Secondary emission isn’t a huge problem in triode tubes (12AX7, 12AY7, etc) used as voltage signal amplifers as only small currents flow and the plate stays relatively cool. However, in a current amplifier – such as in a tube guitar amp output stage – higher currents flow and the plate gets much hotter. In this case, the collective kinetic energy of all those electrons striking the plate causes additional heating – in some cases the plate can literally run red-hot and can glow dull red. The elevated temperature of the plate means that electrons are more easily dislodged and the plate undergoes more wear and deterioration due to outgassing.
Because the plate is enclosed within an evacuated glass envelope it can only cool by dissipating heat as radiation or conduction through the pins. In an effort to improve heat dissipation (emissivity) engineers figured out processes to manufacture plate material that was darker in colour – a truely black surface has the highest emissivity. 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
- Aluminum, 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
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.
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 aluminum pressed onto a steel or iron substrate. When heated sufficiently, the aluminum bonded to the steel and formed a very 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. In the mid 1950s, General Electric developed a five-layer anode metal which had an inner copper layer. This became widey adopted for awhile as copper is a good conductor and distributed the heat over the surface of the plate more evenly preventing hotspots.
However, just to get a perspective on the significance of plate colour as a means for cooling the tube it should be noted that the transparency of the glass envelope to infrared radiation has nearly as much to do with plate temperature as the colour of the plate itself. Therefore, ensuring good air circulation, and avoiding shields whenever possible, make a lot more difference to the performance and longevity of tubes than the specific colours of their plates. So it’s could be questioned whether all these R & D efforts into plate emissivity were perhaps mis-directed, however the engineers were doing best practice – applying physics theory and experimentation to build tubes as well as they possibly could.
For low power preamplifier tubes and even power amplifier tubes in the application of guitar amps I question whether the plate colour has any real, practical benefit on tube life. Other variables such as ambient cooling due to airflow, tube orientation and correct biasing in power tubes have a greater influence on the plate temperature than the colour of the plate. Regarding sonic differences between black and grey plate tubes, I’m skeptical that there are any differences, however, although I’ve been unable to discern between the tubes I have tested and listened to, it’s possible that plate material might have some fundamental effect on the tone. Or perhaps something else in manufacture of black plate tubes affects inter-electrode capacitances, transconductance, plate resistance or leakage – all real measureable effects. It seems more likely that there is a measure of spread of these parameters in black and grey plate tubes and arbitarily swapping tubes in and out of an amplifier can yield subjectively better (or worse) tone. Any guitarist or engineer swapping out a tube in an amp might get lucky and hit the tone jackpot where the tube delivers what they consider to be a subjectively pleasing tone. They might then go on to make a broad generalisations about black or grey plate tubes based on this single, unique ‘happy accident’. Imagine this being repeated again and again by thousands of musicians and experimenters creating a ‘sea’ of subjective opinions based on all those variables – totally unscientific because it’s impossible to reduce all this data down to causes and effects and quantify what phyiscal parameters within the tube affect its tone in a beneficial way.
It is my view that a dominant factor affecting the tone of black plate tubes could simply be nostalgia. Although I regard myself as being scientifically objective I have to confess that I get nostalgic about old amps and tubes – old gear has a straight-forwardness, an honesty about it, harking from an idyllic, golden age in western manufacturing. Just the the look and smell of vintage ampifiers, new old stock tubes still in their original packaging, knowing those engineers from and bygone, better time and place used best practice in the design, that manufacturers built gear to last (and be serviced by a repairman) puts me in a great frame of mind when playing and that has to be something worth having!