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Vacuum tube

[zh ē n k ō ng gu ǎ n]

Electronic Physics Products

Vacuum Tube, representing Glass bottles inside Vacuumize In order to facilitate the flow of free electrons and effectively reduce filament Oxidation loss. Vacuum tubes are widely used in every corner of life.

Chinese name: Vacuum tube
Foreign name: Vacuum Tube

Principle: Flow of free electrons
application area: Electronic physics

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six working principle
seven classification

fundamental theory

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Operating principle of triode

It all starts with electronics. Electronic components are inherently specialized Electronic physics , using the characteristics of materials and structures to form different reactions to electricity. For example, capacitance can be formed by using two metal sheets that are close to each other but do not touch each other; With silicon as the main material, it can be turned into semiconductor after proper manufacturing process, such as Diode 、 Transistor as well as IC Etc; Place the copper wire Insulating paint Encapsulation formation Enameled wire , roll up the enameled wire to form an inductance, add Iron core Then it becomes a transformer and connected together is the Leeds line. There are many other electronic components, which are actually built on the foundation physical phenomenon Exquisite design on.

Invention process

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Current and electron flow

Of course, the vacuum tube is not made of several pieces of metal plates for no reason Vacuumize Of Glass bottles Its development has a story with Edison, the king of invention.

The current flows in the opposite direction to the electrons

Before that, let's ask a small question: Is the direction of "current" in circuit analysis the same as that of "electron" flow? The answer is no, current and Electron flow The direction of is just opposite. Scientists in the past were unable to observe the direction of the flow of electrons, so they said in a unified way that a certain pole of the battery was set as the positive pole, the voltage was positive, and the current flowed from the positive pole to the negative pole to form a closed circuit. Because of the unified statement and practice, there was no conflict for many years, until modern scientists had more sophisticated equipment, and after observation, they overturned the previous statement: "It turns out that electrons are made of batteries Negative terminal Flowing out "! (In other words, the electrons flow out from the negative end of the loudspeaker of the amplifier and return from the positive end of the loudspeaker)

Edison effect

Vacuum tube

As a user, you don't need to care what is true, just act according to the conclusions of scientists. This section is because Edison discovered that the filament of his bulb was always burned out from the positive end after he invented the bulb. So he made further experiments to add a small metal plate to the bulb, connect the metal plate to the meter after lighting the lamp, and apply positive voltage and Negative voltage Observe the current. For the science at that time, it was impossible for a metal plate that was in a vacuum state and not connected to generate current no matter how it was connected, but a strange thing happened. Edison found that a substance (actually an electron) would "jump" from the negative pole of the battery to the positive pole through the metal plate. This discovery certainly aroused greater experimental motivation. This phenomenon was called“ Edison effect ”。 This is the first time that scientists have questioned the direction of current flow, and free electron The phenomenon of flowing in space.

Birth of vacuum tube

Fleming tumb

The reason why metals can conduct electricity is that they have more free electrons, which facilitates the flow of electrons Electronic materials Must be created by Conductivity It is made of good materials. There is another characteristic of the electron negative electricity The electrons of

Phase absorption. It is also known from the Edison effect that when the metal substance is heated, free electrons active around the proton are easy to dissociate, and the high temperature leads to the enhancement of electronic activity. At this time, if there is a positive voltage in the space, the free electrons will flow in the space. Based on these known knowledge at that time, J.A. Fleming made the first diode vacuum tube in 1904, De Forrest (De Forest Lee) improved the diode and manufactured the first triode in 1907. Since the triode was successfully developed, the application of vacuum tubes began to be realized, and the development of vacuum tubes has been going on ever since.

