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The 12AX7 Tube: The Cornerstone of Guitar Tone

Much has been written about the ‘Les Paul’, the ‘Strat’, Fender and Marshall amps. These guitars and amplifiers radically transformed the musical landscape into something energetic and exciting, something electric!—and right there, at the thermionic nucleus of it all, was the ’12AX7′. This ubiquitous vacuum tube was employed by practically every American and British amp manufacturer since the late 1940s; it’s the heart and soul of any self-respecting Fender, Gibson, Vox, Watkins, Laney, HiWatt, Marshall, Boogie or Soldano amplifier and the wave of boutique guitar amps that hit the market more “recently” in the 1990s. In fact, it could be argued that this glowing glass bottle of electrons is the most versatile tube ever made and it’s what made rock and roll possible. So isn’t it about time we showed the 12AX7 a little appreciation? In order to, ahem, forgive the pun, rectify this glaring oversight here’s an article dedicated to the 12AX7—the cornerstone of electric guitar tone!

The First 12AX7

The 12AX7 was developed (development number: “A-4522”) by Radio Corporation of America (RCA) at their Harrison, New Jersey factory. The factory site has a long and fascinating history in vacuum tube manufacture. Edison Light Works (who eventually became General Electric) had been manufacturing light bulbs there since 1882, just six years after Edison perfected the incandescent bulb. RCA purchased the factory from Edison Light Works in 1930. Ever innovative and looking to the future, RCA turned the Harrison plant production lines from lightbulb manufacture to radio tubes, growing the site to become a twenty-four building complex covering 650,000 square feet. Over the course of its history this factory alone produced over three billion radio tubes.

edison_lamp_company_factory — Edison Lamp Company, Harrison, East Newark, New Jersey.

RCA registered the 12AX7 in the Radio Manufacturers Association’s “Electron Tube Registration List” on 15^th September 1947. The new tube also appeared on page 155 in their Receiving Tube Manual ‘RC 15’ in the same year. The 12AX7 was their baby. But they did need a little help from Sylvania and GE (General Electric) to deliver it to the world. RCA supplied the design engineering expertise, and Sylvania’s mighty manufacturing plants supplied the production capability, and materials know-how. It was not a uncommon arrangement to utilise other equipment manufacturers: There are many “RCA” tubes that were designed under patents owned by RCA, but manufactured by Sylvania or GE.

RCA Receiving Tube Manual RC 15 — The 12AX7 and it's medium-mu brother, the 12AU7, both made their first appearances in the *RCA Receiving Tube Manual* in 1947.

On the 12AX7’s release there was no big announcement, no fanfares, no ticker-tape parade. The 12AX7 didn’t even get a passing mention in the “RCA Review“, or any of RCA’s other sales and advertising magazines. And there’s virtually nothing to mark the event in electronics magazines—except this: RCA placed an ad on the rear cover of the January edition of Electronics in 1949. The (colour!) advert shows a 12AX7 is pictured alongside several miniature and other tube types.

You could be forgiven for thinking that RCA’s low-key marketing campaign for 12AX7 just doesn’t add up; given the tube’s enduring popularity and the vital role the it’s played in the audio industry over the last seventy years. Why was it that arguably the world’s greatest tube received so little attention on its creation? Was development number: “A-4522” a classified government project? A top military secret?

Well, no. The simple fact of the matter was that, at the time, the 12AX7 wasn’t deemed newsworthy. RCA’s new tube was just two 6AV6 triode sections housed together in one glass envelope; not really a great technological leap forward. All the real engineering, the development of the glass-to-metal seals, cathode alloys and coatings, electrode spacing and geometry, etc had been done, some time ago. All these technical innovations were inside the 6AV6 (registered on 28^th March 1947) which was already finding service in AM radio receivers as a detector and amplifier.

The electronics industry moves fast: the technology of the 6AV6, and hence the 12AX7, was old news, and so were miniature twin triodes. The 12AU7 had been registered a year or so before the 12AX7, on 18^th October 1946, making it the earliest 9-pin miniature glass based twin triode tube entry in “Electron Tube Registration List”. GE placed a vibrant, two-page ad for their new 12AU7, 12AT7 and with two other miniature tube types in the November edition of Electronics in 1947. These tubes were targeted squarely at the domestic FM radio and rapidly growing television market.

