The 6SN7 Double Triode

In what are often claimed to be the ”good old days” – in this specific case, the years before 1932 – the design and usage of tubes was still quite a simple affair. In general, a tube was simply, well, a tube – one that usually functioned as a triode system. This principle applied to the vast majority of electron tubes used back then. There were few exceptions, such as the one-of-a-kind multi-unit tubes from the Germany radio company Loewe. Looking back, these can actually be considered to be the first real ICs. But these were, at best, exotic beasts intended for rare applications in special devices. In fact, they often served to ”encourage” customer loyalty since a given device could only be operated with one of these special tubes, thus excluding any competing suppliers right from the start.

 

 

Initially – aside from military applications – standard broadcasting and thus civilian radio sets represented the largest and most important market for tubes for a long time. Then, in the early 1930’s, some new and dynamic factors arrived on the scene. ”Talkie” films, high fidelity and television were the buzzwords of the day. In fact, the first two had already become reality, even if still only taking baby steps. The third was still in its test phase, yet showing great promise for the years to come. Furthermore, the devices kept getting smaller, thus requiring even more compact active components. The sound-film industry was largely responsible for stimulating development in the area of power tubes and loudspeakers, while broadcasting and television were focused on the need for more greatly integrated tubes (more systems within one glass unit). These had to be small, easy to use and have readily determined levels of energy consumption.

 

The idea of combining several triode systems into one and the same tube (with each system eventually having its own heating system), actually arose very early in tube development, when, in 1909, Lee De Forest fitted his ”Audion” (then still without a socket base, with open connections at each end) with two heaters. Other manufacturers, such as Cunningham and Moorhead, expanded on this concept in an attempt to extend triode service life. For instance, when the first heater wire (the ”cathode”) eventually ”burned out”, a second, battery-powered one was connected to the system. The true inspiration toward having two separate triode systems within one tube was actually based more on the previously mentioned multi-unit tubes (such as those used by Loewe, TeKaDe, Nelson, Emerson and Speed). There’s no doubt that it was also motivated by the ”twin triodes” that had already become well established in the 1930’s in the United States and England. These were used for Class B amplification and battery operation (notable examples are tube models 19, 53 and 79).

Then, in September 1937, the first true double triode appeared on the scene – the 6F8G from RCA. With this tube, still encased in the conventional ST glass body dating from the late 20’s and early 30’s, one of the two systems still had its grid connection located outside on the top of the glass body, where it was easily accessible for connecting a cable and a grid cap terminal – a last vestige of radio design from the 1920’s/30’s.

When it came to newer, more refined developments for both the civilian and military sectors, the market of the day was demanding a double triode with a moderate transconductance and high linearity. After all, the heater voltage, standardized at 6.3 V in 1934, had to be moderate in its current consumption and the form of the device itself had to be as compact and small as possible. In the late fall of 1939, just a few weeks after the start of World War II in Europe, RCA Radiotron brought out the 6SN7 octal (8-pin) double triode on the American market under the designation 6SN7GT. This event marks the birth of the true common ancestor of all modern double triodes. All double triodes – whether with 9-pin noval sockets or 8-pin octal sockets – that we are now familiar with and use in today’s high fidelity can be directly traced back as evolutionary derivations of the technology of the 6SN7GT. No matter what direction individual developments took – toward greater transconductance, higher µ (µ = amplification or ”gain”), shielding between systems or varying combinations favoring one or more over the others – in the final analysis, they are all variations of this same basic concept.

At about the same time the 12SN7GT, with the same specifications, came out along with the special 12SX7GT for use with the 12.6 V heating, common in many military devices and modified to use very low anode voltages. In particular, the last one was intended to be run off batteries and for use in the 26 V on-board networks used by the US Army Air Corps, the forerunner of the United States Air Force. To ensure reliability, the 12SX7GT was subjected to extreme burn-ins and developed to have high microphone resistance – really not a bad idea at all when you consider the high vibrations constantly present in prop-driven, multi-engined, long-range bombers.

