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Secrets of the Tube Alchemists

Well not so much alchemy as chemistry. And physics too, and, even worse: maths. It turns out all those terrible, soul-crushing subjects my teachers tried to instil into me back at school are of vital significance to guitar tone. But at the time I was eleven years old; my mind was on other things, like larking around and a growing, inexplicable attraction to girls. The classroom did not fully hold my attention—all that dry, lofty knowledge of ages was wasted on me; well, perhaps not completely wasted.

Over time I learned the value of a good education; now I see the world with an electronic engineers’ eyes, a qualified ‘know it all’, equipped with a hard-earned knowledge of science. Science is what makes it possible to determine how the properties and geometry of materials will affect electric guitar tone. For instance, how the resistance and capacitance of scatter-wound coils give vintage AlNiCo electromagnetic pickups their bite, or why polypropylene is an excellent dielectric material for use in audio coupling capacitors. But what about tubes? What makes a good vacuum tube? Although there’s a sea of information to be found on the internet and inside guitar magazines about pickups, electronic components, such as capacitors and resistors, and guitar amp circuits, there’s little to be found on that critical thermionic component at the heart of your amp. There’s no real meat on tube vendor websites either, nothing for a tone hound to sink his teeth into; and the sales literature of modern tube manufacturers is very lean on facts regarding the actual construction and quality of materials to say the least.

Mullard tube drawing with labels — So what is it, exactly, about the construction and quality of materials that affects the tone of a vacuum tube?

So, where else can we look for trustworthy info on tubes? Well, there are vintage datasheets, and then there’s technical manuals of course; but, and this may come as a surprise, you probably know something of the science behind what makes a good tube. That’s right, you know more about this geeky kind of stuff than you imagine. So, imagine for a moment that you’re young again. Think back to when you were a kid, a kid at school… in chemistry class…

The Oxides

crucible_320px — When heated to high temperature, calcium carbonate decomposes to become calcium oxide (*CaCO₃ → CaO + CO₂*). This basic school chemistry also occurs during the manufacture of a vacuum tube.

The year is 1978. In a secondary school somewhere in North Notts, England a first year science class is about to begin. Standing in front of a “white board”, the latest in technological developments for the classroom, is a science teacher. She’s young, barely in her early twenties. Her hair is jet black and cut short, like Jane Wiedlin from ‘The Go-Gos’. Her eyes are black too. Their irises are so dark, darker than the pools of water gathering in the now disused, empty and unlit mine shafts of Whitwell colliery. She’s very pretty. She outlines the “experimental method”, how to conduct a scientific experiment: objective, apparatus, method, results, conclusion. She sets up a Bunsen burner and tripod on top of which a crucible is placed containing a bit of chalk, lights it, using a taper, and fiddles with the stem until the flame burns lilac blue. As the chalk, which is calcium carbonate (CaCO₃), heats up it begins to crumble and decompose into calcium oxide (CaO) and carbon dioxide gas (CO₂)—that’s all chalk’s good for now, now that the old blackboards are gone.

Despite being hopelessly infatuated with my teacher I did learn something in science class, something useful about chemical decomposition. The decomposition in her experiment is one of several processes that occur during the manufacture of a vacuum tube. The cathode in a vacuum tube is coated by spraying it with a solution of “alkaline earth” metal (barium, strontium and calcium) carbonates. These carbonates are decomposed by heating, which drives out the CO₂ leaving an oxide layer coating the cathode’s surface. This oxide-coating lowers the workfunction of the cathode, that is, it reduces the energy required to release electrons, making it electron emissive. The oxide-coated cathode is the prime-mover, the working-end of a vacuum tube: the source of negative ions (electrons) that makes amplification of an electrical signal possible.