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The triode is the most basic vacuum tube

Diode, triode and pentode literally represent the number of basic "poles" inside the vacuum tube. The vacuum tube has three basic poles, the first is the "cathode" (represented by K): the cathode is of course negative, it is the place where the electron flow is released, it can be a metal plate or the filament itself, when the filament heats the metal plate, the electrons will be free and scattered in small Vacuum glass In the bottle. The second pole is the "Plate" (represented by P), which is basically the metal plate at the outermost edge of the vacuum tube. When the eye sees the dark gray or black metal plate at the outermost edge of the vacuum tube, it is usually the plate. The plate is connected to a positive voltage, which is responsible for attracting electrons emitted from the cathode (remember? The principle of opposites attraction), as the end point of electron free travel. The third pole is the "grid" (represented by G). From the perspective of structure, it is like a circle of thin coils, just like a fence, fixed between the cathode and the screen. The electron flow must pass through the grid to the screen, and the voltage between the grids can control the flow of electrons. Its role is like a water tap Generally, it has the function of circulation and blocking.

The vacuum tube light has three poles, so the later vacuum tubes have been continuously improved, and there are also many ways to improve the structure to match with different amplification methods (such as superlinear connection), but the content of this part has been separated from this article and will not be detailed for the time being.

History

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The engine must run with fuel, and the action power of the vacuum tube is electric energy. Among the electrodes of vacuum tubes, the most important one is the cathode, which is responsible for releasing electrons as the basis of all actions. The earliest vacuum tubes, because of their simple structure and theory, directly used the filament as the cathode. In other words, when the filament was lit, the electrons would be released from the filament due to the increase of the filament temperature, passing through the grid and going straight to the screen. This kind of vacuum tube is called "direct heating vacuum tube". 300B, the main character of this topic, belongs to this type of vacuum tube. Compared with other modern five pole vacuum tubes, 300B has simple structure and good performance, output power Also low.

Direct heating

Directly heated vacuum tube

Filament can be made of different materials. Because the directly heated triode directly uses the filament as the cathode, the characteristics of the filament directly affect the performance of the directly heated vacuum tube. Basically, the filament of vacuum tube can be divided into three kinds of materials. The first one is of course high temperature resistant Tungsten wire 。 Transfer the high-purity tungsten wire take a percentage The filament is wound in the innermost layer of the vacuum tube, and can generate temperature after being energized. However, the tungsten filament must be heated to more than 2000 degrees before the electrons can disperse. Therefore, when the vacuum tube made of tungsten filament is ignited, it will emit brilliant brightness, and the temperature is too high. Don't be surprised. It's not that the vacuum tube is going to burn, but that it is! However, it needs more power to light the tungsten wire. The advantage is that the tungsten wire is very durable and is widely used in vacuum tubes with large power or long life. I often hear people say: "That vacuum tube is so bright when it is lit, it must be lit after two or three times". In fact, in some cases, the service life of this vacuum tube can reach tens of thousands of hours. It can be used as a light bulb at home, which is both durable and decorative!

The other filament uses thorium tungsten alloy It only needs to heat the filament to a thousand degrees to work, which saves electricity in comparison. The most commonly used is oxidation Alkaline soil Filament, which is coated with a thick layer of oxidized alkaline earth outside the filament, looks like a white gray substance. It only needs to be heated to about 700 degrees (looks like dark red) to obtain sufficient electrons working temperature It is the lowest and the most power saving. Generally speaking, it only needs to supply about 6.3V DC to work normally.

Direct heated vacuum tubes certainly have their inherent advantages, but there is a fatal disadvantage, that is, the cathode is vulnerable to the temperature of the filament to change the characteristics. When filament Voltage variation , or alternating current When the filament is supplied, the cathode is in an unstable state. Therefore, some people advocate that DC power supply should be used for direct heated vacuum tubes, while others emphasize that AC power supply must be used to avoid damaging the cathode. This debate has long been a controversial topic in the audio industry. The author has no intention of causing a topic here. In any case, all parties have their own reasons. As long as it sounds OK, the pipes are durable and sound good. If you have research experience, the author is very happy to accept.

Parathermal type

Parathermal vacuum tube Stability higher

In order to solve the filament problem of the directly heated vacuum tube, the vacuum tube designer decided to separate the filament and cathode, put a circle of metal sleeves around the filament, let the filament directly heat the metal plate, and the electrons emit from the metal plate. This heating method is called "parathermal vacuum tube".

