Electron tube

ELECTRON TUBE
Electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube). The two principal electrodes of an electron tube are the cathode and the anode or plate. The simplest vacuum tube, the diode, has only those two electrodes. When the cathode is heated, it emits a cloud of electrons, which are attracted by the positive electric polarity of the anode and constitute the current through the tube. If the cathode is charged positively with respect to the anode, the electrons are drawn back to the cathode. However, the anode is not capable of emitting electrons, so no current can exist; thus the diode acts as a rectifier, i.e., it allows current to flow in only one direction. In the vacuum triode a third electrode, the grid, usually made of a fine wire mesh or similar material, is placed between the cathode and anode. Small voltage fluctuations, or signals, applied to the grid can result in large fluctuations in the current between the cathode and the anode. Thus the triode can act as a signal amplifier, producing output signals some 20 times greater than input. For even greater amplification, additional grids can be added. Tetrodes, with 2 grids, produce output signals about 600 times greater than input, and pentodes, with 3 grids, 1,500 times. X-ray tubes maintain a high voltage between a cathode and an anode. This enables electrons from the cathode to strike the anode at velocities high enough to produce X rays. A cathode-ray tube can produce electron beams that strike a screen to produce pictures as in oscilloscopes and video displays. Gas tubes behave similarly to vacuum tubes but are designed to handle larger currents or to produce luminous discharges. In some gas tubes the cathode is not designed as an electron emitter; conduction occurs when a voltage sufficient to ionize the gas exists between the anode and the cathode. In these cases the ions and electrons formed from the gas molecules constitute the current. Electron tubes have been replaced by solid-state devices, such as transistors, for most applications. However they are still widely used in high-power transmitters, some television cameras, specialty audio equipment, and as oscilloscope and video displays. A klystron is a special kind of vacuum tube that is a powerful microwave amplifier; it is used to generate signals for radar and television stations.

A History of the Vacuum Tube

Picture representation of a simple tube.

Tubes really aren't that hard to understand when you realize they are more or less a light bulb. Vacuum tubes were indirectly invented by Henry Woodward in 1874, who was the inventor of the first "light bulb." A few years later the patent was purchase by Thomas Edison, who worked on improving Woodward's invention. Edison's version of the light bulb is almost exactly the same as the bulbs we use today. Look at the bulb that's lighting your room. It has a little wire that's shining. This is called the filament, or heater, and also shines in your tube though not as bright.

Edison, really just a tinkerer, continued his experiments by placing a metal 'plate' inside the glass bulb near its filament. He discovered that if this plate had a positive voltage applied to it the light would work, but if it had a negative voltage the light would not work. This is essential in understanding why guitar amplifiers work, though don't worry yourself with the details yet.

Edison found this incredibly strange since there was no physical connection between the filament and the plate. It made no sense. It was an 'open circuit' inside the bulb, therefore it should have no effect at all! It would be like unplugging your TV from the wall and still getting Girls Gone Wild infomercials. This was years before the electron theory, so the phenomena could not be explained. Edison assumed nothing could come of this oddity and ceased further experiments.

It was later discovered that when certain metals were heated a 'cloud' of electrons would form around the metal. Electrons are the negatively charged particles of the atom, so they are attracted to the plate when charged positively. Opposites attract. This revolutionized the study of electricity. Up to that time, scientists thought that electricity was some type of 'juice' that flowed from positive to negative. This is where the phrase "give it some juice" originated. The discovery also proved that electricity didn't go from positive to negative, but from negative to positive—which is still confusing people to this day.

American scientist Lee DeForest later took the light bulb to a new level by wrapping a thin grid of wires closely around the 'cathode'. (In directly heated tubes the filament is the cathode.) He discovered that by applying a small voltage to the grid he could control the intensity of the light bulb. He could variably control the flow of electricity using electricity itself. Edison showed that the bulb could be turned on or off, but DeForest could actually adjust the voltage to any level. How does this relate to tube amps? Answer: In a huge way! If we have a very large voltage flowing through the tube, we can control this with a small voltage at the grid. Amplification and the vacuum tube was born! Suddenly we were rushed into a time of radio, television, and rock and roll music—which never would have been possible without DeForest's invention. Interestingly enough, Dr. DeForest was also the person who synchronized sound to silent movies.

If you need help visualizing what's going on then think of tubes in this way: Remember that the control grid simply acts like a shut-off valve in a water pipe. It controls the flow of electricity like a shut-off valve controls the flow of water. This is why Brits call tubes "electron valves," or valves for short. It allows us to turn the weak signal coming from our electric guitar into a high voltage wall shaking behemoth with huge gonads. Suddenly nerdy guys everywhere started getting laid. The tube would continue to be improved until the invention of the transistor in the 1940s. The transistor is much cheaper and more reliable than the tube, making it better suited for most electronics applications. Organic and responsive overdriven-guitar tones, unfortunately, is not one of them. This wasn't always believed to be so.

Because transistors cost pennies and don't need output transformers, manufacturers could sell a transistor amp at the same price as a tube amp and double their profits. In the early '70s tube amps started losing popularity, and transistor powered rackmounts started becoming popular. During this period there were magazine ads that attacked and ridiculed guitar players who still used tubes. By the early '80s you literally couldn't give away the same vintage tube amps that are so sought after today.

As the 90s grew near everyone started to wake up, many realized that '80s hair tone really wasn't all that great, and consequently a renaissance occurred. So once again the mighty electron tube reigns supreme in a tiny part of electronics. The tube looks like nothing more than a museum relic to the digital enthusiasts, but is still hailed by HiFi audiophiles and guitar players.