But many months were to pass before either tube could be found on shop shelves. There’s an early sighting of the 12AU7 in Radio-Craft magazine in June 1948. It didn’t come cheap though. At 88¢ the 12AU7 cost as much a 6V6GT power tube! Prices did settle down over the coming months, and by July 1949, when the 12AX7 became commercially available, both tubes could be purchased for 34½¢ apiece from mail order electronics parts suppliers such as Concert Master Radio Tube Company in Chicago. In today’s money that’s a modest $4-50 a tube.

The 12AX7, and 12AU7, are part of a family of 9-pin miniature B9A tube types developed during the 1940s, which include the 12AY7, 12AV7, 12AZ7, 12AD7 and 12AT7. These variants possess different amplification factors (gain), transconductance, grid voltage, plate resistance and current capability. For example, the 12AX7 has a gain of 100, whereas the 12AU7 has a gain of only 20—the 12AU7 does have considerably more “oomph” though; its cathode can supply more electrons, and more electrons means more current. This variation exists between the B9A tube types to enable them to be used as building blocks to construct phase splitter, voltage amplifier, oscillator circuits, etc—see the section on CIRCUITS in the RCA Receiving Tube Manual.

B9A Tube Specification

B9A vital physical statistics The 12AX7 is a 9-pin miniature B9A glass base tube. B9A tubes are significantly more compact than the earlier generation of ‘Bakelite’ base “GT” tubes. The glass envelope measures just 22.2mm diameter and the maximum height should be no more than 49.2mm; although some modern tube manufacturers do seem to have a problem adhering to this!

The 12AX7’s nine pins are arranged in a circle of ten: the pins are evenly spaced in an arc with one pin omitted. This arrangement provides keying, meaning the tube can only be inserted in one orientation, the correct orientation. The circle on which the pins are arranged is 11.9mm in diameter. Each pin is of 1.02mm diameter and there’s a 36° angle between each pin counting from 1 to 9; apart from the gap between pins 1 and 9, where the angle is 72°. According to B.S. 448:1953 “Electronic-valve bases, caps and holders”, the force required to insert/remove a tube into/from its socket should be no less than 1.36Kg and no more than 5.9Kg.

Although the 9-pin miniature tube types described above are different electrically, their physical dimensions are all the same. And their pins all exit from the base of the glass envelope. This is what enabled manufacturers to miniaturise the tube. The smaller glass envelope was more resistant to shock and vibration, and this improved reliability. Further, the miniature glass base tubes consumed considerably less power than the old octal tube types—the 6SL7-GT seemed antique in comparison. The absence of the Bakelite base eliminated a costly operation in manufacture and the possibility of dielectric losses, which extended the high frequency response of the tube.

Fender Princeton model '5D2' — The Princeton '5D2' was the first *Fender* amp to use the 12AX7 [Photo taken by Heritage Auctions].

These killer vital statistics made the 12AX7 the go-to tube for audio equipment manufacturers. All guitar and hi-fi amplifiers employ a preamp of one sort or another to boost low level signals, and this is exactly what the designers of the new high-mu twin triode intended it to be used for. Leo Fender made good use of the 12AX7 in many of his guitar amplifier designs, which he based on example circuits outlined in the RESISTANCE-COUPLED AMPLIFIER SECTION of the RCA Receiving Tube Manual—the ‘Champ’, ‘Bassman’, ‘Vibrolux’, ‘Showman’, ‘Deluxe’ and ‘Twin Reverb’ amps and the ‘Princeton’ all have their humble beginnings here (on page 200). The revised model of the ‘Princeton’ (model ‘5D2‘) was the first Fender amp to utilise the 12AX7.

The 12AX7 was made of the right stuff. It was in great demand by equipment makers. It wasn’t too long before other tube manufacturers got in on the action, and by the early 1950s 12AX7s were also shipping out of the factory doors of General Electric, Tung-Sol and CBS-Hytron. And over the coming decades the 12AX7 dominated the vacuum tube market.

If you look closely through the 12AX7’s glass envelope you can see the two separate triode sections within. The two identical metal structures are the plates of each section. The grid, cathode and heater are hidden inside the plate—the 12AX7 really is two 7-pin miniature 6AV6 tubes shoehorned into one 9-pin miniature tube. It may not have been especially ground-breaking, but this simple combination of two high gain triode stages in a compact package turned out to be really useful, for audio equipment manufacturers.

The mark I version of the 12AX7 was not perfect. Far from it. Its “ladder” plates were tall and thin, which made the tube sensitive to picking up external vibration and sound.

