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Black Plate Tubes

  1. HomeKnowledge BaseBlack Plate Tubes

Black Plate Tubes

by Dr Phil Taylor

Black Plate Tube
Brimar 25Z4G half wave rectifier tube with black plates [click image to enlarge].

Have you ever wondered why some vintage tubes have black coloured plates whereas others are grey? Or why black plate tubes command such stratospheric prices, and are prized by tone hounds? Could it be that the plate colour imbues these tubes with superior sonic properties? Or perhaps it’s the material they’re made from? Or, is their coveted status merely a side-effect of the sales hype put out by tube vendors over the years to make a few quick bucks?

As always, Effectrode are on the case, sifting through ageing technical tube literature, studying the withered and yellowing pages within, searching for arcane knowledge, some vital key to solve this thermionic enigma, anything that might tell us whether the sonic properties of black plate tubes are real and physical or just a figment of the mind…

A Nickel's Worth of Physics

Now, going back to the early development of the vacuum tube, engineers understood that the plates of receiving tubes had to be kept cool in order to reduce secondary emission to extend tube life. Secondary emission occurs when electrons from the cathode strike the plate at high velocity, causing electrons (secondary emission electrons) to be dislodged from metal atoms in the plate. The process is a little like pouring water into a bucket or a bowl; the faster the water is poured the more chance there is of water escaping the surface tension and being thrown up as tiny droplets.

Secondary emission isn’t normally a problem that’s associated with the tubes in a guitar amp’s preamp section. These tubes (12AX7, 12AY7, etc) are operating as voltage signal amplifiers and only small currents are flowing so the plate runs cool. However, in a current amplifier—such as the push-pull 6L6 output stage of a Fender ‘Twin Reverb’ amp—higher currents flow and the plate runs much hotter. The collective kinetic energy of all those electrons striking the plate causes additional heating; in some cases the plate begins to glow dull red, (“red plating”) running hotter than a hooker’s doorknob on nickel night!

During normal operation the plate in a power tube is working at temperatures close to its vapour point.  Pushing the temperature beyond this results in “outgassing”, where metal atoms literally boil off the plate surface to form a vapour. These metal atoms—the vapour—inevitably wind up being deposited as a thin film on cooler surfaces, such as the grid and mica supports. The metallic film creates current leakage paths, which manifest themselves as a sporadic “sputtering” noise when the tube is operating in an amp. The sound is like bacon frying or wood crackling and spitting on a fire grate. Ultimately, if this thermal runaway is left unchecked, the intense heat causes the plate metal to soften; the metal begins to bend and buckle until the glass envelope melts and collapses around the plate, sealing it within a translucent, vitreous tomb.

6L6 Power tube "red-plating"
6L6 Power tube "red-plating" [Photograph taken by 'Upscale Audio']

Now, before the tube meets its hot and sticky end, it is emitting a great deal of heat. The plate is enclosed within an evacuated glass envelope, so it can only cool by dissipating this heat as radiation. However, the radiated heat also warms the glass envelope, which means convection—the flow of air currents around the glass—also plays a significant part in cooling the tube. Heat loss via conduction through the pins is minimal because the pins are relatively small and only connected to the plate by thin wires.

Thermal radiation can experiment
Thermal emissivity by radiation tin can experiment. [photo taken by Auburn University]

In an effort to improve heat dissipation, tube manufacturers realised that the plates needed to be darker in colour; dark surfaces emit, and absorb, heat radiation much more effectively than light ones do. This fact is often demonstrated in schools using the simple apparatus pictured on the left. It consists of two identical metal cans, one shiny silver and the other matt black. Both cans are filled to the same level with hot water and a thermometer placed in each. At set intervals, the water is stirred with the thermometer and the temperatures taken. After a few minutes it can be seen that the water in the black can therefore cools more rapidly: The black can possesses high high thermal emissivity and is an excellent heat radiator; whereas the thermal emissivity of the shiny can is low meaning it’s a poor heat radiator. These same physical principles—thermal emissivity—also apply to the plates of a tube.

Emissivity is the ratio of radiation emitted by a surface to the radiation emitted by a complete radiator (black body) at the same temperature and under the similar conditions. The emissivity can never be greater than unity. Listed below are a selection of emissivity values for metals from the Color Scale of Temperature Handbook 1955, page 2692:

  • Silver, highly polished 0.02
  • Aluminium, highly polished 0.08
  • Nickel, polished 0.12
  • Cast iron 0.25
  • Monel metal, oxidised 0.43
  • Brass, polished 0.60
  • Oxidised steel 0.70
  • Black gloss paint 0.90
  • Lampblack 0.95
  • Carbonised Nickel 0.87-0.97

Practically all commercial receiving tubes manufactured before World War II were constructed with nickel or nickel alloy plates. Low power signal tubes, such as types used in portable radio sets, had natural colour plates as there was no need to dissipate large amounts of heat. In higher power applications where high emissivity darkened plates were required, the plates would be first treated by heating them to high temperatures and then placed in a hydrocarbon-rich atmosphere prior to assembly. This process would carbonise the plate giving its surface a charcoal black colour.

Cool Running

The transparency of a tube’s glass envelope to infrared radiation has nearly as much to do with plate temperature as the colour of the plate itself. Therefore always ensure adequate ventilation around tubes for air circulation and avoid using shielding cans if possible—this will help them run cooler and maximise their life. And, it goes without saying, make sure your amp’s power tubes are biased correctly.

