Even though they’ve been only introduced at the dawn of World War II and oft used to drive power tubes much older than them, are the 6SL7 and the 6SN7 family of small signal thermionic vacuum tubes the most venerable of their kind?
By: Ringo Bones
Even though they have became popular to the point of ubiquity during the thermionic vacuum tube amplifier revival in the hi-fi scene of the 1990s and were often asked to drive output power vacuum tubes that were developed between the 1920s and the 1930s like the PX25 and 300B output power tubes and the 211 and 845 transmitter power tubes pressed into audio use, though all are directly-heated triode types and the 300B is probably the only one of the tubes specifically designed for audio amplification use. Historically, the 6SL7 and the 6SN7 family of small-signal thermionic vacuum tubes were originally released in 1939 and were officially registered in 1941 as the glass-cased 6SN7GT. During World War II – a slightly improved 6SN7 was developed then a more rugged 6SN7W was also developed a bit later for military use.
At the height of World War II, the 6SN7 was one of the most important components of the first programmable electronic digital computers, the ENIAC, which contained several thousand 6SN7 tubes. The SAGE computer systems used hundreds of 5692 – an ultra high reliable version of the 6SN7 famous for its bright red base and much often used for missile and space exploration applications at the height of the Cold War and the USA versus USSR Space Race – uses hundreds of 5692 vacuum tubes as flip-flops, also known as RAMs.
Major Western consumer electronic manufacturers considered the 6SL7 / 6SN7 family of tubes virtually obsolete around 1964 and ceased its production because the more modern 12AX7 vacuum tube was starting to replace it in most consumer electronic applications. Though the US Department of Defense and the then NACA and then NASA were still using ruggedized military spec ultra reliable versions of the 6SL7 / 6SN7 family of tubes around the mid 1960s, they probably building up their stock of these types of tubes via existing – but slowly dwindling – NOS or new old stock supplies and sources. The renewed interest of single-ended triode zero negative feedback audio amplifiers in Japan during the start of the 1970s and the single-ended triode vacuum tube hi-fi amplifier revival of the 1990s eventually triggered a renewed interest and remanufacture of such family of tubes.
Even though the ultra reliable ruggedized military spec American NOS 6SL7 and 6SN7 vacuum tubes famed for their unmatched sound quality probably only became widely available to post-Soviet Russian tube hi-fi amplifier enthusiasts way after the disestablishment of the US Strategic Air Command back in June 1992, such small-signal tubes are indispensible in the construction of zero negative feedback single-ended triode amplifiers.
The 6SL7 tube is an Octal based high mu double triode with a 6.3 volt heater. Like the 6SN7 tube, the 6SL7 tube has been around for a long time – since 1939 in fact - and there are quite a few versions to choose from. The 6SL7 would be used in similar applications to the 12AX7 – or to those in the EU, the ECC83 – as phase splitters, low-level preamp stages, etc. Linearity is good but as with other high mu vacuum tubes, large voltage swings aren’t possible. As a driver, the 6SL7 will perform better than a 12AX7 tube but not by much because its anode impedance is too high.
Although its amplification gain is slightly less than that possible from a 12AX7 tube, a stage gain of around 50 is available from the 6SL7. Generally, microphony and excess noise is low but as its gm is low at 1.6 milliampere per volt, moving coil input stages aren’t really practical. There are military versions of the 6SL7 which are ruggedized, ultra reliable and of course ultra expensive. The 5691 vacuum tubes are the missile silo / Cape Kennedy version of the 6SL7 tube and is regarded as the best example. Used in a lot of older American hi-fi audio equipment, the 6SL7 has a warm but very clear sound. The later remanufactured versions of the 6SL7 have a different presentation compared to older types as with the sister 6SN7 tube. Modern hi-fi amplifier manufacturers who use the 6SL7 tube include Cary and Audio Note in their single-ended triode zero negative feedback power amplifiers.
The 6SN7 (the B65 tube is the British version) is an Octal based double triode with a 6.3-Volt only heater, the 6SN7 ranks along with the 12AX7 / ECC83 as one of the most popular tubes of all time. A quick flick through the Radio Designer’s Handbook show its pages littered with 6SN7 tube based designs. It is one of the most linear tubes of its class. A gain in the region of 15 for one stage is quite normal.
Generally, the noise and microphony of the 6SN7 tube are low making this tube suitable in preamps as well as power amp driver stages and most competent audio designers would use it in place of a 12AU7 / ECC82 tube in almost every application. The US Department of Defense used 6SN7 tubes by the truckload and had special versions made. There is even an ultra high reliability version for telemetry kits to analyze missile trajectories and space exploration applications during the 1950s – like the 5692, famous for its bright red base, but don’t try to buy any unless you’ve got a bulging wallet!
The sound quality of most 6SN7 tubes is excellent, but like wine, it tends to get mellower with age. The later 6SN7WGB or STC types have a lean, clean sound. Earlier manufacture versions of the 6SN7 have a slightly softer but all have a very open, natural sound quality. The best known application of 6SN7 tubes during the Golden Age of Stereo was the British Williamson amplifier which used GEC B65 tubes.