There was no getting round it—it didn’t matter whether the plates were manufactured from black (carbonized nickel) or grey (aluminium-coated steel)—the early 12AX7s were often found to be too microphonic. This meant the tube wasn’t so ideal for use in phono preamp stages or amplification of very small input signals.

And it was noisy. So, as well as being microphonic, the early 12AX7s do tend to generate excessive white noise (undesirable hiss), “flicker” noise (sporadic bursts of noise), crackles and pops. There’s considerable variability; some tubes aren’t so bad, but too many are too noisy. These noise problems essentially have two root causes. One: impurities not being removed when pumping out the air and other gases during the manufacturing process to create the vacuum And two: the composition of the nickel base alloy in the cathode.

American metallurgical engineers had become aware that their cathode alloys didn’t just have shortcomings, they had fundamentally serious faults. The main problem being silicon content. Silicon reacts with barium in the cathode coating causing the build up of a resistive layer, which over time results in a decline in gain and increasing noise. This drastically shortens the useful life of the tube.

Finally, if that wasn’t enough, the cathode heater introduced low frequency mains hum. This, along with the 12AX7’s other issues, imposed severe limitations on the use of the tube. The 12AX7 could not be used in mission critical applications that required long-term reliability. And it was inadequate for use in high-sensitivity voltage amplifier circuits, such as hi-fidelity phonograph amplifiers for low-output magnetic pickups. Keep in mind that the 45RPM singles and “microgroove” LPs were the primary music medium of the time and that this industry was expanding, rapidly. The need for a low-noise high-gain tube was driving engineering companies to come up with a better tube. RCA still had work to: it was time to go back to the drawing board and stamp out a few bugs.

Code Cracking

The alphanumeric name printed on the glass, “12AX7”, isn’t just a model number, it’s a code that also describes the tube’s vital statistics. The “12” indicates that the heater (the part that glows orange when the tube is powered up) requires 12 volts to operate; the “A” indicates the tube is an amplifying component; the “X” identifies the tube’s electrical characteristics, that is the gain factor, plate resistance, etc; and the “7” the number of active pins.

Tube Bugs

In principle curing the 12AX7’s microphony simply comes down to vibration control. If the electrodes are made from a stiffer material they become more rigid. This limits inter-electrode movement between the grid, plate and cathode thus reducing the amplitude of any signals induced by external sound and vibration. This is exactly the approach taken by car manufacturers to reduce noise levels inside vehicles. They stiffened and damped door panels to prevent them vibrating and radiating sound. And it worked—extremely effectively; cars are much quieter than they used to be.

Inter-electrode movement can also be reduced by machining the parts—plate, electrodes and micas—more precisely. In theory, their tolerances can be tightened up so that all the parts are fixed rigidly in place and nothing can move. With no room for movement sensitivity to external vibration would be eradicated entirely; in theory.

But in practice, the precision and accuracy of machining needed to achieve this were beyond the technology of the time. Manufacturers could not produce uniformity consistent tubes, and some manufacturers were more consistent than others.

Further, stiffening panels, and electrodes, increases their resonant frequency; in the same way the pitch of a guitar string changes when its tension is increased when tuning up. It’s possible to push resonance outside the audible spectrum so that it cannot be heard. This approach was taken by GE. They came up with a short, stocky “pinched” (“folded”) plate design, which proved to be exceptionally stable and low in microphony. They used the design in their 5751 avionics and “five star” line of receiving tubes. This design geometry was quickly adopted by other manufacturers, including RCA and Sylvania. Tubes made with short folded plates, for the most part, are less sensitive to amplifying vibration than longer “ladder” plate types.

Like microphony, hum induced by the heater was essentially a mechanical engineering issue. And, as always, the boffins at Sylvania had a few ideas. In 1950 Sylvania engineers Gehrke, Huntington, and Granger filed a patent for a helically wound heater. Their patent (US2677782-A and US2,867,032) highlights longer tube life as major benefit of the invention, however the helical heater also minimises AC heater supply noise in the tube’s output. According to the July, 1962 edition of Electronics World [page 39] the first tube to incorporate a helically wound heater was the 12AY7.

Improving the cathode alloy was a much tougher nut for science to crack though—even tougher than machining parts to a precision that was beyond current technology. It was not obvious, at all, what factors limited cathode life and caused some cathodes to generate more noise than others. The best part of the 1950s was spent grappling with the problem. It took chemistry, physics, metallurgy and a focused international research effort make any headway in understanding the complexities of the cathode; but ultimately, the problem buckled under scientific pressure to yield its secrets.