 

A Younger Brother is Born

In 1941 came the 6SL7GT with a much higher µ factor and correspondingly less current. Both tubes were essentially a success right from the start, especially in military applications. After all, given world events at that time, the military would remain the dominant market sector in the coming years. The octal double triode passed its acid test with flying colors. The extreme demands of wartime applications resulted in great technical precision and ruggedness right from the start of series production. This early production phase, in full compliance with military standards, produced outstanding results that were never bettered in later years of production. Around 1940, RCA Radiotron, Sylvania and Tung-Sol were the largest manufacturers of the 6SN7 and 6SL7 double triodes. Sylvania initially tried to interest military and civilian customers in its loctal base design, a variant of the octal base, but didn’t meet with much success. That’s actually too bad since the symmetrical design (pin geometry) of loctal bases and sockets does permit some very elegant wiring. Shortly thereafter, on the other side of the Atlantic Ocean, Mullard and soon also MOV brought out their ECC30 series of octal triodes on the UK market. Almost without exception, these early English octal double triodes proved to be rather distinct units, which, even though they were technically based on the two American double triodes, could not serve as direct equivalents for them, at least not for the 6SN7GT.

When World War II came to an end, the focus of production returned to the civilian sector, with an emphasis on the then newly emerging entertainment technologies. The war was over and won; people wanted sources of amusement and distraction and to enjoy being consumers again after enduring the rationing and privations of the war years. It was then that television and high fidelity sound systems became the driving forces behind technical developments in electronics.

 

 

From 8 to 9 –

From Large to Small

Miniaturization of electronic components continued unabated, finally resulting in the 9-pin noval-base tubes that appeared in 1947/48. These were known as the 12AT7, 12AU7 and 12AX7 in the US and, respectively, as the ECC81, ECC82 and ECC83 in Europe. The relatively tiny noval-base double triode was born. It was much smaller and lighter, used less material and came in greater technical variations suitable for more applications. The 12AU7 was supposed to serve as a miniaturized 6SN7 and take its place in circuit boards and amplifiers. But since their specifications were not really identical, the 6SN7 was simply made smaller and installed in a noval-style glass body – Voilà, the 6CG7 had suddenly arrived! The additional ninth pin in the miniature base was used to create a shield between both of the double triode systems, which were now, of course, packed in much more closely together. However, this internal shield soon fell victim to hyper-vigilant bean counters. And that’s how the ”new and improved” 6FQ7 came about. At virtually the same time, production of the European counterparts, the ECC8x series, was started at Philips.

With the appearance of the ECC81, ECC82 and ECC83 and their American equivalents, we now have finally arrived at the ”modern” version of the double triode. All later versions, even the ECC88/6DJ8 and their (great-) grandchildren are really just technical variations of those first three ECC8x models. Just like the first noval double triodes, in terms of R&D these are also direct descendants of the venerable octal 6SN7. It was with the 6SN7 that tube technology took a big step forward into the modern era. It represents nothing less than a technological quantum leap in miniaturization and simplification. Another, similar leap would come only 15 years later with development of the transistor.

 

 

The Long-Distance Runner

in Tubes

Now that we’ve taken this short side trip over to noval tubes, which really do deserve their own separate story, let’s get back to the octal double triodes. Both high fidelity and certain television applications required compact double-triode tubes with high linearity and low distortion. The fact that the 6SN7 was and is considered to be the ideal voltage amplifier (the ”studio standard”) ensured that this tube would be produced for an unusually long time. In fact, it was only in 2005/2006 that the US armed forces finally released their supplies and sold off their huge reserve inventories. The resulting sale fueled the surplus tube market in a massive way. Among the tubes disposed of were tens of thousands of 6SN7 and 6SL7 double triodes that had been made in the US during the 1980’s. Since the 6SN7 is still being made to very this day (in Russia and China), this father of all modern double triodes used in high fidelity amplifiers has now been on the market for over 70 years. Only very few other tube models have been produced for such a long period of time. Driven by different specifications standards from the US military (army, navy, army air corps) there were, in addition to the two ”Joint Army Navy” (JAN) designations used during WWII, some other designations for the first octal double triodes.

The early production runs of the 6SN7GT for the US military bore the designation VT-231 on their bases. Similarly, VT-229 was the military designation for early 6SL7GT units. Some manufacturers, such as Sylvania, concluded their own separate contracts with the armed forces and built variants of the 6SN7 with special mechanical and/or electrical features for military customers. Some common examples of these are Sylvania’s 6SN7W and 6SN7A built for applications in the US Navy. Some of these early versions have metal collars around their bases, had extremely large getter deposits and special internal reinforcements to increase their shock resistance. Now and then, some units with a shield coating on the glass tube body even turn up.

