Vintage Radio
1:00 pm
Mon November 4, 2013

Vacuum in a Tube Enabled Sound from a Box

Credit Paul Litwinovich

In my humble opinion, the vacuum tube rates among the most important technological inventions of the past century. It is considered by most to mark the birth of the electronic age, and until the invention of the transistor in 1947, it stood alone as the heart and soul of just about every electronic device developed.  

Opinion varies over just who should receive credit for inventing the device. The debate focuses on four well known inventors;  Americans Thomas Edison and Lee DeForest, Englishman John Fleming, and the German physicist Wilhelm Conrad Roentgen. For the purpose of discussion, let us define a vacuum tube as a vessel most often made of glass, porcelain or metal, from which as much air as possible has been pumped out, containing a number of electrical elements and performing a useful function. Based on that definition, all of them qualify in one way or another.

In 1895 German physicist Wilhelm Conrad Roentgen discovered that by placing two electrodes in an evacuated glass tube and applying high voltage to these electrodes, that not only would electricity flow through the vacuum, but that x-rays would be generated by the impact of the electrons striking the steel plate. Mr. Roentgen was not the first to observe that electrons could be made to flow in a vacuum, but his invention of the x-ray tube was probably the first useful and practical application of the phenomenon. As a result of his discovery, he was awarded the first Nobel Prize in Physics. Both of the elements in Roentgen’s original tube were unheated. It is not easy to get electrons to flow in a pure vacuum between unheated elements. Fortunately for Mr. Roentgen, vacuum pumps were not of the highest quality in 1895, and traces of remaining gasses became ionized (electrically charged) allowing current to flow.

Thomas Edison invented the light bulb in 1879. He did so by placing a carbon filament in a glass enclosure and pumping all of the air out of it. His early bulb had one major problem. Carbon seemed to evaporate from the filament and deposit itself on the glass, darkening the glass and thus reducing the useful light output. He noticed that more carbon seemed to evaporate from the end of the filament attached to the negative terminal of his power source. A year later he placed a second element in the bulb, a small metal plate near the filament. He observed that electricity would flow through the vacuum from the hot filament to the plate. He was unable to explain this effect but was hoping that the carbon would also be drawn to the plate instead of darkening the inside of the bulb. When this did not happen, Mr. Edison discontinued research on the phenomenon,  but not before obtaining a patent for the “Edison Effect”, hoping that it might have some commercial application in the future.

John Fleming was an engineer, physicist, and professor who taught and lectured at a number of British universities. He presented an important paper on the theory and design of electrical transformers in 1892. He crossed the pond to take a job in  Thomas Edison’s lab and became curious about the “Edison Effect”. Later, while working for Marconi Wireless he began conducting further research and discovered something that Mr. Edison had overlooked. He noted the fact that electrons would only flow in one direction, from the hot filament to the cold plate, and only when the filament was made negative, and the plate positive. In 1904 he patented this device as the “Fleming Valve” for use as a detector of radio waves. The English still refer to vacuum tubes as “valves” to this day. Ironically,  his first vacuum tube was replaced by the first “solid state” device, the gallium crystal (henceforth the term “crystal radio”)  about a year later. Larger versions of his “valve” would later become rectifier tubes used to convert AC to DC.

Lee De Forest obtained a Ph.D from the Sheffield School of Science at Yale University. His dissertation was on the subject of radio waves. Despite his degree, he remained more of an intuitive inventor like Thomas Edison, than an academic theorist. Upon inventing the vacuum tube as we now know it, he was quoted as saying he “didn't know why it worked, it just did.”

Triode vacuum tube
Triode vacuum tube
Credit 1942 Audels Radioman’s Guide

What Mr. De Forest did in 1906 was to place a small wire grid between the hot filament and the plate of Fleming’s valve. Think back for a moment to 6th grade science class and the demonstration using static electricity or magnets to show that likes repel and opposites attract. That's what  happened in De Forest’s tube. Electrons (negative charge) were emitted by the hot filament and attracted to the plate (having a positive charge). If the grid was made negative, it would repel the electrons before they could reach the plate, turning the current flow off.  If the grid was made positive, it would not only let them pass but accelerate them as well, turning the current flow on. Just a small change in grid charge resulted in a large change in current flow through the tube. In other words, the tube could amplify. De Forest patented his device as the “Audion”, implying that his first application for it was audio amplification.  Soon after, the first patent  lawsuits of the electronics age were filed as both Edison and Fleming claimed infringements against their own inventions. Mr. De Forest was credited with nearly 300 inventions over the course of his lifetime including the technology for talking movies. He lost most of his fortune on legal bills resulting from patent litigation mostly related to the vacuum tube (De Forest VS Fleming), and his claim to have developed the regenerative receiver (De Forest VS Edwin Armstrong). Both of which he eventually won.

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