Further development

Operating principle of vacuum tube

In this way, the vacuum tube seems to be much more stable because of the volume and heat storage capacity of the metal sleeve Much greater than For the traditional filament, even if the temperature of the filament changes temporarily, or even stops heating for a few seconds, the temperature change of the metal plate is limited, which is the main reason why some expanders can sing for more than ten seconds after they are turned off. Since cathode and filament are independent, cathode plate must be composed of filament Indirect heating So the filament was changed to tungsten again, in order to Durability And a layer of white magnetic field is coated on the outer layer of the tungsten wire. On the one hand, it is insulated, on the other hand, it also has the effect of shaping. Due to the poor indirect heating effect, the cathode metal plate will be coated with thorium, barium or other substances conducive to electron emission. Therefore, the metal plate of the vacuum tube always looks gray and black, unlike the normal metal plate, and because the production and assembly must rely on hand, there are always many small scratches left on the metal plate, so buyers need not worry when buying the vacuum tube.

Vacuum tube difference

For ordinary users, the difference in the use of direct heating vacuum tube and side heating vacuum tube is not necessary to care about the difference between direct heating vacuum tube and side heating vacuum tube, but for designers, the side heating vacuum tube is indirectly heated, Filament current Generally, it is large, and the cathode metal plate must be heated for the parathermal structure, so there is a slow heating period after the startup. If it is the front stage, the delay design must be done to prevent the startup pulse from hurting the rear stage.

According to the development process, the earliest vacuum tube is of course a direct heating design. Diodes were first developed. The functions of diodes are just like those of today Diode With rectifier and radio With proper design, the diode can also become Manostat , such as the current Zener Diode. Since the operating principle of the vacuum tube is very simple, many scientists have joined the research and development work since the first vacuum tube was successfully manufactured. The first triode was successfully manufactured by an American scientist in 1907, which ushered in the advent of the radio era. Farewell phonograph , entering the era of expansion.

structure

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The vacuum tube has a cathode (K) that emits electrons and an anode or plate (P) that is usually applied with high voltage during operation. The filament (F) is a very thin Wire The current passes through it, making the metal wire generate light and heat, which will excite the cathode to emit electrons. The grid (G) must be placed between the cathode and the plate. Grid voltage is used to restrain the amount of electrons passing through the grid, so it can play a role in the current between the cathode and anode Control function 。

In order to maintain the vacuum state in the tube, the vacuum tube is equipped with an object called Degassing Agent. It is generally made of barium, aluminum, magnesium and other active metal alloys. After extracting the air from the tube, heat the elements and degassing agent in the tube to Red fever In this way, the gas contained in the electrode in the tube can be absorbed. Use one around the pipe High frequency electromagnetic field The degasifier will sublimate rapidly, and the degasifier will absorb the gas in the pipe. After reaction, glass tube Silver degassing agent coating is accumulated on the inner wall. If the Tube body When the glass tube is broken or leaks, the silver degassing agent accumulated on the inner wall of the glass tube will fade, which also means that the vacuum tube cannot be used.

working principle

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Vacuum tube

Now, let's take a closer look at the working principle of the simplest vacuum tube. Tidy up As just mentioned, the vacuum tube has several electrodes, from the innermost layer to the outermost layer: filament, cathode, grid, and screen. Disassemble a vacuum tube and draw it in the attached figure, From Figure It can be seen that when the filament is lit, Filament temperature Gradually increase, although in a vacuum state, the filament temperature Radiant heat When the temperature of the cathode metal plate reaches the temperature of electron dissociation, electrons will rush out of the metal plate. At this time, the electrons are negatively charged. If a positive voltage is applied to the plate, the electrons will be attracted and fly towards the plate, passing through the grid to form an electron flow. Just now, the grid is like a switch dead The flow of electrons will stably pass through the grid and reach the plate. When a positive voltage is added to the grid, it is attractive to electrons and can enhance the speed and power of the flow of electrons; On the contrary, if a negative voltage is added to the grid, electrons must detour to reach the screen. If the grid structure is huge, the electron flow may be blocked completely.