After World War II, the VT-231 and VT-229 JAN designations gradually disappeared from the bases of the military versions of the 6SL7GT and 6SN7GT. At first they were still present in small print. Then, in the 1950’s, both original JAN designations finally disappeared completely. An additional prefix identifying the OEM manufacturer of the given tube generally preceded the actual JAN designation itself. Here are some common examples: CKR stood for Ken-Rad (Kentucky Radio), CNU for National Union, CRP for Raytheon, CRC for RCA Radiotron and CHS for Sylvania.

 

 

The Sonics Factor

For today’s audiophiles, the tubes from the early production years are the most desirable ones. That’s because the 6SN7 and 6SL7 tubes from that period, especially those made during World War II, have certain special qualities. In addition to rugged mechanical construction and very good levels of vacuum, many of the military versions of both double triodes exhibit high levels of production consistency. In general, at the start of a given product cycle there seems to have been a great tendency toward favoring a very rugged design with over-sized components. The tubes were made of very good materials and, to ensure initial product success with no early failures, they were made in a production process emphasizing high quality, both of which increased costs. In fact, the military versions of models 6SL7 and 6SN7 from the 1940’s represent an unparallel period of high quality during the entire production history of these tubes. In 1948, RCA even came out with models 5691 and 5692 (part of the famous ”red-base” series), which are special versions of the 6SN7 and 6SL7 designed to further meet the extreme static and dynamic forces encountered in avionics applications. In all other cases, in the years after the war the production of both octal double triodes and their derivations after seems to have been focused on making them as inexpensively as possible. After all, the civilian market sector didn’t require such high quality.

So, why not cut more than just a few corners?

 

The Quality Factor

Starting in the early 1960’s, production of tubes for the consumer sector became unprofitable and only production of replacement parts for military use remained worthwhile. Exclusively militarily-oriented production, which continued until the late 1980’s, is characterized by its extremely high quality. From the earlier production runs, more exactly those of the 1940’s, tube fans have collected ”round plates”, ”black plates” and ”smoked glass” of the VT-231 and VT-229 models from Tung-Sol as well as Sylvania 6SN7W/6SL7W models with extremely high getter-deposit and metal collars around their bases. Such rarities also include the VT-231 and VT-229 RCA Radiotron tubes with ”coated/smoked glass” and ”black plates”.

Tung-Sol and RCA made their early military versions with tube-like anodes, a design which favored consistent emission. In addition, the round design permitted very precise production of these systems. Beyond that, these tubes were able to withstand the impact of high mechanical forces (shocks, constant vibration). Strictly in terms of performance measurements, you’ll hardly find any differences between these and later production runs with tack-welded metal bases and ribbed design. However, when it comes to the sonic performance, there is said to be worlds of difference between the early VT-231/229 models and those from the production runs of the late 50’s and 60’s.

For a long time, Sylvania maintained high-quality production in the US. Even in the early 50’s, the company brought out models capable of producing astonishingly high sonic qualities (unintentionally, of course). As a general guideline for those interested in, perhaps even obsessed with, the subject of ”tubey magic”, the following statement applies: The older the production date, the better the tube. In particular, the previously mentioned VT-231 and VT-229 tubes yield really captivating sound quality, noted for its extreme clarity, transparency as well as natural, realistic sound reproduction lacking any unnatural, system-added side effects (”discolorations”). The good symmetry offered by both units is also evident in channel-separated applications, where no problems/conflicts arise. If you look for objective reasons for the presence or lack of the aspects mentioned above, you’ll always end up attributing it to the high-quality production standards and methods: purity of materials, smooth anode surfaces and a very consistent emissions field made possible by tube-shaped anodes along with thick mica insulating washers, numerous mechanical supports, excellent vacuum and a generously sized getter coating.