Plates of Steel

During World War II, nickel was in very short supply in Germany, so the German electrical industry developed composite plates which consisted of a very thin layer of aluminium pressed onto a steel or iron substrate. When heated sufficiently, the aluminium bonded to the steel and formed a dark grey surface which worked about as well as carbonised nickel in tubes. Following the war, the process was adopted and refined by American tube companies, where it gradually displaced carbonised nickel.

The transition from using carbonised nickel to aluminised steel wasn’t without its problems though, as Sylvania Electric found out in June 1948. At this time Sylvania were engaged in improving the endurance of 7AD7 pentode tubes used in the Whirlwind I, a digital electronic computer under development for the U.S. Navy for use as a flight simulator. The machine contained in the region of 5000 tubes and if just one of these tubes failed the whole system failed. Reliability was paramount, and to that end Sylvania reengineered the 7AD7 incorporating a high-purity tungsten heater filament to negate “cathode poisoning”. The new long-life tube eventually became the 7AK7.

Along the way to creating the 7AK7 Sylvania’s engineers took it into their heads to tinker with several other aspects of the tube’s construction, including fitting new aluminised steel plates. As it turned out this was not a great idea. Testing revealed an increase in gas pressure within the tube envelope: A real problem because any residual gas in a vacuum tube also causes cathode poisoning and kills the tube dead. So Sylvania reverted back to using the old carbonised nickel plates—at the time, they had various other military and commercial contracts vying for their attention, so probably would not want to divert their resources into investigating why the new aluminised steel plates might be “outgassing”.

The outgassing may have been due to the release of adsorbed gases from within the aluminium surface coating or adverse chemistry occurring between the aluminium and the steel. It’s well understood that “hot-dip” aluminising of mild steel results in the formation of an iron aluminide (Fe₃Al) layer between the steel and the aluminium. This intermetallic layer is relatively very brittle and consequently the outer layer of aluminium cracks and peels off from the surface during forming operations and machining. Either way, the vacuum was being compromised and these gas molecules were ending up being deposited on the other electrodes within the tube, including the cathode. And, the aluminising process was not without its technical issues as this old US patent shows.

Iron aluminide (Al3Fe) interface layer
Iron aluminide (Fe₃Al) interface layer.[Source: AIP Conference Proceedings 1711, 040002 (2016)]

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‘.

The Colour of Tone

All this chat about physics and history is well and good but what do black plate tubes sound like? Is their tone “better” than their grey equivalents? That’s not easy to answer because there are so many other factors affecting the tonal colouration of a tube. 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 what effect the plates might have these other 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 as near as dammit.

Sylvania black and grey plate12AU7 tubes
Sylvania black and grey plate12AU7 tubes.

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.

In Conclusion

There are many audiophiles who hold an unshakeable belief that black plate tubes possess improved tonal characteristics. And they’ll often resort to using vaguely defined terms like “bloom”, “dimensionality” and “soundstage” to describe what they’re hearing.  Audio electronic engineers are unable to measure such things as no test equipment exists to do so. Because of this engineers are usually less than impressed by the use of such woolly terminology, which is not objective, not scientific and these things cannot be measured and plotted. The engineer in me tends to agree. But, the musician in me wonders if there might be something in what audiophiles are attempting to describe with their unscientific, yet more poetic vocabulary.

Perhaps there are second or third order effects introduced by the plate that affect tube tone in some subtle way; things engineers can’t measure. Perhaps these effects are buried beneath other dominating factors, such as inherent non-linearity, microphony, self-noise and the placebo effect, yet some people have the ability to resolve them. I can’t say for sure, as science is only best guess. I am sure about one thing though: nickel’s a great deal more expensive than steel, and despite this Sylvania still continued to utilise it to manufacture the plates for their aerospace grade tubes, even after the advent of aluminised steel in the 1950s, almost right up until they shutdown their Emporium tube plant in the late 1980s. Now why would they do that?

 

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The entire effectrode.com website is copyright © 1963-2022 by EFFECTRODE THERMIONIC. All Rights Reserved. No part of this website may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without prior permission in writing of the author.

In This Section

  • Black Plate Tubes
  • Chemical Highlights of Tube Manufacturing
  • Cryogenic Treatment of Tubes: An Engineer’s Perspective
  • Developments in Trustworthy-Valve Techniques
  • Evolution of the Tube
  • Foil Those Tube Forgers
  • Microphonics
  • Mullard ECC83 (12AX7) Reissue vs Original – A Physical Comparison
  • Mullard ECC83 (12AX7) Reissue vs Original – An Electrical Comparison
  • Noise
  • Oxide Cathode Life: Investigations into the Causes of Loss of Emission
  • Signal Tubes
  • Speed, Efficiency & Perfection – Aims That Have Built a Mammoth Factory in 16 Years
  • Subminiature Tubes: The Future of Audio!
  • That’s a Sylvania tube, the print is green, no, it’s blue
  • The ‘Magic Eye’
  • The ’12AT7′ Tube
  • The ’12AU7′ Tube
  • The ’12AX7′ Tube
  • The 12AX7 Tube – The Cornerstone Of Guitar Tone
  • The 6SN7GT – the best general-purpose dual triode?
  • The Accurate BSPICE Tube Models
  • The Cool Sound of Tubes
  • The Inner Workings of Vacuum Tube Buffers
  • The Tube Family Tree – Part 1
  • The Tube Family Tree – Part 2
  • The Tube Family Tree – Part 3
  • Tube Vendors
  • Tubes: The Old Verses the New
  • Vacuum Tubes and Transistors Compared
  • Valve Microphony Part 1: Production of Microphony and Methods of Investigation
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