What Are Vacuum Tubes?
- Michael LeRoy (electronics, programming)
Generally speaking, a vacuum tube is a glass tube with at least one heater element (filament). The air is pumped out, and an additional metal piece is connected to the tube.
Vacuum tubes were replaced by contemporary transistors and have performed the same kind of work. The difference is that vacuum tubes are more temperamental and bulkier. They also need to be replaced when burnt out. All the tubes are slightly different from each other and require a proper biasing to set the necessary parameters, while modern transistors are almost identical.
Vacuum tubes take some time to heat up before they begin to operate within their effective zone. If you don’t want to wait, their characteristics will be unstable until settling after some time.
These are nonlinear devices. Some musicians prefer vacuum tubes over transistors, claiming that such amps produce a particular sort of sound that is impossible to imitate with transistor amps.
The principle of operation is similar to transistors. Both types of devices force electrons to change their position from point A to point B and produce the same effect.
There is a specific type of tubes that don’t have a filament, using external sources, such as radioactivity sensors, for charging. An example of such tubes is the Geiger-Müller tube. It’s filled with gas ionized by a ray of radiation. It’s a specialized type of diode which is not technically vacuum.
Before people invented LEDs, there were many methods of producing displays. One of the approaches involved the use of devices similar to vacuum tubes: nixie tubes and the VFD (vacuum fluorescent display). These two types rely on a different mechanic. VFDs are actually vacuum and have electron-emitting filaments, while nixie tubes are filled with inert gas. They discharge energy into the gas making the grids glow. Both types of devices are quite similar to vacuum tubes.
- John R. Smith, engineer
A vacuum tube, which is also known as a “tube” in North America and a “valve” in the U.K., is a device with an evacuated container that controls electric current between electrodes. It emits electrons from a heated cathode or hot filament, which is thermionic emission. Such tubes are called thermionic tubes. However, there are also phototubes, which rely on the photoelectric effect. Not all electron tubes are vacuum tubes. There are also gas-filled tubes which usually operate at low pressure, using the electric discharge in gases.
Diodes are the simplest vacuum tubes. They only contain a heater, plate (anode), and a heated electron-emitting cathode (sometimes, the filament itself acts as a cathode). Current can only flow between the two electrodes in one direction because electrons emitted move in the tube and are collected by the anode. The current can be controlled by adding control grids within the tube. Tubes with grids have many applications, including switching, amplification, oscillation, display, and rectification.
Vacuum tubes were invented by John Ambrose Fleming in 1904 and used to be a basic component for a range of electronic devices during the first half of the 20th century. For example, they were used in sound recording, radio, television, telephone networks, industrial process control, analog and digital computers. The invention of vacuum tubes was the main reason why all these technologies became popular around the world. However, a semiconductor was invented in the 1940s, allowing for the development of solid-state devices, which were more reliable, efficient, durable, and cheaper than vacuum tubes. Starting from the 1960s, transistors and other solid-state devices replaced tubes in all industries, and only CRT (cathode-ray tubes) remained the basis for video monitors and TVs. Nevertheless, tubes are still used in areas where they demonstrate better results than semiconductors. For instance, tubes are used in high-frequency amplifiers and microwave ovens.
- James Tillerson, Marine Corps League
The vacuum tube was fully developed thanks to Lee De Forest, who invented what he called the “audion.” It not only moved current in one direction but also increased the current. De Forest has also put a metal grid in the tube and used a small input current to change the voltage, being able to control the flow and to increase the initially weak current after it comes out of the main electrodes.
- Alexander O’Donnell, hair stylist
It’s a device that generates microwaves. A filament glows red if you apply a 6-volt current (AC or DC). The filament is connected to the negative output of the high voltage power supply, while the whole body is positive. These devices work similarly to CRTs, as the electrons run from the anode to the cathode.
CRTs were more complex and required a higher voltage (up to 35 kv in the largest CRTs). A control grid inserted between the cathode and filament allowed for precise modulation of the number of electrons. The resolution of computer monitors at that time was 1920✕1024, so the grid should react to a frequency of 250 MHz, while the regular TVs had a standard resolution of 640✕480 and required less than 6 MHz.
There are also magnetrons in everybody’s kitchen. In the magnetron, electrons are induced by donut-shaped magnets instead of flying straight. The cathode guides electrons with 8 cavities that have antennas. A microwave oven has a frequency of 2.45 GHz because electrons create 8 pulses per rotation, rotating 306.25 million times per second.
- Mark S. Cooper, professor, physics and astronomy
Vacuum tubes are tubes with all the air pumped out. Electrons are pulled out of a hot metal surface and then accelerated by voltages, allowing tubes to work as transistors.
Before transistors were invented, vacuum tubes were used to build electronic devices, including computers. However, given a huge difference in size between tubes and transistors, these computers were really big.
Some audiophiles claim that the best amplifiers are made out of vacuum tubes, which is a controversial topic in the music world.
- Chris D.
