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Cryogenic Treatment of Tubes: An Engineer’s Perspective

by Dr Philip Taylor

As an electronics engineer and someone who’s in the business of designing tube audio equipment (effects pedals) I’ve a burning interest in anything that might help improve the sound quality and reliability of vacuum tubes. Over the many years I’ve worked with these glowing glass devices I’ve been aware that there can be variability in microphony and electrical noise between tubes, even tubes from the same factory batch, and have often wondered if there might be some kind of a fix for this as, to me at least, it always seemed a shame to bin a completely functional vintage tube just because it was subjectively slightly noisier or a little more microphonic than another. It crossed my mind that cryogenic treatment (cryo-treatment) might perhaps reduce the inherent white and fluctuation noise that tubes generate. After all, there are several tube vendors that offer this service with claims that it “causes metallurgical molecular changes in the metals within the tube which enhance tone and increase overall life”. Does enhancing tone mean lowering noise and microphony? And can cryo-treatment really improve a tube’s reliability? Here’s my nickel’s worth on the matter.

What is Cryogenic Treatment?

Broken gear tooth

Cryogenic treatment is used to improve the durability of moving steel parts.

Cryogenic treatment is a process of cooling steel alloy down to low very temperatures (−190 °C) to increase its surface hardness to improve its resistance to wear, the practical application being to extend the life of cutting tools, gear teeth, moving engine parts, that kind of thing. Cryogenic treatment alters the crystal structure of steel by completing the conversion of austenite to martensite making it harder. Now, I recall making a screwdriver in metalwork class at secondary school many years ago. The tip of the screwdriver was hardened by heating it up with a blowtorch and then cooling it rapidly (‘quenching’) by plunging it into a bucket of water. It’s my understanding that quenching the steel in this way and not allowing it to cool down slowly prevents austenite from forming and makes the steel much harder and more brittle. The process isn’t 100% perfect though as steel still contains some austenite crystals. Apparently cryogenic treatment completes the conversion to further harden it.

Effect on Vacuum Tubes?

Effect of underwear on tone.

Two different crystal structures that occur in steel which have absolutely no measurable effect on tone. [picture by Tom Duerig]

So cryogenic treatment can work to improve the hardness of ferrous metals such as steel, but what about a vacuum tube—a tube is not a lump of steel—like a wristwatch, it’s a delicate and complex mechanism composed of many different parts, which in turn are made from different types of materials. The metal electrodes are mainly high purity nickel for the plates and heater cover (cathode), tungsten alloy for the heater wire, molybdenum (sometimes with gold plating as in Sylvania ‘Gold-Brand’ tubes) for the grid wire, copper support posts and the heater is coated with a mixture of strontium/barium oxides. All these tiny metal parts are supported by thin mica discs within a glass envelope. I can only speculate on what effect chilling a tube down to extremely low temperatures will have on the materials that make up its many, intricate internal parts. There are no published noise measurement figures or life tests comparing cryogenically treated and stock tubes—it seems a serious scientific research investigation is needed here.

I did manage to find a couple of research papers describing how cryogenic treatment hardens aluminium so maybe it’s possible that the process will also harden copper, nickel and the other metals utilised in the construction of tubes too? There are no moving parts within a tube, and hence no mechanical wear, but hardening the metal electrodes could in principle make them stiffer, and stiff electrodes are good thing in tube. Any minuscule displacement or bending of the grid, plate and cathode relative to one another results sensitivity to vibration pickup—stiff, and accurately machined electrodes limit this movement keeping microphony to a minimum. But it should be kept in mind that cryogenic treatment is ordinarily employed to complete the conversion of austenite to martensite in hardened steel as described earlier; the metal electrodes, and other parts inside a tube aren’t quenched to make them super hard to begin with, which means there is no conversion to complete. So, if cryo-treatment doesn’t affect the hardness or stiffness of the electrodes then what benefits does it impart on an electron tube?

Cryogenically treated Mullard 'reissue' tube

Taking a tube down to extreme low temperatures runs the risk of ruining tube permanently and irreversibly unless precautions are taken to protect it from the stresses of the procedure. [photo courtesy of 'TheTubeStore.com']

Well, cryo-treatment might not benefit tubes at all, in fact, it could be argued that the severe cooling process could, if anything, degrade the performance of these thermionic devices. Subjecting a tube to this kind of wide variation in temperature will result in differential rates of contraction (and expansion) of the parts that make up the electrode assembly, undoubtedbly inducing unnecessary thermal stress on it and the glass envelope in which it’s housed. Tube manufacturers never designed or intended tubes to be stored at cryogenic temperatures. It’s not recommended practice to treat electronic components like this and it’s worth noting that electronics component manufacturers publish datasheets that specify temperature ranges for storing and operating their devices at—none recommend storing their components at cryogenic temperatures.

