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Sep 17 2019

Basic Vacuum Tube Structure

Introduction

An electron tube is an electronic device that generates current conduction in a hermetic closed  glass container, and utilizes an electric field to attract electrons in a vacuum to obtain signal amplification or oscillation.

That is, a cathode electron-emitting portion, a control grid, an acceleration grid, and an anode (screen) lead enclosed in a glass container are soldered to the tube base, and the electric field is used to inject the electronic modulation signal into the control gate in vacuum, and the signal data of different parameters after amplification or feedback oscillation of the signal are obtained at the anode.

tube symbol

Structure

Cathode: A source that emits electrons, but at normal temperature, the cathode does not emit electrons, and it needs to be heated to emit electrons. This type of heating is called electron emission. All tubes emit electrons in this way.

Filament: It used to heat the cathode, the directly-heated type of filament is the cathode, and the heater-type filament is only responsible for heating the cathode, not as an cathode.

Anode: Receives electrons emitted from the cathode and connect the positive electrode of power supply.

Gate(Grid): Controls the amount of electrons reaching the anode, in addition, adding negative voltage on suppressor grid, and positive voltage on screen grid.

Core column: Supports each electrode in the tube and leads each electrode out of the tube.

Mica sheet: Fixs electrodes position and sustains the relative position between electrodes to prevent the oscillation damage.

Getter: It is used to absorb the residual gas in the tube, so that the tube is always in a high vacuum state. The purpose of this is to ensure that no gas ionization to change carrier properties. And the characteristic curve becomes worse, and the electrode life is shortened. Therefore, the leaky tube is a damaged tube, and can’t be used continuously.

Base Pin: The pin is mounted on the leading foot to fix and install the tube and connect it to the external circuit.

Shell: It used to form a vacuum area to fix the internal structure.

vacuum tube structure

A basic vacuum tube typically has three poles, a cathode (K) for emitting electrons, an anode (A) for absorbing electrons emitted by the cathode, and a grid (G) for controlling the flow of electrons to the anode. The basic conditions for the cathode to emit electrons are: the cathode must have heat. The cathode is divided into two types: one is a directly-heated type, which is an electric current that directly passes through the cathode to cause the cathode to generate heat; the other is the heater-type. The cathode is generally a hollow metal tube with a filament wound into a spiral shape. The filament voltage causes the filament to heat up, so that the cathode generates heat and then emits electrons. Most of the tubes used today are such tubes. The electrons emitted by the cathode pass through the gap between the gate wires to reach the anode, and the gate is much closer to the cathode than the anode, thus changing the gate potential has a much greater effect on the anode current than when the anode voltage is changed.

small vacuum tube structure

To increase the amplification factor of the electron tube, a gate is additionally added between the anode and the control gate, which is called a screen grid. Since the screen grid has a positive voltage much higher than the cathode, it is also a very powerful accelerating electrode that allows electrons to reach the anode quickly at a higher rate, so that the effect of the control gate becomes more pronounced. However, due to the acceleration of electrons by the screen grid, high-speed moving electrons hit the anode. The high-speed electrons have great kinetic energy, and so-called secondary electrons will be emitted from the anode. Some of these secondary electrons will be absorbed by the screen grid to form a curtain grid. The current causes the curtain grid current to rise, resulting in a drop in the screen grid voltage, and a drop in the anode current.

Due to the simple structure, the earliest electron tube directly used the filament as a cathode. In other words, when the filament is lit, the electron is released from the filament due to the increase in the temperature of the filament, and goes straight through the gate to the screen. This kind of electron tube is also called directly- heated type tube.

electron tube structure

The Filament can be made of different materials. Since the directly-heated tube directly uses the filament as the cathode, the characteristics of the filament directly affect the performance of the it. In general, the filament of the electron tube can be mainly divided into three materials: first one is the tungsten wire with high thermostability. The high-purity tungsten wire is drawn into a filament, wound in the innermost layer of the electron tube, and the temperature is raised after being powered. However, the tungsten wire has to be heated to more than two thousand degrees, the electrons can be emitted. Therefore, the tungsten wire needs to consume a large amount of power. Tungsten wire is very durable and is commonly used on high power or long life tubes. In some cases, this vacuum tube can last for tens of thousands of hours and is used as a light bulb in the home, which is both durable and decorative. Another type of filament is made of tantalum-tungsten alloy, which only needs to heat the filament to more than one thousand degrees to work, which is power-saving. The most commonly used is oxidized alumina filament, which is made by coating a thick layer of yttrium oxide outside the filament. It only needs to be heated to about 70 degrees, you can get a sufficient amount of electrons under the lowest operating temperature and the most power-saving state. In general, it only needs to be supplied about 6.3V DC, the tube can work normally.

Looking the tube insider, a mercury-like film can be seen sticking to the glass wall, which is a design that used to extend the life of the tube. Except for a very small number of low-voltage tubes (not low operating voltages, but low-pressure gases inside the tubes), most of the tubes must be evacuated for proper operation. The pins of the tube are metal feet. Although sealed in glass, there is still a chance of air leakage between the glass and the metal pins. But the Metal evaporated plating in the glass tube acts with the gas, and it exists to absorb the gas to maintain the vacuum state. After this thin layer of metal is oxidized, it will turn white, indicating that the tube has leaked, so if the tube is broken, this layer of vaporized material will also turn white, so when buying a tube, it is necessary to see the vapor deposition condition. If it looks pale or fall-off, which means the tube has aged.

vacuum tube diagram

3 comments

    • Brain Willett on 2019-10-16 17:17:29

    From the article, we can know the basic structure of vacuum tube, that is, a basic tube typically has three poles, a cathode for emitting electrons, an anode for absorbing electrons emitted by the cathode, and a grid for controlling the flow of electrons to the anode.

    • Rob Perse on 2019-10-18 10:15:32

    Hello, I have an little problem, why are most vacuum tube housings made of glass?

      • author on 2019-10-18 14:27:59
        author

      Re:

      Because glass is a heat-resistant, well-processed, and economical material, not only for the pass  but also for today.

    • Hugo Lee on 2019-10-22 17:01:25

    From the article, we can know the basic structure of vacuum tube, that is, a basic tube typically has three poles, a cathode for emitting electrons, an anode for absorbing electrons emitted by the cathode, and a grid for controlling the flow of electrons to the anode.

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