The first commercial cathodes were coated with barium oxide. Before long tube makers began developing new formulations based on blends of barium, strontium and calcium metal carbonates in an effort to lower the workfunction further, that is, decrease the operating temperature needed for efficient electron emission. Oxide-coated cathodes operate at a relatively cool 800-1000°C, orange-hot, rather than white hot, like the old directly heated tungsten cathodes (essentially a modified filament lightbulb); a lower operating temperature improved the tube’s reliability. These formulations were either double or triple mixes of the alkaline earth metal carbonates. The double carbonate mixes were those of barium and strontium, and the triple carbonate mixes contained an additional small quantity of calcium carbonate; for example, Walter H. Kohl describes a mix of 13% CaCO₃ + 31% SrCO₃ + 56% BaCO₃ in Materials and Techniques for Electron Tubes on page 552. This formula is still used in tubes made today.

periodic_table_320px — There’s something about the “alkaline earths” that works for tubes: a precise blend of barium, strontium and calcium carbonate applied to the cathode maximises electron emission.

But formulating a cathode coating isn’t as simple as throwing together a few carbonates in a beaker and mixing them with a spatula. Oh no, it wouldn’t be, would it. There’s the particle size to consider. The carbonates are milled into a powder where the particle size ranges from 1-9 microns. But which particle size yields best cathode performance? And then there’s the “binder”, the solution the carbonates are suspended in so they can be sprayed onto the cathode. How do you select the ideal solvent? And how much solvent do you use to dissolve the powdered carbonates? How thick should the coating be sprayed? Questions, questions, questions. But they all need answering because all these factors play a vital role in the quality of the cathode. In an attempt to exercise more control over these complexities, and at least establish some kind of repeatability, tube manufacturers chose to mix their own carbonate cocktails, rather than relying on external suppliers; to eliminate variation in the manufacturing process.

So, all tubes are not the same, because all oxide-coated cathodes are not the same. In part, it’s why a Sylvania 12AX7 doesn’t sound exactly like a Brimar 12AX7, even though their technical specifications are the same on paper. And it’s why tone hounds froth at the mouth over the Tesla E83CCs (the closest thing to a genuine Telefunken ECC803s), or the Sylvania ‘Gold Brand’ 5751. There really are differences in the tone and durability of similar tube types made by different manufacturers. This is not something imagined, not snake-oil, market hype or nostalgia—it’s the result of different engineering approaches taken in the design of the tube. Take Sylvania Electric, for example. They introduced ‘Duplex’ cathode coatings in their ‘Gold Brand‘ tubes. Their Duplex coatings were a unique mix of different emissive materials, which became progressively “activated” over time, greatly extending the usable life of the cathode. Clever stuff… no wonder some manufacturers chose to keep their cathode coating formulas closely guarded secrets.

Activation

“Activation” is achieved by heating the cathode above its normal operating temperature. In some cases a current is drawn whilst at this elevated temperature. This results in impurities in the nickel base “reducing” some of the barium oxide coating to create free barium atoms (a single barium atom for every one-hundred barium oxide molecules) within the oxide coating. This process vastly improves the cathode’s emission capability. Activation is just one of several manufacturing processes the cathode undergoes to enhance its emission performance and life span. Whilst being heated the tube is also being continuously vacuum pumped to purge it of any residual gases, particularly oxygen, and other contaminants released from the metal electrodes during heating—if these are not removed they will react chemically with the cathode gradually “poisoning” it to cause a decline in its emissivity. Further, a current is drawn during activation which affects the distribution of barium atoms in the cathode to enhance its emission performance. It’s vital that the tube undergoes this “aging” process. And it must be adhered to rigidly. If not the tube will perform poorly, very poorly; in fact, in many cases the tube will be useless in hi-fidelity audio applications.

The Alloys

It’s not just the oxide-coating formulation that determines the performance of a tube: the composition of the alloy from which the cathode is fabricated plays an equally vital role, perhaps more so. The relative quantities of certain trace elements or impurities within the nickel alloy can literally make or break a tube. Even the smallest excess, or lack, of certain elements causes a significant degradation in the tube’s thermionic performance and adversely affects its operational life. Too many impurities will result in the formation of a high resistance “interface” layer between the base nickel and oxide coating; this builds up over time causing emission to decline, shortening cathode life. However, too few impurities prevent the cathode from being properly “activated” during manufacture, which results in poor emission from day one. Like Goldilocks’ porridge, the trace impurities in the nickel alloy must be “just right” to make the ideal cathode.