Cooling vacuum tubes, or any other electronic component, down to extreme low temperatures is a risky business, a game of chance that risks ruining tube permanently and irreversibly unless precautions are taken to protect it from the stresses of the procedure. The temperature must be cycled (decreased and increased) slowly for the reasons outlined above and the humidity has to be maintained at near zero to prevent condensation forming and oxidising exposed metal parts. Additionally, the utmost care needs be taken when handling a tube whilst it is at cryogenic temperatures as the physical properties of the materials from which the tube is constructed alter. At at around −200 °C many materials become fantastically fragile and prone to fracture or shattering, meaning cryo-treatment increases the odds of loosening something within the electrode assembly or worse still, embrittlement and fracturing of precisely engineered and delicate internal parts. There’s a dramatic scene in the “Terminator 2″ movie where a single bullet fired by Arnold Schwarzenegger shatters the frozen T-1000 terminator into a thousand pieces, however take a look at this short video of a platinum cup being submerged in liquid nitrogen for a real experimental demonstration of how embrittlment drastically alters the properties of metals.

Effect on Tone?

Tube text book

There are no tube text books that even mention cryo-treatment, let alone advocating the procedure as a technique for improving the audio qualities of tubes.

There’s no written evidence, no research papers, no text books, no technical support data or any documentation produced by any tube manufacturer at any time that even mentions cryo-treatment let alone advocating the procedure as a technique for reducing noise and microphony in tubes. And there’s nothing of substance online that indicates how the process might possibly improve the ‘tone’ of tubes either. Does it change the way electrons are emitted from the cathode? Does it somehow change the way electromagnetic fields form inside the tube? Does it improve the bonding of the oxide coatings to the cathode or remove residual contaminants to make the tube electrically quieter? Does cryogenic treatment affect the tube in any measurable or audible way at all?

There are a handful of tube vendors pushing cryo-treatment, their rhetoric based on a smattering of science about how they cryogenically treat tubes, but where’s the hard science of what cryogenics actually does to improve the performance of tubes? Their websites are often littered with astounding claims stating that their cryo-treated tubes possess “tighter focus from top to bottom”, “more holographic 3D soundstage”, “more subtle inner resolution extracted from recordings”, “tighter bass”, “increased dynamic range”, “faster transient response” and even references to NASA’s research and great scientists such as Albert Einstein. All impressive stuff, but does cryogenic treatment have any more effect on the audio qualities of a tube than the positions of the stars and planets above? I’ll put my money on the scientific method, rather than astrology, every time. If this extreme freezing process really does have a measurable effect on tone then, as an engineer, I’d expect them to support their claims with independently assessed noise and microphony tests, preferably accredited by an external body such as the National Physical Laboratory.

Marketing Hype 101

At this point it would be easy to write-off cryo-treatment as Tono-Bungay, flimflam, a fake or a scam or, at the very least, a placebo. But let’s just go back a step: if cryo-treatment is actually damaging tubes as speculated earlier on, then might this reasonably explain it’s apparent effectiveness? Could it be that subjecting tubes to the stresses of extreme cold provides a test methodology reveal potential early failures—a kind if negative null test? That is, the cryogenic process doesn’t improve performance, it’s simply that people are listening to hand-selected tubes because all the weak tubes would have been weeded out. Could this be the reason why some people can hear a difference? A good engineer would want to know the truth (it’s out there somewhere). More than that, they’d need to know. Like Albert Einstein they’d want to understand the physics. And with their sights firmly set on making a genuine contribution to the art of vacuum tube manufacture, they’d perform experiments, make measurements, collect and analyse data and then subject their findings to rigorous peer review to find out if “better tone through cryogenics” is the real deal and not merely phantasmagoria—this is the way real advances in science, and tone, come about.

Didn’t Mullard Know About It?

Mullard Blackburn oygen and hydrogen plant.

Oxygen and hydrogen production plant on the Blackburn site – Mullard had the motivation and resources to explore cryogenics.