 

 

It’s a Question of Price – NOS

Given this situation, the price structure on the world market for ”New Old Stock” (NOS) and ”good used” tubes covers a whole lot of ground. In some cases, the prices for genuine, early-production NOS VT-231 and 6SN7W/A tubes have reached several hundred US dollars. In contrast, NOS 6SN7WGTA/B tubes in their original packages from the 60’s and 70’s are worth at most around five dollars. The far rarer British variants – the ”coke bottle” CV181, CV1988, ECC31, ECC32, ECC33 and ECC35 also attract very high prices on the NOS tube market. Lively, animated discussions about the ”sound” of certain tubes can greatly drive up their prices. However, in such cases, it’s often overlooked that these highly touted sonic characteristics only apply to a specific environment and within certain circuitry, not to mention the fact that the subjective perception and personal preference for ”the right sound” are totally dependent on the taste of the respective listener.

 

 

The New Trend: ”Tube Rolling”

The fact that hi-fi freaks on the search for sonic nirvana will spare no efforts and walk or even run down almost any path, is something that we all know all too well from our own experience. Because of easily removed housing covers and easy access to the tubes in use, over the last 15 years fans of tube-based audio amplifiers have come up with yet another interesting hobby called ”tube rolling”. This hip, insider American slang term refers to the practice of repeatedly trying out different tubes in the hope of eventually finding ”the one” that makes the amp sound (even) better. It’s become common practice to attempt to optimize amplifier sound by installing ”the right” version or construction of a given tube model. The current price structure for NOS double triodes (and also some pentodes, tetrodes and high-performance triodes) is a direct result of the demand generated by this trend. Although ”tube rolling” may be a relatively new phenomenon in the western ”audio hemisphere”, the practice of tracking down and trying out ”the best” vintage audio tubes has been pursued with great fervor in the countries and city states around the Yellow Sea since the early 70’s. Japan, South Korea, Taiwan and Hong Kong have all imported large quantities of the relevant top-notch, highly regarded NOS tubes from Europe and the United States. Interestingly enough, this is all happening in a time when back here at home the number of zeros after the decimal point for the intermodulation distortion (IMD) specifications of latest Japanese transistor generation is being subjected to close, even ardent examination.

Searching for the right version of a tube for use in the input or driver stage of an amplifier is, of course, a reasonable endeavor. However, it’s important to keep in mind that it’s really nothing more than ”tweaking” of the unit in an attempt to refine its sonic characteristics to meet one’s personal preferences. In fact, when it comes to models of the 6SN7 and 6SL7 it’s simply not possible to try out every version since some models have certain special electrical parameters that greatly limit their wider use. Some of these special versions, especially ECC3x models, have great differences in transconductance, respectively amplification or gain, compared to their American counterparts even though they do have the same sockets and heatings. Similarly, not a single Mullard or MOV ECC-31, 32, 33, 34 or 35 is really the same as an apparently comparable 6SN7 or 6SL7. Differences in gain or grid capacity, the inverse amplification factor or internal resistance will be perceived as changes in sound, for better or worse, by users. However, in reality, they only represent shifts and changes in the operational bias settings.

RCA’s famous ”red base” series, which includes the very carefully manufactured and particularly rugged variants of the 6SL7 and 6SN7 known as the 5691 and 5692 and intended for super-duty avionic and rocket applications, require greater levels of heating current. So, it’s quite possible that they will overload a line transformer or draw unregulated heating power supplies way too far down! At the same time, they’ll also result in significantly lower maximum values for anode voltage and power dissipation. This means that without, at the very least, knowing the maximum heating current or anode voltage of the tube under consideration, ”tube rolling” can produce extreme distortion and, in the worst case, will damage the amplifier. The different suffixes after the model designations on American octal double triodes – GT, W, A, GTA, GTB, WGT, WGTA, WGTB – each stand for certain design features and maximum permissible parameters. As a rule, manufacturers of tube amplifiers select basic tube models and then optimize their circuitry for use with them. Before trying out another tube that supposedly produces better sound than the one that originally came with the amplifier, it’s a good idea to use the specifications for the original tube as a reference point to avoid unpleasant, even expensive, surprises.

 

 

They’ll Never Grow Old

The 6SN7 and 6SL7 are classics in the very best sense. The fact that both of these octal double triodes are still being used in new high-end amplifiers as input and driver tubes 70 years after their start of production has nothing to do with nostalgia or not knowing any better. It’s simply because they are tubes of outstanding design that remain the ideal solution for meeting the operational requirements of high-end amps. Good sound comes from mechanically and electrically proven components, regardless of their design vintage. That’s what it all really comes down to.

 

D. H. Kurt