Scientific studies made during the 1950s showed that controlled amounts of “reducing agents” must be added to the nickel alloy during fabrication in order to get the cathode to work properly at all. There’s that word again: “reducing”. What does it mean?

Well, there’s an old schoolbook chemistry definition that states reduction is the reverse of oxidation. And what is oxidation? A well known example is the rusting of iron, where the iron reacts with oxygen in the air to form iron oxide, a.k.a. rust. So reduction is the reverse: the removal of oxygen. And it has to be removed from the cathode, otherwise it will literally poison it and ruin its ability to function as an effective electron emitter.

The reducing agents most commonly used in nickel cathode alloys are magnesium, manganese, carbon and silicon. The quantity of reducing agent added to the base nickel can be anywhere between 0.01% to 0.2%; a trace amount that makes a huge difference to the performance of the cathode. The first cathode nickel alloy that became commercially available was “220” nickel; manufactured by Canadian giant Inco mining company; the world’s largest producer of nickel for the best part of the 20th century. Inco tweaked their cathode alloy formula, adjusting the the relative quantities of reducing agents, just a little, to improve it, and “330” followed.

Nickel: “The Devil’s Copper”

In the 16th century, in far eastern Germany on the Czech border, miners unearthed a very strange mineral deposit. It looked uncannily like copper ore, however after extracting, crushing and roasting the ore, the miners were completely baffled when smelting didn’t yield copper, but instead a bright, silvery metal, so hard it could not be reworked. Their bewilderment turned to horror as men, handling and working the ore, became ill with severe stomach pains, vomiting and diarrhea. Some miners even suffered disorientation and became delirious, which resulted in accidents and deaths.

If the ore wasn’t copper what could it be? It was very similar, almost as if it had been cunningly disguised. But disguised by who? Now there were tales of a mischievous goblin known as “Nickel”, or “Old Nick”, who lurked in the dark caves and tunnels of the mine. These were superstitious [medieval] times and miners concluded the ore must be Nickel’s doing; so they named the ore “kupfernickel”, which literally translates as “copper-nickel”, the Devil’s Copper. The reason the nickel ore was so highly toxic is that it contained over 50% arsnic!

Reducing agents weren’t the only additives used in cathode alloys. During the 1950s Superior Tube Co. perfected their Cathaloy^® series alloys (US Patent 593,537), a range of specialised cathode alloys engineered for high strength, high activity, low sublimation, freedom from interface impedance, or combinations of these desired characteristics. Examples include, Cathaloy A-31 for extreme stress applications, containing 4% tungsten, making it twice as strong as tungsten-free alloys. And Cathaloy A-33, an all-purpose cathode alloy, which combines high emission with freedom from sublimation and interface impedance.

In 1952 Superior Tube Company filed a patent for their 'Cathaloy' cathode alloy.

In fact it was common to add tungsten to strengthen the cathode to improve the tube’s resistance to vibration and shock; high strength cathodes can contain as much as 40% tungsten blended with the nickel. Manufacturers were often directly contracted by the US armed forces to improve metallurgy or ruggedise tubes, such as the “JAN” (Joint Army Navy) tubes [Getting the Most Out of Vacuum Tubes by Robert B. Tomer pages 72-75]. In 1958 Superior Tube filed a patent (US Patent 2,836,491) for a tungsten-titanium-nickel alloy with enhanced resistance to deformation at cathode operating temperatures. Soon after, they developed a tungsten-zirconium-nickel alloy for the US Navy, which they dubbed X-3012.