The gate can easily control the flow of the electron flow, connect the input signal to the gate, add an appropriate bias voltage, and attach a resistor to the plate string to achieve the purpose of signal amplification. The vacuum tube is also connected with Transistor Similarly, it has a variety of amplification configurations (in fact, the amplification configuration of transistors is an application extending from vacuum tubes). It can create ever-changing electronic products by combining different electronic materials such as resistors, inductors, transformers and capacitors. Don't forget, the first computer was made of vacuum tubes. Of course, it can only do simple addition and subtraction. So far, the basic working principle of the vacuum tube has been reported. What is missing? Please observe the inside of the vacuum tube wall, there is a piece similar to Mercury This is a design to extend the service life of vacuum tubes. Except for a few low-pressure vacuum tubes (not working voltage Low refers to the existence of vacuum tube inside Low pressure gas ）Most vacuum tubes must be vacuumed to work properly. The connecting pin of the vacuum tube is a metal pin, although Glass package However, there is still a chance of air leakage between the glass and the metal connector. Metal in glass tube evaporation Things (i.e Getter ）, will interact with gas, and its purpose is to absorb gas to maintain vacuum degree 。 This thin layer of metal will turn white after oxidation, indicating that the vacuum tube has failed to leak. Therefore, if the vacuum tube is broken, this layer of evaporated material will also turn white. Therefore, when purchasing an old vacuum tube, you should also pay attention to the evaporation of the plating material. It is better to have something like mercury. If it starts to pale and flake, it means that the vacuum tube has entered old age.

Operating principle of diode

Users who use the 300B vacuum tube must have an experience. Turn on the power supply of the amplifier and turn off the indoor lights. At this time, the 300B filament will emit a faint yellow light. At the same time, at the top of the vacuum tube, mysterious blue light like aurora will sometimes appear. The blue light looks thin, soft and slightly mysterious. Like the aurora, it sometimes twists and waves, and seems to shine in the vacuum tube. People who see blue light for the first time are curious about it. Some people say it doesn't matter. Others say it is abnormal. Basically, blue light is generated based on several factors. 1. There is low-pressure gas inside. 2. The vacuum tube is poorly designed or manufactured. 3. The plate voltage is too high.

The main source of blue light is still electrons. When the design and coating of the plate are not good enough to attract the electron flow to the plate, electrons will flow everywhere. The blue light seen by the vacuum tube is the result of the electron flow in the vacuum tube. Blue light looks beautiful, but it may produce radiation, but the author is not sure whether it is harmful to human body. The appearance of blue light is also closely related to the vacuum tube brand. Continental tube and Soviet Union Although Sovtek has a greater chance of blue light than others, the three 300B amplifiers I use myself use four Continental Tubes and two WE300Bs. Only the Continental Tubes will emit blue light, which is considered normal after a long time.

1916 Wired telephone The triode made for use is the most simple direct heated triode with a luminous filament, such as Palisade The grid of is between the filament and the screen, and the screen is located at the bottom, which is a metal sheet.

classification

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According to heating mode

Vacuum tubes can be divided into two categories: direct heating and side heating.

Direct heating The vacuum tube was born earlier. It has a fatal disadvantage that the cathode is vulnerable to the temperature of the filament to change its characteristics. When the filament voltage changes or the filament is supplied with AC power, the cathode appears in an unstable state.

Side heating type The vacuum tube is relatively stable. Because the volume and heat storage capacity of the metal sleeve is far greater than that of the traditional filament, even if the filament temperature changes temporarily, or even stops heating for a few seconds, the temperature change of the metal plate is limited, which is the main reason why some expanders can sing for more than ten seconds after they are shut down, because the power supply part has a large capacity capacitor Internal residual power is not discharged completely.

according to container structure classification

Most commercially available vacuum tubes have glass walls. The military and other special types are made of metal.

Differentiate according to structure and use

Vacuum tubes can be divided into diode 、 triode 、 Tetrode , quadrupole tube bundle Pentode and Composite pipe And so on. Common models according to use: