So, my advice when buying tubes is simply 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 vacuum tubes made by the likes of Mullard, Sylvania or the other giants from the golden age of tube manufacturing—genuine N.O.S. tubes can still be found and there are even a few good 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 bona fide tube guru knowing what’s important in tube design and, what’s not so important.
Anyway, that’s my two cents worth on tube cryo-treatment. To summarise, there is variation in the construction of vacuum tubes and these variations have a direct effect on a tube’s tonal characteristics. They’re due to engineering limitations, or to put it another way, the tube manufacturers’ ability to fabricate these complex thermionic devices consistently and accurately. Mullard (and Sylvania), in their heyday, with their large-scale research and manufacturing facilities and a wealth of expertise, got as close as anyone ever could to building the perfect tube. However, despite this they were unable to attain the level of precision required to ensure their tubes possessed uniformly low microhpony and self-noise. Over the decades Mullard made numerous changes to the construction and materials used in their tubes in an effort to iron out these problems, but they never considered deep freezing them; why would they?—there was nothing to indicate it would work.
It would be magical if simply putting tubes in the freezer was a ‘silver bullet’ that could reduce inter-electrode movement and kill microphony; improve the insulation properties of mica spacers to prevent noisy current leakage paths; and somehow restore, revitalise and smooth out fluctuations in electron emission from the cathode oxide coating and make the tube as good as new. But I cannot begin to imagine how this “cure-all” works—’magical’ really is the right word to use here—because cryogenic treatment of tubes is not science. There are no solid scientific explanations describing how the process might affect the electrical or mechanical characteristics of a tube in any beneficial way. Nor are there even the most sparse comparative test results to validate claims for improved sonic performance—by the way, ‘V2’ was the cryo-treated tube in the table of tests shown earlier on.
The bottom line is that cryo-treatment is no remedy for inferior materials, manufacturing defects or deterioration caused by ageing, and, it won’t transform a cheap rebranded modern manufacture tube into a genuine vintage Mullard tube either—it just doesn’t work that way. And, those cryo-treated tubes we tested came highly priced. Not a great financial investment, but perhaps an investment in knowledge; because we’ve learned cryo-treatment has about as much influence on the tone of a tube as the positions of the stars and planets above—and I’ll put my money on science, rather than astrology, every time. As for my heap of dodgy old tubes, well, I’ll just have to put them back on ice…
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.