Several of the big tube manufacturers got in on blending their own cathode alloy formulas too, including G.E. (General Electric), Sylvania and RCA, who produced their “N-series” alloys, like N-132 for example. And, across the pond, Philips developed their own magnesium-nickel alloy blend. The magnesium-nickel alloys demonstrated excellent low flicker noise (low-frequency, sporadic bursts of noise) performance, significantly better than the American cathode alloys being produced at the time. Principally this is because much lower levels of silicon are present in magnesium-nickel alloy. The problem with silicon is that it has a tendency to react with barium in the oxide coating to form a silicate layer of barium orthosilicate between the base alloy and the oxide-coating. This interface layer can reach several hundreds of ohms resistance resulting in degradation of the tube’s transconductance and increased flicker noise. Cathode alloys containing magnesium as a reducing agent, rather than silicon, do not suffer from this problem.

Magnesium-nickel alloys were widely used by other European tube manufacturers, including Mullard in Great Britain—it’s worth noting here that Mullard essentially were Philips: in 1925 Stanley Mullard sold half the shares in the Mullard company to Philips in Eindhoven, Netherlands, with the remainder of the company being acquired by Philips in 1927. However, as vast and mighty as the Philips empire was, there were other mighty tube manufacturers, not too far away, producing their own, arguably superior, cathode alloys. If you’ve ever tried Telefunken (Germany) and Tesla (Czechoslovakia) tubes then you’ll know exactly what I’m talking about. They sound exceptionally quiet and beautifully smooth. Is it the absence or addition of some vital trace element in the base alloy, or perhaps the particle size of the carbonates? a different solvent for the binder? or maybe it’s something to do with the ageing process? I don’t know. We may never know… All the alchemists who knew the secrets of how to make a good cathode are gone, the tooling scrapped and the design documentation lost, perhaps forever.

Absolute zero… effect

A quick word on the oddball practice of cryo-treatment applied to vacuum tubes. No tube manufacturer, at any point in history, ever, has recommended freezing tubes down sub-zero temperatures. Extreme low temperature has zero effect on the composition of the oxide-coating, zero effect on free barium, zero effect on base alloy impurities and reducing agents, zero effect on thermionic emission and therefore absolutely zero effect the tube’s sonic characteristics. But cryogenic mistreatment will put stress on the glass-to-pin seals. And it will decrease the fatigue resistance of the internal spot-welds to weaken them. Further, it causes embrittlement of the glass envelope, the metal electrodes, micas; it will loosen grid wire, which is thinner than a human hair; and it will create strain on the entire ‘cage’ assembly. Such damage is irreparable; it will permanently impair the tube’s ability to function. In short, cryo-treatment’s benefits are ZERO.

Summing-up

This short article has barely scratched the oxide-coated surface of vacuum tube manufacture, and yet has still ended up being overly technical. I apologise for that. There’s still so much more to discuss too—the heater, the grid, the micas, the anode and the glass envelope… still at least you now have an idea of the complexities, and subtleties, of tube manufacture. And you know where to look for more information: technical tube books, preferably vintage technical tube books. The important take home message from all this technobabble is: pay attention in science class and don’t get distracted, no matter how smoking hot your science teacher is! Oh, and this. Tube manufacture was taken really, really seriously back in the day. It had to be. Because they were used in extreme applications, such as military comms equipment, precision laboratory instrumentation, computers, radar, jet fighters, missile guidance systems, space and other mission critical stuff. Failure was not an option. Companies, such as Sylvania and Mullard took their business so seriously that they synthesised their own chemical additives, mixed their own cathode coating formulas and blended their own alloys, to ensure that their tubes were of the highest quality. A vacuum tube is the culmination of a world-wide engineering effort spanning many decades; and its audio performance is largely dictated by the quality of the oxide-coating and nickel alloy used in the cathode.

Did you enjoy this article? If so, you might be interested to find out more about the devilish details of vacuum tube manufacture in this old film produced by T.E.O. Department Film Unit, which contains excellent footage of the various wire making, chemical plants and assembly workshops situated on the Mullard Blackburn site—the largest and most advanced tube manufacturing facility in Britain and, for that matter, Europe. The raw materials were transported in and everything was made on site. Tubes fabricated entirely in Great Britain!

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Secrets of the Tube Alchemists