By the mid 1950s, General Electric had developed an improved five-layer anode which had an inner copper layer. Copper is an excellent thermal conductor and distributed the heat more evenly over the plate surface preventing hotspots and—more importantly—prevented the formation of the brittle Fe₃Al layer. G.E.’s new plate material was soon adopted by other big American tube manufacturers, such as RCA, Raytheon, Tung-sol and Sylvania. Economically this made more sense as steel is significantly cheaper than nickel, a lot cheaper. But it’s interesting to note that Sylvania still continued to utilise nickel plates in their subminiature tube types such as the ‘6021‘ and ‘6112‘.
All this chat about science and history is well and good but what do black plate tubes sound like? Is their tone “better” than their grey equivalents? Well, there’s no straightforward answer to this because there are other things that also affect the tubes’ tonal colouration. Things such as sensitivity to external vibration (microphony), secondary emission, variations in cathode coating formulas, physical dimensions of the plates, inter-electrode capacitances, transconductance, plate resistance or leakage or how much care was taken assembling the tube “cage” (mica washers and support pillars) on the production line; all these things can influence the tonal characteristics of a tube. To make any kind of meaningful assessment about the influence of the plates these variables need to be removed from the equation. It’s difficult to zero them all out completely, but let’s see how far we can go.
Sylvania did produce variants of certain tube types, such as the two 12AU7s shown in the photo below. Although the construction of the short folded black and grey plates isn’t exactly identical, their geometry and size are very similar.
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—I undertook my own investigation; electrically, mechanically and acoustically testing several hundred vintage black and grey plate Sylvania 12AU7 tubes. The result? Well, I couldn’t hear any obvious differences between them; certainly nothing that could be attributed to the plate colour. That’s not to say they’re aren’t any sonic differences, but if there are, I couldn’t perceive them.
I have to confess to being more than a little sceptical about the idea that the plate material can alter the tone of a tube; for one simple reason: I cannot imagine a physical mechanism to describe how it would do so. However, despite this and my findings, I don’t want to simply write-off the opinions of those who swear by their black plate tubes. So where does this leave us? Can these conflicting views be reconciled?
