vintage radio

The Vacuum Tube

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.

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, 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.

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 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.

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 with Edison Light 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. 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.” 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”. 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.

I'll spend more time on the evolution of the vacuum tube as it relates to advances in radio technology in future articles.

This month’s radio is a Polle Royal, manufactured by the Royal Radio Corp. of Providence, Rhode Island. I estimate the date of manufacture to be circa 1923. I have been unable to locate much information about the company other than it was one of the many small manufacturing companies to come and go during the infancy of the radio industry. Many of these small independent radio manufacturers were quite successful for a few years only to fail during the great depression. Many turned out a high quality product. This particular set appears to have been targeted at the higher end of the market and was technologically advanced for its day. It consists of a 5 tube bakelite chassis using #01 tubes and a bakelite front panel featuring silk screened markings with gold leaf paint. The knobs feature machine etched pointers also filled with gold leaf paint. The cabinet appears to be pine or poplar with an ebony stain. Like all radios from this time period, it is a battery powered set. Unlike last month’s radio which was designed to maximize battery life, this set was designed for maximum performance at all times and was probably sold to consumers who were not as concerned with the price of batteries.

Pictured with the radio is an RCA Radiola horn type loudspeaker from the same time period. The earliest speakers were of the horn design. They use a coil and metal diaphragm almost identical to headphones from the same period and used the megaphone characteristics of the horn amplify the sound. This particular speaker has a horn made out of a primitive plastic (probably a hard rubber product) and would have been considered a basic horn sold with cheaper radios. Most likely someone purchasing a radio such as the Polle Royal would have purchased a more expensive speaker featuring an ornate wooden or polished bakelite horn. The importance of the loudspeaker was that it allowed the entire family to listen to a program rather than having to take turns with the headphones. The horn speaker would soon be replaced by the paper cone speaker which offered superior sound. By 1926 horns were on the way out.

How it works: This radio uses a triple stage tuned radio frequency (TRF) circuit design with independent tuning for each stage and is referred to by collectors as a "three dial TRF" for this reason. The triple stage TRF replaced the regenerative circuit and would remain the circuit of choice until the early 1930s. Later designs would gang the three stages together so that all would be simultaneously tuned by one tuning knob. If we look inside the cabinet which features a hinged top with a holding device, we see the five tubes and the three tuned stages. Each tuned stage consists of one tube with its own tuning coil and capacitor. The two remaining tubes serve as a detector and an audio amplifier. Note the colorful design of the coils, and the fact that you can see the reflection of coils and tubes in the polished bakelite chassis. The manufacturer also concealed the wiring and minor components such as resistors beneath the chassis. This was done for good reason. This radio would have been considered a status symbol. It was probably operated with the cover open to display its “high tech” goodies and the fact that it had five expensive tubes instead of the two or three that most regenerative sets had. It was designed to look nice, inside and out.

Despite the three tuning knobs, the set would have required far less skill to tune than a regenerative receiver. Each of the three knobs would be tuned to produce the loudest and clearest signal. It would have had a more natural sound (limited mostly by the horn speaker) than the regenerative circuit as well. The set was quite sensitive and would have easily pulled in distant stations when hooked to a good antenna. It tunes the standard AM broadcast band. Selectivity was fair, but this would not have been a problem at the time due to the limited number of stations on the air. This set is original and unmodified in anyway. It is in working condition.

Collector’s hint: These vintage battery sets make great starter sets for new collectors. Although rare models such as the Polle Royal can be a bit pricey, there are quite a few of the more common brands on the market, find them on E-bay, in antique shops, and at flea markets. They display well even if you don’t restore them to working condition. Their circuitry is simple enough that a beginner hobbyist can usually get them working without too much effort. They don't operate on the higher voltages found in the AC sets, making them safer for the less experianced technician to learn hands on troubleshooting. Other than a bad tube, not much can go wrong with them. Wire wound resistors and mica layer capacitors rarely deteriorate with age unless physically damaged. Look for sets that are complete, avoid the set with the hole in the front panel where a control used to be. Tubes for these sets can still be found but may cost as much as $30 or more each, so keep that in mind before buying a radio with empty tube sockets.

For demo and test purposes, most will run well on 45 volts, which can be obtained by connecting five 9 volt batteries together in series and using a 6 volt lantern battery for the filaments. For more frequent use, AC adapters producing these voltages are available from antique radio parts dealers such as tubesandmore.com. Although these sets work best with an outdoor long wire antenna, a modern ferrite stick antenna from Radio Shack will pull in local stations.

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