Vacuum tubes perform the same functions as a MOSFET or transistor. These are active electronic components (capacitors, inductors, and resistors are inactive). Tubes were used from the 1910s to the 1960s, and were almost completely replaced by transistors in the 1980s. Modern devices (amplifiers, digital circuits, oscillators, etc.) have microchips, which are pieces of silicon with many transistors. Vacuum tubes were the first solution that became the basis for modern electronics.
- Mick Brown (self-employed)
Vacuum tubes preceded solid state electronics. A vacuum tube is, basically, a glass tube in a vacuum with a heater element, like lightbulbs. Such tubes can be used as oscillators, amplifiers, rectifiers, detectors, or switching devices. They are used in guitar amps, preamps, and home theater systems. Tubes have nothing to do with bipolar transistors. I hope, this helps.
- Robert Greene
Vacuum tubes, also known as thermionic valves and electron tubes, are active electronic devices that move electrons through a vacuum. There are many kinds of tubes, and they were the basis for many electronic devices until the mid-1960s when transistors and other solid-state devices replaced them. The most common tubes look like tubular glass bulbs with a terminal point on the base and may also have additional terminals on the sides or at the top of the bulb.
There are also three-element tubes (triodes), which consist of an anode (“plate”), a heated cathode, and a grid in between them, which controls the flow of electrons. A small difference in voltage applied to the grid results in a big difference in the current. Therefore, triodes are used for amplification.
Triodes may be complemented with additional grids and other elements, to improve their characteristics and to adapt these tubes to other applications. 4-element tubes are called tetrodes, tubes with 5 elements are pentodes, and so on.
- Jeremy Samberg, old nerd
A vacuum tube is what the Britons call a “valve radio” or “valve.” It’s a glass tube with metal electrodes. Tubes were used in electronics before transistors were a thing. A cavity magnetron is a specific kind of tube, which is used in microwave ovens.
- Sam Lawrence, engineer
These are tubes with no air inside. They are similar to lightbulbs but serve different functions. A basic tube is called a triode — it has a cathode, control grid, and anode (plate). The cathode is heated by a heating element, or the heating element itself can be a cathode. The control grid changes the flow of electrons and is able to amplify a signal because it’s located closer to the cathode.
The whole construction is assembled into the base, inside a glass tube with the air pumped out. The vacuum allows for eliminating corona discharges and arc-overs. Given that these devices rely on electronic repulsion and attraction, high voltages are necessary.
- Harry Brown, electrical engineering
Previous answers are great. This is an answer for those who lack knowledge about basic circuits and transistors.
Let’s say, we have a circuit that transmits a signal. We can measure the signal in volts or current (I’ll go with volts as they are more familiar for most people).
The voltage of the signal can be expressed as a number, just like the speed of a jet. Let’s say that 1 volt corresponds to a jet going 100 mph. 2 volts mean that the jet is going 200 mph. The magnitude of the signal fluctuates.
Suppose we need to amplify the signal, multiplying it by a number. For instance, if we multiply a 1-volt signal by 2, we’ll get 2 volts.
This task is easy if we always need to multiply by the same number, but if the task is to amplify the signal by another signal, which also changes, it gets far more difficult.
That’s exactly what vacuum tubes and transistors do — they multiply a signal by another signal. Before transistors were invented, this was possible only using vacuum tubes.
- David Grimes
Vacuum tubes created electronics as we know it. They looked similar to light bulbs but were used in oscillators, amplifiers, rectifiers, and other devices. They had a base with several pins to connect to a circuit. Inside the tube, there were metallic parts connected to the pins. The electronic thing was put in the tube and sealed. After this, the air was pumped out from a hole at the top, and this hole was also sealed. The vacuum is necessary to prevent burning up. Today, the only common type of vacuum tube is a magnetron oscillator, which is used in radar systems and microwaves. Tubes had also been used in TVs until the 1980s.
The largest tube I’ve seen was nine inches round and two feet tall, the smallest one was ¼ inch round.
- Greg Short, NCR
The most common vacuum tubes are CRTs (cathode ray tubes). They were used in old monitors, which is a reason why monitors made twenty years ago were much bigger than the modern ones.
Tubes take some time to heat up and require a lot of energy to make the filament hot. In the old TVs, a small current was applied to the filaments all the time so that they could turn on faster.
- Rick Perry, vacuum technology expert
Vacuum tubes were used before transistors and consisted of a glass tube with electronic components inside. The tube was evacuated and sealed because the electronic components would fail in the atmospheric conditions. The first radios, TVs, and computers were made using vacuum tubes.
- John O’Rourke
Thomas Edison has noticed that ions were expelled from the hot filament and deposited on the inner lining of his lightbulbs. Edison considered it a problem and tried to reduce this effect because the bulbs got cloudy. However, Flemming got interested in this phenomenon and managed to control the flow of electrons by adding a cold plate with a small wire screen attached to a different voltage. His invention became a vacuum tube, which was called a “Flemming valve.”