In my last post, I talked about the basics of vacuum tubes and one of the very first tube types, the diode. Diodes were used to rectify household alternating current (AC) into the direct current (DC) needed by many home electronic devices, and they were also used to detect incoming radio signals so that the information carried by these signals could be heard.
In 1906, Lee DeForest, an American inventor, was experimenting with vacuum tubes. He took a diode and added a third element, called a grid, between the cathode and the plate. This early grid was simply a wire bent back and forth several times. Later grids took the form of a coiled wire that surrounded the cathode.
Remember that in a tube, we boil electrons off the surface of the cathode. For a diode, If the plate is positive with respect to the cathode, it attracts the electrons and a current flows. If the plate is negative with respect to the cathode, it repels the electrons and no current flows. Now, in a triode, we have the grid between the cathode and the plate. We connect a wire to the grid and bring it outside the tube. Let’s apply a voltage to make the plate positive with respect to the cathode and leave the grid disconnected for now. What happens?
Well, nothing different really—we get almost the same current flow we would have in a diode. There is plenty of space between the grid wires for electrons to pass through on their way from the cathode to the plate. A few electrons may strike parts of the grid and be scattered, but this has little effect on the overall flow. However, if we now apply an additional voltage to make the grid negative with respect to the cathode, the effect is dramatic. Even though the plate is at a positive potential, the grid starts repelling electrons and the current flowing to the plate drops rapidly. If we make the grid negative enough, we can stop the flow of current completely! This is called cutoff.
Inside the tube, the grid is not connected to anything, and we are not using it as a source of electrons like the cathode. This means that it requires very little current to swing the electrical potential of the grid positive or negative. This is a groundbreaking discovery! We now have a device where a tiny current (grid current) can control a much larger current (current between cathode and plate). This is the basic principle of an amplifier. Before amplification, early radio enthusiasts constructed huge antennas to capture incoming radio waves. The right design with a large antenna created the largest possible alternating current in the antenna wire from the incoming signal, however, this current was still very weak. At best, it could operate a pair of headphones. Even with a big antenna, radio operators strained to hear faint signals.
Now, imagine that we connect a strong current source (rectified current from an outlet, or a high-voltage battery) to the cathode and plate of a triode. We connect the very weak radio signal coming from the antenna to the grid of that same triode. Voila! The weak radio signal causes small changes in the grid potential relative to the cathode, and these changes control the much stronger current flowing between the cathode and plate. We now have a strong current (cathode to plate current) that is changing exactly the way the weak radio signal was changing! We have amplified the weak signal. Not only was this new form of amplification enough to make a weak signal easier to hear in a pair of headphones, it could even power a speaker so that everyone in the room could enjoy a radio program!
As tube development progressed, tubes became more powerful in terms of both amplification and the ability to handle larger currents, thus bringing in even weaker stations and powering larger speakers. Researchers and developers created tetrode tubes with four elements (cathode, plate, and two grids), and pentode tubes with five elements (cathode, plate, and three grids), to improve the electrical properties of tubes. The number of radios and broadcast stations skyrocketed, and millions of people began enjoying this new form of entertainment.
In addition to amplifying signals, engineers also started using tubes as oscillators, devices that create alternating currents. This was useful in creating radio broadcasts, and also in radio receivers with the superheterodyne circuit invented in 1917—a circuit so good at bringing in weak radio stations that you can still buy AM radios that use it. Oscillator tubes were also a key component in TVs, as was a new development in tubes: the picture tube. Also called a kinescope or cathode ray tube (CRT), the picture tube once again boiled electrons off a cathode, but in this application, the electrons were sprayed toward a phosphor screen that lit up to produce a picture as the electrons struck it.
Finally, tubes played a role in the development of faster, more powerful digital computers, which required hundreds, then thousands, then tens of thousands of electronic switches to perform binary logic and arithmetic operations, and route data. How can a tube be a switch? Easy! The grid is the switch control: set it negative enough so that the tube is in cutoff when you want to stop the flow of electrons and turn off the switch. You may have heard of the ENIAC computer, which had almost 18,000 tubes—imagine trying to find which one was bad!
Tubes are still around today, in use by audio enthusiasts who enjoy the sound they produce in a hi-fi stereo system, and musicians who enjoy the sound of music played through a tube guitar amp. Collectors restore and use vintage tube radios, TVs, and test equipment. And, even the heavy, bulky picture tube has made a comeback as vintage computer and game console enthusiasts rescue obsolete TVs from the dumpster so they can display their favorite programs with the scanlines they remember. Tubes helped enable and shape the electronic world we live in–next time you listen to the radio or watch a video, imagine the warm glow of the tubes that was part of so many families’ experience in the early days of electronic entertainment.