This is a DeForest Audion tube…a type T version. It dates to about 1916 and is a typical triode of the day.
When an electric current flows through the filament, it becomes hot as in a typical lightbulb. In addition to visible light, electrons are emitted. Early incandescent lamps began to discolor…some sort of impingement of particles upon the internal glass surface was taking place although it was not understood at the time. Fleming added a plate inside of lamps in order to study the so-called “Edison Effect” and discovered that a current flow was taking place. This resulted in the diode. A tube with two elements: filament and plate. Diodes allow current to pass in one direction only and are useful in a wide variety of circuits, including the detection of radio waves. Adding the third element, the grid, as Lee DeForest did, created an ability to control the flow of electrons between the filament and the plate. By selecting the polarity and magnitude of the voltages applied to the elements it was found to be possible to not only switch but actually amplify the applied signal. This was revolutionary and with widespread ramifications…..it was the birth of modern electronics!
The construction of the type T tube is pretty basic and straightforward, even crude in some regards. For example, the plate (sometimes referred to as the “wing” in DeForest’s own notes) does not fully encompass the spiral grid and filament structure. The result is a loss of some efficiency in its ability to amplify. DeForest, the inventor of the triode, did not truly understand exactly how the device worked. For example, he believed that some level of gas needed to be present inside–in actuality, a hard vacuum is far superior. It was Edwin H. Armstrong and others who researched and sorted these mysteries out and made many great discoveries along the way. Discoveries such as that not only could the triode amplify, it could also be driven to oscillate–thereby becoming a generator of radio waves. Such discoveries played into what resulted in a decades long legal battle between DeForest, Armstrong, and others. Such was the early days of the electronic industry. It was every bit as rapidly changing and tumultuous as the heyday of Silicon Valley’s beginnings or the gold rush of the 1850’s.
Looking closely at this lowly tube, crude even by the standards of those which hit the market within just a couple of years, it becomes apparent that this was-at the time, a truly revolutionary device. Every bit as state of the art as your best microchips are today. Originally the type T was supplied licensed for amateur and experimental uses as a tube with leads to be connected into whatever circuit the user decided to try. Upon close inspection you can see the hand blown envelope, glass to metal seals, spiral grid, bent plate, and a simple straight filament running through the center. The tubulation, where this tube was sealed to the pumping system to be evacuated and heated to bake out moisture and impurities and then sealed off with a torch, protrudes proudly out the top. This was a device made with the utmost care at a time when the exact science of how to best do it was not fully understood by those making the tubes. It was in a very real sense learning, creating, and testing the theory as you go. This tube was made before thoriated filaments to increase electron emission….it was made before getters to achieve higher vacuums…..it was even made before the use of induction heating to quickly bake out the internal metal parts. Looking closely at this particular example it becomes readily apparent that some early radio experimenter decided to solder in the wires and use some Plaster of Paris type material to cement it into a 4 pin base of the type that would become commonplace on factory supplied hardware of the 1920’s. This adaptation is not entirely surprising and also yields a little more information. It tells us it must have been a good one, because variances were common at the time and this particular example was evidently performing well enough for its owner that he or she chose to utilize it in some later design that made use of the 4 pin base and socket arrangement. Tubes were expensive and sought after at the time. The type T Audion cost $11.00 in 1916. One hundred years ago, this represented a princely sum–more than a week’s wages for many. No wonder its owner made efforts to utilize it as long as possible.
Aside from its own merits as a tangible piece of early radio history, it is of special interest to me because it represents an example of the simplest form of construction of a functional device of its type. As a person who has a technical streak and glassblowing/neon experience, the temptation to reproduce such a device in the home shop is very real and I have succumbed to this temptation more than once. Today almost anyone can get a tube and build a replica of a single tube regenerative radio receiver–they even sell kits for this, but how many are going to bother trying to construct a tube from scratch? …for some of us, that is all the reason we need and I have more than once dreamed of what it was like to be working at the cutting edge in a shop not much different than the one in my spare room. So for me, looking at this, it represents not only history and discovery, but also possibility.
This tube is remarkable in another way as well. For whatever reasons, it has survived. Survived recessions, depressions, two world wars, scrap drives, recycling, and multiple generations — any of whom could have just tossed it into a bin as old junk without recognizing any of its significance. As such I owe a debt of gratitude to whomever the last generations of technological hoarders were.
