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The Tube Is Dead. Long Live The Tube
THE VACUUM TUBE HAS BEEN OBSOLETE FOR DECADES—AND IT’S HERE TO STAY
Fall 2002 | Volume 18, Issue 2
IT’S 1960 OR SO, AND YOU’RE A 12-YEAR -old living in a suburb somewhere on the East Coast. It’s late on a school night and you’re in bed, but you’re not sleeping; you’re listening to a big old Atwater Kent radio that you inherited when your mother got one of those new transistor radios for the kitchen. Tonight you’re tuned to WOR in New York at 710 AM, and you’re listening to this guy named Jean Shepherd who’s telling a funny story about when he was a kid and blew up his ham radio one day, scaring his mother and almost burning down the house. Then he starts talking about radio and vacuum tubes, and he says that those new things called transistors may be good for some stuff, but they’ll never replace tubes altogether: “When you get down to putting out 50,000 watts of radio frequency, no sir. You gotta call in the big boys with the fans blowing on ‘em. With the water running through ‘em to keep ‘em cool.”
Then you look over at the nightstand, and see the orange glow behind the Atwater Kent giving the only light in the room, and even under the sheets you can feel the reassuring warmth of the radio’s tubes, and you know Shepherd is right. Your mom’s new transistor set doesn’t sound nearly as good as the Atwater Kent, or a real hi-fi with tubes. How could some dinky little gadget that doesn’t even light up replace all that?
What you didn’t know at the time, of course, was that the writing was already on the wall. Even while the vacuum tube was still the undisputed king of the electronics world, in the 1950s and early 1960s, engineers were diligently experimenting, learning how to build solid-state circuits with the recently invented transistor. As the six-ties dissolved into the seventies, transistors and integrated circuits pushed tubes aside in all sorts of consumer electronics, from stereo systems to radios to television sets, until by 1980 you would be hard pressed to find a tube in any device in your home—except for the single, huge picture tube in the TV. Yet the vacuum tube remains on the scene, stubbornly hanging on almost a century after its invention. Is this mere nostalgia? Perhaps, or at least in part. But nostalgia or not, there are still some jobs for which the obsolete vacuum tube is the better choice.
Like so many other inventions, the vacuum tube can be traced back to Thomas Edison. In a sense Edison invented the very first tube, one that you use every day, the ordinary light bulb. In electronics, however, the term vacuum tube (also called, in the United Kingdom, a valve) describes a device that controls the flow of electrons through a vacuum. Edison discovered in 1883 that if he placed another electrode, in addition to the filament, inside one of his light bulbs, electricity would mysteriously flow through the vacuum from the filament to that second electrode. With an uncharacteristic lack of foresight that he later regretted, he found the phenomenon interesting but useless, and he did no more than make it be known as the “Edison effect.”
The Edison effect remained nothing but an oddity until the British scientist John Ambrose Fleming began experimenting with it early in the twentieth century. By 1904 he had developed the Fleming diode, which used the Edison effect in a vacuum tube containing two electrodes, called the cathode (or filament) and the anode (or plate). Applying electricity to the cathode, heating it, caused current to flow to the anode. Because only the hot cathode, and not the cool anode, gave off electrons, current could flow in only one direction. Fleming had invented a rectifier, a device to convert alternating current into direct current. The Fleming diode found immediate application in the detection of weak radiotelegraph signals.
USEFUL AS IT WAS, HOWEVER , Fleming’s diode could only pick up signals, not amplify them. That came soon after, when the American engineer Lee de Forest inserted a third electrode between the cathode and anode, thus creating a “triode.” When applying a current to this third electrode, called a grid, de Forest discovered that he had created the world’s first device capable of actually amplifying a signal. He patented his Audion tube in 1907, giving birth to the science of electronics. In the following decades thousands of tube types were devised for radio, radar, television, and audio: tetrodes (with two grids between the cathode and anode), pentodes (three grids), and various other multiple-grid designs. All were variations of de Forest’s basic Audion concept.
Despite the great ingenuity of their designs, though, tubes had problems. They were fragile. Drop one and break its glass envelope, and just like a light bulb, it was dead. They were expensive to manufacture, because of the difficulty of achieving a sufficient vacuum. Tube equipment was physically bulky and heavy. Tubes needed big, heavy transformers to manage the high voltages they required, and the transformers not only could be unwieldy and expensive but also could affect the signals the devices handled. Most troublesome was the heat problem. By their very nature, vacuum tubes reach temperatures of hundreds or even thousands of degrees. Pack a lot of them into a small space, and the resulting heat can adversely affect other components and pose a hazard for anyone who happens to touch the equipment in the wrong place. Finally, tubes have a finite lifespan. Inevitably they burn out and need replacement.