The 7025

In the late 1950s RCA announced their RCA-7025. According to one of several full-page adverts they placed in magazines at the time the 7025 was “…specially controlled for noise and hum.“. And had a, “…helical hairpin-type heater,” and, “…exceptionally sturdy cage assembly featuring short, stiff stem leads, oversized side-rods and micas of special design to improve noise and microphonics.“—keep talking, whoa keep talking. Fuel injection cutoffs and chrome-plated rods, oh yeah. The 7025 was a reengineered 12AX7 specially developed for resistance-coupled preamp applications… the purpose the 12AX7 was originally designed for, but never quite worked very well at. A few years later in 1960 RCA released the 12AX7A [which, as far as I can tell, is essentially the same as the 7025].

RCA’s mark II 12AX7 was better than their first attempt, however there was still room for improvement. Despite efforts to tighten up tolerances the “ladder” plate tube still had a tendency to be microphonic. Some tubes from a batch would be good, some not so good and some unusable. It was a problem that RCA engineers never quite got to the bottom of and resolved to their satisfaction.

The ECC83

But meanwhile, across the pond, minds vast and cool watched… and slowly, and surely, they drew their plans to—move in and take a slice of the action. The versatility of the 12AX7 represented an excellent business opportunity for Philips-Mullard. Philips could see that there was plenty of scope to improve the 12AX7 and tasked their mighty R&D department to begin work on developing the finest 12AX7 ever made. Meticulous attention to detail was paid to the precision of the control grid windings, electrode metallurgy and improvement of emissive materials used in the cathode resulting in the much sought-after Mullard ECC83 [pictured right]. Note: the European nomenclature for the 12AX7 is ECC83; the 12AT7 is ECC81; and 12AU7 is ECC82.

The Mullard ECC83 is a superlative example of 1950s precision engineering. High grade materials and tight machining tolerances ensured the lowest possible theoretical self-noise and exceptionally low microphony. And Philips’s legendary quality control processes ensured that their ECC83 was manufactured to consistently high standards. There was uniformity between tubes too—something RCA had not been able to attain. Further, the Philips ECC83 utilised a magnesium-nickel cathode alloy. This alloy was superior to the American silicon activated alloys of the time which suffered from bad chemistry. Over time a resistive “interface” layer built up between the alloy and the emissive oxide-coating, resulting in a reduction in gain and increased self-noise, shortening he usable life of the tube. Philips’s cathode alloy did not suffer from these problems.

BEL ECC83 — If it looks like a *Mullard*, it is a *Mullard*: *BEL* ECC83 manufactured with *Mullard-Philips* tooling.

The ECC83 shown above originates from Mullard’s Blackburn factory Lancashire. The Blackburn site covered an area of 43 acres, employed over 6000 people and was arguably the most technologically advanced vacuum tube facility in the world. But Mullard were not a British company. They were once, many, many years ago, until Philips acquired them in 1927. Over the decades Philips invested heavily in new tooling and factories, which is why the Blackburn site exists. The Philips empire had a long and mighty reach. Their Blackburn site was one of many worldwide. Philips set up factories in Britain, Holland, France, and on the distant shores of Japan (Matsushita), Australia (Philips Miniwatt, Hendon, Alberton) and India (BEL, Bangalore).

The tubes from these factories were often labelled as “foreign made”, however they were all manufactured using Philips/Mullard tooling and expertise through their Transfer Of Technology (TOT) program. ECC83s made with TOT were physically identical to their British made counterparts in every way—they had the same ladder plates, star micas, seamed glass envelope, spot welding, grid wire spacing, electrode geometry, and the same cathode metallurgy and chemistry.

As the vacuum tube era drew to an end, Philips came to dominate the market. They bought out Sylvania, who’d bought out RCA, who’d bought out GE. These companies were the undisputed heavyweights of tube manufacturing and Philips ruled supreme. In the last days Philips held all the cards, but nobody wanted tubes any more. But I am getting ahead of myself—we’re not quite done with our story of the 12AX7 yet.

The ECC803S

So let’s go back to a time when anything seemed possible: the dawn of the Space Age. When humanity was on the cusp of manned spaceflight, harnessing the power of the atom and developing the frame grid tube: the holy grail of tubes. A frame grid tube has high transconductance, high gain, low noise and low microphony. All these desirable properties are made possible by the frame grid.

