Visual inspection only reveals so much though, as the other internal parts, grid, cathode and heater are hidden beneath the plate. There’s no way to tell if these parts were packed nice and tightly within the cage during assembly or whether it was a “Friday afternoon job”. Testing for microphony might though. So, the next thing is to test the sensitivity of the tube to external vibration pickup, to ensure it’s not adversely microphonic.
So, if this checks out we’re done, right? Well, not quite. There’s also the cathode coating to consider: are the cathodes coated with the same formulation? Probably. Well, possibly. Sylvania refined their electron emissive formulations over the years; there’s a chance they could be different. Without obtaining the technical data that was held in Sylvania’s research and development dept. back in the day we’ll never really know for sure. This is the best we can do to minimise any tonal effects caused by physical differences in tube construction—we now have at least some kind of basis on which to undertake a subjective comparison.
And—being naturally curious about such things—Effectrode undertook our own investigation; we electrically, mechanically and acoustically tested several hundred vintage black and grey plate Sylvania 12AU7 tubes. The result? Well, I confess that I couldn’t hear any glaring differences between them, and certainly nothing we could attribute to the plate colour. That’s not to say they’re aren’t any sonic differences, but if there are, we couldn’t perceive them. And I must also confess to being a little sceptical about even entertaining the possibility, for one simple reason: I cannot imagine a physical mechanism to describe why they should sound different. Anyway, that’s my nickel’s worth.
Despite these findings, it’s unreasonable to simply disregard the fact that there are many, many musicians and hi-fi enthusiasts who swear by their black plate tubes. So where does this leave us? Can these conflicting perceptions be reconciled? Well, there’s one last part of the equation that’s not been taken into account: the “placebo effect”. The placebo effect is defined as, “a beneficial effect produced by a placebo drug or treatment, which cannot be attributed to the properties of the placebo itself, and must therefore be due to the patient’s belief in that treatment.” The effect is so powerful (sometimes even more powerful than the beneficial effects of the drug under test) that scientists design “blind” tests to null out its influence on the results.
The placebo effect exists outside of medical science too. The belief in things, such as the healing power of crystals or magnetic bracelets, cure-alls and snake-oils sold by charismatic travelling salesmen in the Old West, voodoo, cryogenic treatment of vacuum tubes to improve their tone and even the colour of the plates in a vacuum tube can produce a perceived beneficial effect, although there is no physical mechanism to describe why. And there’s a great deal of voodoo in vintage gear and tubes. They hark from a bygone, idyllic, golden age in electronics manufacturing when things were built to last. And they looked good too. In fact, old electrical equipment looks better than good; it’s almost always beautifully styled and pleasing to the eye. Just the look and smell of a gorgeous vintage guitar amp or new old stock tubes, still in their battered, but colourful original boxes is enough to get most tone hounds salivating… craving… needing. This gear looks so cool, it must sound cool too.
Often it does, but not just because it is better—in some scientifically measurable way—but because the listener believes it to be better. This is the power of placebo. Science may have taught us that we live in a real world containing objects that possess certain physical properties, however we are creatures of the mind and therefore subject to its irrationalities… or is the world we see merely just a thin veneer hiding the true nature of reality?—your next stop, the Twilight Zone! Whatever the case, the placebo effect is powerful, so powerful it competes, and even eclipses, the other measurable physical phenomena occurring within the tube. It alters the listener’s perception of its sound quality, making it sound better than it actually is.