Vacuum Tube Construction and Amplification Principle

                                 Vacuum Tube Construction and Amplification Principle

The advancement of science and technology is getting faster and faster, and new electronic products are constantly being released every day. However, equipment parts are also constantly being developed, replacing many old equipment and components, but only some components cannot be completely replaced by current technology. , vacuum tubes are an example. Anyone who has a certain preference for music will know that the pitch of vacuum tube speakers and general speakers will be known by listening.

History of Vacuum Tubes

In 1904, John Fleming of England invented the vacuum diode, opening the door to the application of electronics. In 1907, Lee de Forest invented the vacuum triode, which can control the activity of electrons. Since then, electronic technology has advanced rapidly. Today, electronic products are various and bring convenience to life. The contribution of vacuum tubes cannot be denied. In 1939, Dr. John. V. Atanasoff and his student Clifford Berry used vacuum tubes to complete a 16-bit adder. This was the first vacuum tube computer. In 1946, J. Presper Echert and Dr. John W. Mauchly, professors of the University of Pennsylvania, completed the first computer ENIAC (Electronic Numerical Integrator and Computer) with vacuum tubes as parts. Known as the electronic numerical computer, it is the world's first important digital computer. It shared 18,000 vacuum tubes, weighed about 30 tons, and was 30 meters long. It took about two classrooms to fit in, but it never reached its full potential because the tubes kept burning out during calculations. Vacuum tubes are quite inconvenient to use due to their large size, high power consumption, difficult heat dissipation, and low reliability. They are also expensive and low in popularity. After 1970, they were gradually replaced by transistors. Today, vacuum tubes are only used in audio equipment and high-power high-frequency communication systems.

How Vacuum Tubes Work

The principle of a vacuum tube is that the cathode metal plate emits electrons and is received by the anode metal plate at the other end. Generally, there will be a grid between the two, which is used to control the flow of current in the vacuum tube. Taking the cathode voltage as 0 V as a reference, the anode voltage is generally a positive voltage (+V) of hundreds of volts, and the grid (barbed wire) electrode is normally negative voltage (-V). Vacuum tubes can be divided into direct heating and indirect heating. When heated, the cathode of the vacuum tube emits electrons. Because vacuum tubes take time to heat up to operate, speakers using vacuum tubes take a long time to start up.

vacuum multipole

Common vacuum tubes can be divided into diodes, triodes, tetrodes, and pentodes, which literally represent the number of basic "poles" inside the vacuum tube. The vacuum tube has three most basic poles. The first is the "cathode" (Cathode, represented by K): it is the place where the electron flow is released. It is a metal plate or the filament itself. When the filament heats the metal plate, the electrons will Free out, spread in vacuum glass bottles. The second pole is "Plate" (represented by P), which is basically the outermost metal plate of the vacuum tube. When the eyes see the dark gray or black metal plate on the outermost layer of the vacuum tube, it is usually the plate. The screen electrode is connected to a positive voltage, which is responsible for attracting the electrons emanating from the cathode, using the principle of attraction of opposites, as the end of the free travel of electrons. The third pole "Grid" (Gird, represented by G), from the structural point of view, it is like a circle of thin coils, just like a fence, fixed between the cathode and the screen, and the electron flow must pass through the grid As for the screen electrode, the flow of electrons can be controlled by applying a voltage between the grids. It acts like a faucet, with the functions of flow and blocking. However, the triode, tetrode and pentode all have the same basic function of amplification.

Function and Comparison of Vacuum Tubes

Both vacuum tubes and transistors are electronic switches that can be used to control the flow of electricity, or to amplify or amplify electrical signals.

Ideal amplification characteristics

Assuming that the screen voltage is constant, gradually change the grid voltage (because the input impedance of the vacuum tube is very high), it can be clearly found that when the bias voltage increases from 0V to 5V, every time the bias voltage increases by 1V, the flow through the screen The current of the pole will not increase linearly, but when it is reduced from 0V to -5V, the current will decrease by 2ma for every 1V bias reduction until there is no current, which explains why most vacuum tubes operate at negative bias state.

Although many original components have gradually replaced vacuum tubes, only a small number of products have not been replaced. However, the most common vacuum tube products on the market are vacuum tube speakers. The sound quality of vacuum tube speakers is more beautiful than that of other components. Although vacuum tubes and transistors have their own advantages and disadvantages. However, if the advantages of vacuum tubes are combined with those of transistors, it will be of great benefit to the design of high-end audio products.