Before we discuss the frame grid in more detail, here’s brief primer on transconductance. I’m sure guitarists will be familiar with the concept of gain and noise, but what about transconductance? Well, it’s simply this: the change in output current caused by a change in input voltage on the grid. If you’re a tube amp owner, then that voltage on the grid is the tiny signal from your guitar pickup; and the output at the plate is a larger, or amplified, version of your guitar signal. You can think of a tube as being like a water tap or a valve, however, instead of controlling the flow of water, it’s controlling the flow of electrons. In fact, in Britain a “tube” is called a “valve” for this very reason.

Telefunken ECC803S — *Telefunken* ECC803S frame grid twin triode.

If you take a look at the datasheet for a 12AX7 you’ll see it’s transconductance specified there amongst the tube’s other vital statistics, such as plate resistance, amplification factor, etc. Transconductance is basically a measure of how well a tube can amplify a signal. Now the tube’s ability to amplify is primarily determined by the cathode-to-grid spacing. The closer the grid is to the cathode the more influence it has on the flow of electrons, and the larger the transconductance. But the distance required for optimum operation is tiny, less than a hundred micrometers.

Such a small grid-cathode spacing is not practically realisable with a conventional grid assembly—such as that used by RCA in their 12AX7—due to limitations imposed by its design and construction. A conventional grid is fashioned by stretching and wrapping fine wire onto the two parallel, notched copper rods using a grid-winding machine to form a spiral as shown in Figure A below. The cathode is then positioned in the centre of this grid and both held in position by two mica supports at either end.

The accuracy and precision of machining, stamping and hand assembling these various parts sets a limit on how close the grid can be placed to the cathode without them touching. What happens in practice is that we end up with a grid wire that can vibrate like a plucked guitar string. This can result in microphonic pickup of external sound and vibration. But worse, in extremely high vibration environments, the grid can actually come in contact with the cathode, just like a guitar string hitting the fretboard when plucked hard. Not good.

Fig. A Conventional notched and swaged grid.

Enter the “frame grid”. In a frame grid [shown in Figure B] an invisibly-fine wire is wound at high tension onto a rigid precision-stamped molybdenum frame—molybdenum is used because it’s hard, strong, has a high melting point and low coefficient of thermal expansion. Because the grid wire is stretched and under tension its freedom of movement is constrained. Further, the resonant frequency of the grid wire is shifted outside the audible spectrum, which makes a frame grid far less sensitive to external vibration than a conventional wound grid.

The machining tolerances of a frame grid are a great deal tighter than those of a conventional grid. This allows the grid wires to be placed in close proximity to the cathode without any danger to shorting to obtain the desired transconductance and low noise performance for audio reproduction. This is the grid Telefunken used in their ECC803S and Tesla used in their E83CC. These frame grid tubes possess superior noise and microphony to tubes with conventional swaged and notched grids.

The Telefunken ECC803S represents the pinnacle of 12AX7 tubes. Over the decades to come there was no other tube manufacturer that could ever get close to duplicating their level of quality. And it was not for want of trying. RCA attempted to reverse engineer the ECC803S and unlock its secrets, but by their own admission it was beyond them. Bell Telephone Laboratories engineer, Edward J. Walsh’s patent [US2,567,415] gives an idea of the complexities and precision required to fabricate the frame grid.

The End?

In hindsight the golden age of tube manufacturing seems terribly brief—just a little over a decade, from the early 1950’s to mid-1960s. The television market is essentially what kept tube sales at a staggering figure of almost half a billion (≈400,000,000) tubes a year during this period. Television created an insatiable demand for smaller, more reliable vacuum tubes, such as the 12AX7 and the other miniature 9-pin tube types. But it was telephony and communication that fueled further development and refinement of the 12AX7 to make it better, quieter and less sensitive to vibration. However, even in those halcyon days, when they were refining and improving their tubes, manufacturers knew the vacuum tube’s days were numbered—waiting in the wings was a small, low-power, heater-less device—the transistor.

To be continued…

A previous version of this article was published in March 2016 edition of ToneQuest Report. The RCA/Edison factories were demolished relatively recently in 2013. You can find more photographs of the demolition in progress on The Observer Online website and further historical information about RCA on the Hagley Museum and Library website. And finally here’s a fascinating glimpse of RCA back in the heyday in 1959. This promotional video contains film footage taken from within RCA Victor television manufacturing factory and the Harrison tube manufacturing plant. You’ll discover that they really do not make tubes like they used to.

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The 12AX7 Tube: The Cornerstone of Guitar Tone