplifiers of the high-fidelity type, up to and including the most expensive and esoteric. It seems there is a 6SN7 tube for every taste and budget, and some of the vintage versions are being hunted to extinction, especially those made for the military. I will try to explain the differences between some of these tubes, and list specifically what I have in stock. I expect some of these tubes will skyrocket in price worldwide as more and more audio designers find out that this preamp tube is a design that really can't be improved upon....and that the fine vintage tubes just cannot (and probably will not) ever be duplicated by a current production tube.

Three envelope sizes are common in the full octal base version: SHORT, glass is 1.5 inches high; MEDIUM, glass is 1 3/4 inches high; TALL or "TALL BOY", glass is 2 inches high.

VT-231:
The VT-231 was the military purchasing and inventory control number up until about 1945 or so. The tubes were often also marked with the JAN-xxx prefix as well. It seems that the VT-231 number was dropped around the time the WWII effort was winding down.  But there was a lot of variation even before 1945. I have Raytheons made in 1943 which have only VT-231 marked on them, and the same vintage only marked JAN-CRP-6SN7GT.  Raytheon is CRP, Sylvania CHS, RCA CRC, CBS-Hytron JHY, Tungsol CTL, KenRad CKR.  It's also worth knowing that there is a common misconception about those VT-231 numbered tubes. It's nothing more than a number. If you find a 6SN7GT of the same vintage, it will be identical to the VT-231 numbered version. I have here Sylvania 6SN7GT from 1945 identical to the VT-231 numbered tube, and Raytheon 6SN7GT 1944 identical to the 1944 VT-231. Also RCA's, Tung-sols, and so on. There is no magical significance to the VT-231 number. But some people think there is, because salesmen have told them so. It's become popular folklore in tube circles.  It's only so because VT-231 branded tubes are so common today, as the military kept thousands in warehouses collecting dust for decades until some bureaucrat figured out there was no equipment that would ever use them. The equivalent commercial branded tube was never kept in warehouses for decades, and in any event, most production of commercial tubes was severely rationed to keep war production up - so there were few produced with the commercial 6SN7GT branding.

6SN7GT / G:
This is the original version of this tube. It may be found in all three sizes of glass envelope. The Sylvania Tall Boy types have two triangular shaped plates facing each other mounted high in the tube, with either a top or bottom getter. The early Sylvania types had a top getter and silver flashed the entire top of the tube, sometimes called "chrome tops". Many brands used this design, but the high mounted triangular plates means it most likely was made by Sylvania. The early RCA "GT" types were usually in either a medium or short envelope and have a grey RF shield sprayed on inside the glass. The GE and Ken-Rad types were similar, but the inner glass coating is deep black. These tubes usually have flat black plates, like large 12AX7 black plates. The military versions of these are the VT-231 and are in very high demand today.

6SN7GTA / GTB:
This is the later improved version, and the improvements were primarily geared toward the TV chassis designers who demanded more max. plate wattage and voltage, where vertical circuits can put up to 1500 volt pulses at 7 watts on the plates. For hi-fi audio use, these improvements are meaningless. These are fine tubes, nonetheless. Most are found in the short glass envelope, but some brands like Raytheon used the Tall Boy style. Sylvania made tubes have the triangular plates mounted lower, and set at angles to each other, although some of the very earliest examples of the GTA types still used the dull blackplates facing each other. RCA used the flat black plates "sideways" to each other, and GE used the grey plates parallel to each other. The Sylvania types in demand have a large top getter patch. The earliest Sylvania GTA types have a green label and heavy top getter flashing extending down the sides of the tube. The early GTB types are similar, but have a yellow or red label. The versions from the 1960s have the getter just covering the top of the tube. All of these have the same black triangular plates and are excellent tubes. The GE types have a side getter and large parallel grey plates, and the RCA used both bottom or side getter. Most other vintage USA brands you find will have been made by these three vendors, and will be one of the above variations, regardless of brand on the label. A few were made with brown micanol bases, the RCA types being very dark brown, the GE types being a lighter, marbled brown. Many audiophiles prefer either the Sylvania chrome tops, the RCA, or the GE side getter over any others, even over the military or redbase types. These are all a great bargain now, and are sure to be rare and expensive tomorrow.

6SN7W / WGT / WGTA:
This type indicates a military spec tube that was not made for consumer use. These usually have extra support posts on the plate structure, heavy mica spacers, and brown micanol bases. These also have the famous triangular shaped black plates. They have been ruggedized to withstand shock, the brown low loss micanol base resists mold, mildew, and fungus growth, and they can withstand heat even if used upside down. The early Sylvania 6SN7W types from the 1940s had a metal collar around the base, then later a black base. These look internally alot like the early Sylvania GTA types listed above. Sylvania made these for other brands as well, including RCA. Easy to spot as they have the triangular black plates facing each other in a short bottle, with a very heavy chrome top flashing extending down the sides of the tube. These are RARE and worth seeking out! The WGT and WGTA types all had brown bases. Only the WGTA types made in the late 70s and 80s reverted back to black bases then coin or wafer bases. These are all wonderful tubes, are long lived, and sound fantastic. No wonder they are fast disappearing from the vintage market.

CV1988:
Another fine military version of the 6SN7GT, this time from Western Europe. These don't often turn up in the USA. The few I have seen have been British and manufactured by Mullard. This tube is usually medium glass, dark brown full octal base, and has a dark grey inner glass coating which hides the plates from view. I suspect these are cylindrical plates, as most of the European 6SN7 varieties have been this type. These are rare, and worth seeking out. Not only do they sound as good as the best industrial 5692, but they have the higher specs of a 6SN7GTB with regards to plate current and voltage, and have the long heater life and overall ruggedness of the US military types. A winning combination!

5692:
This is the premium commercial / "industrial" grade of 6SN7. The most famous of these are the RCA "red base" series, with their distinctive red marbled full octal bases. These are always in the short glass envelope style, and most have four support rods joining the four corners of the top and bottom mica spacers and supporting the plate structure. These have 10,000 hour life filaments and are very resistant to shock and vibration, making them low in microphonics. The RCA versions have the "stop sign" label on the top of the tube with the number 5692 inside, and on the older tubes this was often smeared off, being printed with a chalky white paint. ALL of the red base versions of this tube were made by RCA, regardless of the brand on the label. Tung-Sol, GE, and Raytheon can often be found branded on the label, but RCA made them all. Other brands like CBS/Hytron and Sylvania made brown base versions of this tube, but I suspect all were actually made by RCA, as they are identical inside. These are all really fine tubes, but have been hunted nearly to extinction.  They are worth the higher cost in overall long life and great sound.  (however, some said 6SN7 are fine tube and there is no reason to buy this expensive one!  The judgment is yours, then!)

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