Cryogenic Treatment of Tubes: An Engineer’s Perspective
by Phil Taylor
I’m not a metallurgist nor a physicist but as an audio electronics engineer I’ve a vested interest in anything that might help improve the sound quality and reliability of vacuum tubes. There is varability in microphony or electrical noise in batches of tubes and I’ve often wondered if there might be some kind of a fix for this as it seems a shame to reject a tube – which is an expensive component and completely functional – just because it’s subjectively slightly noisier or a little more microphonic than another. It briefly crossed my mind that cryogenic treatment (cryo-treatment) might perhaps reduce the inherent white and fluctuation noise that tubes generate. After all, there are many 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 simply cooling a tube down low temperature improve its characteristics? Here are a few thoughts on the matter.
What is Cryogenic Treatment?
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 at school in my early teens. 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?
So cryogenic treatment works to improve the hardness of ferrous metals such as steel, but what about a vacuum tube – a tube is not a lump of steel – it’s a delicate and complex component made up of many different materials. The metal electrodes are mainly nickel for the plates and heater cover, titanium for the heater wire, molybdenum for the grid wire and copper support posts. The heater is coated with strontium/barium oxides. The electrodes are supported by mica washers within a glass envelope. One can only guess as to what effect cooling a tube down to so such low temperatures will do to it as there’s been no serious scientific research investigation into this. Additionally, tube vendors have done little in the way of publishing noise measurement figures and life tests comparing treated and untreated tubes. Typically all you’ll find are references to surface hardening, maybe nice magnified images of the surface of cryogenically treated steel and even rhetoric about NASA and Einstein on vendor websites. All this is about as relevant to your guitar tone as the colour of your guitar cable or your underwear. Could it be that cryo-treatment is pants?
I did find several scientific papers indicating that cryogenic treatment hardens aluminium. Perhaps it hardens copper, nickel and the other metals used in the construction of tubes too. Although there are no moving parts in a tube and no mechanical wear, hardening the electrodes would theoretically make them stiffer. Stiff electrode structure is good thing in a tube as it should reduce microphnony, which is caused by the electrodes moving relative to one another. However let’s keep this in context. Cryogenic treatment is just a finishing process used to complete the conversion of austenite to martensite in steel as described earlier. The metal components inside a tube aren’t quenched to make them super hard in the first place so there is no conversion to complete. At this level cryogenic treatment achieves nothing and I’m inclined to think that it could potentially damage the tube. Cooling a tube to cryogenic temperatures would put undue thermal stress on the glass envelope and delicate internal parts. The manufacturers never designed or specified tubes to be stored at such low temperatures. The act of putting one of these delicate devices through cryogenic treatment will cause contraction and expansion of the different materials and increase the chance of loosening some of the internal parts. The mica may no longer stay in secure contact with the glass envelope causing the tube to become more microphonic.
Effect on Tone?
At best, exposing tubes to the stresses of cryogenic temperatures provides a test methodology to reveal potential early failures – a kind if negative null test. It does not improve performance, however it might explain why some people can hear a difference – it’s simply because they’re listening to a hand-selected tube. There is no evidence for the process actually improving noise or microphony though. Nor can I find anything of substance online that indicates how cryogenic treatment might possibly improve the ‘tone’ of tubes either. Does it change the way electrons are emitted from the cathode? Does it change the way electromagnetic fields form inside the tube? Does it improve the bonding of the oxide coatings to the cathode to make it quieter? Does cryogenic treatment affect the tube in any measurable or audible way at all? Tube vendors give us the science of how they cryogenically treat tubes, but what about the science of what it does to improve the performance of tubes, eh?
You’ll find plenty of fluff on vendor sites such as claims that our cryo-treated tubes have “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”, etc, etc. If cryogenic treatment really does have an astounding and revolutionary on tube performance then the world needs to know about it. The skeptics need to be convinced. This is too big to remain on the fringe. Claims need to be supported with independently assessed noise and microphony measurements, preferably accredited by some external body like the National Physical Laboratory? If I were in the business of cryogenic treatment I would consider it a matter of some urgency to have this data to backup my claims – it would give me confidence that my work was of value. If this really is cutting edge stuff, why not apply for a research grant, write a thesis on the subject and make a genuine, significant contribution to the art of vacuum tube manufacture? What is there to lose?
Didn’t Mullard Know About It?
I don’t want to labour the point but it’s worth considering this, if there was any merit in the cryogenic treatment of tubes then surely manufacturing giants Mullard or Philips 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 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. Their 43 acre site at Blackburn, Lancashire 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 any benefits to cryogenic treatment of vacuum tubes, Mullard must have known about it. I suspect that somewhere there is, buried in some university basement library or an old radio ham’s attic, 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 reckon it’s just a question of time before this paper shows up. What do you think the Mullard engineer’s findings would have been?
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 I couldn’t resist that one) with all kinds of nonsense such as cryo-treatment, tube dampers, tube coolers and even rebranding modern manufacture tubes under the names of once great manufacturers – cheap gimmicks that are no subsitute for real engineering. All this nonsense makes purchasing good quality tubes more complicated and difficult than it should be. My advice when buying tubes: 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, Philips, Sylvania or the other tube manufacturing giants from the golden age of tube manufacture – 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. There’s plenty of excellent reading material online 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. For example, 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). These two texts provide a fascinating, nostalgic glimpse into the world of the 1960s, a time when manufacturers went to unprecedented lengths to make tubes as near perfect as possible. After just an hour or so’s reading from either book you’ll be in the know (a tube guru!) as to what is important in tube design and what’s not so important.
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 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 can not 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. And if cryo-treatment really worked, I’d be a happy man as I have several boxes containing hundreds and hundreds of tubes, which have been rejected over the years because of subjective failures. I’m as motivated as anyone could possibly be to fix them – it would be a dream to be able to somehow restore them to optimum performance and use them. But that’s what it will ever be, a dream. The reality is that cryogenic treatment is not a remedy for inferior materials or defects in tube construction. It simply doesn’t work.
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.