Why do some old radio tubes have black plates and others with gray plates? Why are black plate tubes worth more expensive and preferred by music lovers? Does the black plate tube have a better or more reliable tone or is it coveted solely due to sales promotions made by some tube vendors? I conducted a thorough investigation of this matter and explained it as is.
In the initial production of the tube, the engineers knew that the receiving tube plate had to remain cool to reduce secondary emissions which would extend the life of the tube. Secondary emissions occur when electrons from the cathode flow through the plate at high speed. This causes electrons to detach from metal atoms in the plate. This process causes a process that shortens the operating life of the tube.
Secondary emissions are not a problem in triode tubes used as signal amplifiers because only small currents flowing and plates remain cold. But in today’s amplifiers it is like in the output stage of a tube guitar amp where the current flows higher and the plate gets hotter. In this case the overall kinetic energy of all electrons that hit the plate causes overheating and in some cases the plate can actually heat up the red light fades. The increased temperature on the plate means that the electrons are more easily dislodged and the plate has greater wear.
All commercial radio tubes produced before World War II are made with nickel alloy plates. Low power signal tubes, such as those used in portable radio sets, have natural color plates because they do not need to remove large amounts of heat. In higher power applications where dark, high emissivity dark plates are needed, the plate will first be treated by heating it to high temperatures and then placed in a hydrocarbon-rich atmosphere before assembly. This process will make the carbonate plate give its surface a charcoal black.
To get a perspective on the importance of the color of the plate as a means of cooling the tube, it must be noted that the transparency of the glass sheath against infrared radiation is almost the same as the temperature of the plate such as the color of the plate itself. Therefore it must be ensured that good air circulation and avoid shielding if possible make more difference in tube performance and longevity than the specific color of their plates.
For low-power preamplifier tubes and even power booster tubes in applying guitar amps, I questioned whether the color of the plate has practical and tangible benefits in tube life. Other variables such as ambient cooling due to airflow, tube orientation and correct refraction in the electric tube have a greater influence on the plate temperature than the plate color. Regarding the sonic differences between black and gray plate tubes, I am skeptical that there are differences, however, although I cannot distinguish between the tubes that I have tested and listen, it is possible that the plate material may have some fundamental effects. in tone. Or maybe something else in making a black plate can affect inter-electrode capacitance, transconductance, plate resistance, or leakage – all tangible effects that can be measured.
It seems more likely that there is a measure of the spread of these parameters in black and gray plate tubes and swapping tubes that in and out of the amplifier can produce a better or worse tone subjectively. Every guitarist or engineer who exchanges tubes on amps may be lucky and suppress tone jackpots where tubes provide what they perceive as subjectively pleasant tones.