Electronic tube is one of the earliest electrical signal amplification devices. The cathode electron emission part, control grid, acceleration grid, and anode (screen electrode) lead, which are enclosed in a glass container (usually a glass tube), are welded to the tube base. Inject electronic modulation signals into the control gate in vacuum using an electric field, and obtain different parameter signal data after signal amplification or feedback oscillation at the anode. Initially used in electronic products such as televisions, radio amplifiers, etc., it was gradually replaced by amplifiers and integrated circuits made of semiconductor materials. However, in some high fidelity audio equipment, low-noise and high stability electronic tubes are still used as audio power amplification devices (Hong Kong people refer to the use of electronic tube power amplifiers as "amplifiers").

The cathode of an electronic tube

The cathode is a component used to emit electrons, which is divided into oxide cathodes and thorium tungsten carbide cathodes. Generally speaking, the oxide cathode is a side heating type, which uses a specialized filament to heat the cathode body coated with barium oxide and emit hot electrons. The lifespan is generally between 1000 and 3000 hours. Thorium tungsten carbide cathodes are generally direct heating type, which can generate hot electron radiation by heating, so it is both a filament and a cathode. In theory, thorium tungsten carbide cathodes have a much longer lifespan than oxide cathodes, generally ranging from 2000 to 10000 hours. The most widely used high-power transmission tube is thorium tungsten carbide cathode, and oxide cathode is generally used in transmission tubes with output power below 1kW.

In recent years, there have been more high-power emission tubes using mesh cathodes. The mesh cathode is made of thin thorium tungsten wire into a cylindrical shape, and its advantages are: 1) because it is woven with many thorium tungsten wires, the conductivity coefficient is relatively high. 2) Easy to achieve smaller cathode gate spacing, which is beneficial for improving transconductance. 3) Due to the mesh structure of the filament, the current of a single filament is small, and the local magnetic field is weak, resulting in a smaller AC sound generated by the cathode current.

The gate of an electronic tube

The gate of an electronic tube is divided into a first gate and a second gate according to their different roles in the tube, sometimes also known as a control gate or a curtain gate. The main function of the first grid is to control the cathode current, while the function of the second grid is to shield the influence of the plate electrode on the first grid. The gate structure is related to its mechanical strength and heat dissipation effect, and to whether the tube can work stably. In order to reduce the transit time of electrons, the gate to cathode distance is made very short, even less than 1mm. Therefore, manufacturers often use materials with high mechanical strength, high thermal conductivity, good radiation coefficient, and high melting point to make the gate, in order to prevent thermal collisions from occurring at very small distances. The first and second grids should be strictly aligned with each other, so that the curtain grid can intercept electrons less, reduce curtain grid losses, improve current distribution, and enhance performance.

The anode of an electronic tube

The anode is the electrode that collects most of the electrons emitted by the cathode. When the electron tube is working, a large amount of thermal energy is generated at the plate electrode due to the impact of the electron tube on the surface of the plate electrode and the thermal radiation from other electrodes. The dissipated power density of the plate electrode is tens to hundreds of watts per square centimeter, and natural radiation or conduction cooling is no longer sufficient for such a high power density. Therefore, forced cooling method must be adopted. Commonly used methods include air cooling, water cooling, and evaporative cooling.