And it’s worth considering this: if there is any merit in the cryogenic treatment of tubes then surely manufacturing giants Mullard-Philips or Sylvania would have utilised the process. They were in the business of making tubes, not just for guitar and hi-fi amplifiers, but for mission critical military, aerospace and scientific instrumentation applications. They were highly motivated to improve their production processes. They had vast pool of scientific and engineering resources at their disposal. Mullard’s 43 acre site at Blackburn, Lancashire, Great Britain had it’s own liquid oxygen and liquid hydrogen manufacturing plant. The site employed almost 7000 people amongst which were all kinds of specialists including physicists, chemists and metallurgists, resources far beyond the small handful of eastern tube factories that serve the guitar industry today. If there had been anything in cryo-treatment then surely Mullard must have known about it. Perhaps they did; perhaps somewhere, buried in some university basement, library or an old radio ham’s attic, is a technical paper written by a Mullard engineer in the early 1960s titled ‘An Investigation into the Effect of Cryogenic Temperatures on Thermionic Emission’. I’ll leave it to the reader to decide how long the odds are of that paper ever seeing the light of day and if it would put the big freeze on the cryo-treatment industry for vacuum tubes.

How to Avoid Tube Marketing Hype 101!

The vacuum tube industry today certainly does seem to suffer from more than its fair share of marketing hype. Like the washing-up powder market, the tube market is awash (sorry!) with all sorts of flimflam, not just cryo-treatment, but tube dampers, tube coolers and even rebranding modern manufacture tubes under the names of once great manufacturers—cheap (sometimes not so cheap) gimmicks that are no substitute for real engineering. All this marketeering noise and nonsense achieves is to make the task of purchasing a good quality tube more challenging than it really should be. So, my advice when buying tubes would simply be this: invest in tubes from a reputable vendor that checks and matches them on a tube tester and guarantees them or, better still, seek out vintage N.O.S. vacuum tubes made by the likes of Mullard, Sylvania or the other giants from the golden age of tube manufacturing—we’re fortunate that they are still supplies of N.O.S. tubes available and there are deals to be had.

On a final note, the best means of avoiding being duped by marketing hype is to educate yourself. I can highly recommend taking a look at Materials and Techniques for Electron Tubes (1960) by Walter H. Kohl the Senior Engineering Specialist of Special Tube Operations at Sylvania and Electron Tube Design by RCA (1962). You’ll find nothing about cryo-treatment in these texts only information relating to the construction methods and quality of materials, that is, the stuff that’s genuinely relevant in the design of a good quality tube, and, these texts also provide a fascinating, nostalgic glimpse into the world of the 1960s, a time when manufacturers went to unprecedented lengths to design and construct tubes that were as near perfect as possible. After an hour or so’s reading from either book you’ll be in the know, a bonafide tube guru knowing what’s important in tube design and, what’s not so important.

In Conclusion

There is variation in the noise and microphony of vacuum tubes because of engineering limitations or to put it another way, tube manufacturers’ ability to accurately and consistently fabricate these complex devices. Mullard (or perhaps it was Sylvania) in their heyday, with vast resources and a wealth of expertise at their disposal got as close as anyone to manufacturing the perfect vacuum tube, however even their tubes were subject to variance in component tolerances which allow electrode movement and sensitivity to microphonic pickup; DC leakage paths in the mica insulation spacers that allow small currents to flow where they shouldn’t and fluctuations in thermionic emission from the cathode oxide coatings, both of which result in self-noise. It would be magical if cryogenic treatment were some kind of ‘silver bullet’ that improves the insulation properties of mica spacers, reduces inter-electrode movement and improves the emission of the cathode coatings to reduce noise and microphony. But I cannot begin to imagine how this works—’magical’ really is the right word to use here because cryogenic treatment of tubes certainly isn’t science—there are no well-considered explanations describing how the cryogenic process works to improve the electrical charateristics of a tube or even the most sparse comparative test results to validate claims for improved performance. The bottom line is that cryogenic treatment is no remedy for inferior materials or defects in tube construction and it won’t transform a rebranded modern manufacture tube into a genuine N.O.S. Mullard a or Sylvania tube—it just doesn’t work that way.

If you’re interested in what the Mullard Blackburn factory was like in its glory days then do take a look at the following article ‘Speed, Efficiency & Perfection – Aims That Have Built a Mammoth Factory in 16 Years’ originally published in 1954 in the ‘Blackburn Times’ not long after the factory opened.