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Oct 25 2019

Vacuum Tubes: Types of Vacuum Tubes Tutorial

 Vacuum Tube Definition

The vacuum tube, also called electron tube or valve, is one of the earliest electrical signal amplifier components in electronics. A cathode electron-emitting portion, a control grid, an acceleration grid, and an anode (screen) lead enclosed in a glass container (generally a glass tube) are welded to the tube base. The principle is, an electric field is used to inject an electronic modulation signal into the control gate in the vacuum, and different parameter signal data is obtained at the anode after signal amplification or feedback oscillation. The inside of the glass bottle is evacuated to facilitate the flow of free electrons, and the oxidation loss of the filament can be effectively reduced. 

Vacuum Tubes / Valves : In this Video We Will Go Through the Top 10 Things You Need to Know about Vacuum Tubes.


Ⅰ Vacuum Tube Definition

Ⅱ Vacuum Tube Classifications

Ⅲ Specific Vacuum Tube Details

Ⅳ Common Vacuum Tubes for Sale

Ⅴ Question Related to Vacuum Tube Types and Going Further

5.1 Question

5.2 Answer

Ⅱ Vacuum Tube Classifications

To use the tube well, you must know what classification the tube has. Here is a brief introduction.

  • According to the application

According to their different uses, the electron tube can be divided into voltage amplifying tubes, power amplifying tubes, inflatable tubes, thyratron, igniting tubes, frequency conversion tubes,  rectifier tubes, detection tubes, tuning indicator tubes, voltage regulator tubes, and so on.

  • According to the number of electrode

The electron tube can be divided into the voltage amplifying tube, triode, tetrode, pentode tube,  hexode, pentagrid, octode, enneode and composite tube according to the difference of the number of the electrodes. More than three poles of the electric tube is also referred to as a multi-pole tube or a multi-grid tube.

Table 1. Common Valve / Electron Tube Types

Valve Type

Number of Electrodes

Number of Grid Electrodes



















  • According to the external shape

The tube can be divided into the bottle-shaped glass tube (ST tube), an “acorn” tube, cylindrical glass tube (GT tube), large glass tube (G-type tube), metal porcelain tube, small tube according to its shape and housing material ( also known as MT tube), tower tube (light tube), ultra-small tube (pencil tube) and so on.

  • According to the anode heating mode

The electron tube can be divided into the filament-type tube (current directly passes through the cathode to reach a thermal electron emission state) and the cathode heater tube (heating the cathode through a filament beside the cathode) according to the heating mode of the cathode.

  • According to the shielding method

The electron tube can be divided into a sharp cut-off shielding tube and a remote cut-off shielding tube according to the shielding method.

  • According to the cooling method

The tubes can be divided into water-cooled tubes, air-cooled tubes and natural cooling tubes according to the cooling method.

 6V6GT vacuum tube

Ⅲ Specific Vacuum Tube Details

The following details about several vacuum tubes:

   1) diode

The metal plate (cathode), the heating source (filament), and the forward voltage plate (anode) are packaged in a suitable shell, that is, the glass (or metal, ceramic) package, and then evacuated to a vacuum state. It is an electron diode.

It should be noted that due to the manufacturing process, the adhesion of impurities and the material itself, a small amount of residual gas remains in the tube, and the finished tube will coat with a getter in the internal tube. The getter is generally a nitrogen-doped evaporation zirconium aluminum or zirconium vanadium material, in addition to special applications (such as UHF and high voltage rectification, etc.). 

   2) triode

The structure of the diode determines its unidirectional conductivity. When a pole with an appropriate voltage is applied between the cathode and the anode, this voltage changes the surface potential of the cathode, thereby affecting the cathode hot electrons flowing to the anode. This is the modulation pole, which is generally a spiral grid made of wire, so it is called the grid, playing a role in the function of the valve. It can be known that when the amplified signal voltage is applied between the gate and the cathode, the anode current will change correspondingly due to its change, and the anode voltage is much higher than the cathode, although small voltage change between the cathodes also causes the anode to produce a corresponding tens to hundreds of times the voltage change, which is the principle of triode amplifying voltage signal.

   3) beam tetrode

The purely meaning tetrode has only appeared as a verification tube in the history of the development of the tube and has not entered the practical use. In the commodity amplifiers, more than half of the models used are beam tetrodes. It is a kind of power tube, and the requirement for the power tube is to generate as much anode current as possible. In addition, it has some special arrangements on the structure of the electrode, so that it can form a larger anode current than other power tubes under the requirement of keeping the volume difference with other power tubes.

  • structure characteristics

A. The cathode is elliptical, which increases the effective emission area, thereby increasing the amount of hot electron emission.

B. Like the triode, a curtain grid is added between the suppression gate and the anode of the beam tetrode.

C. A pair of bow-shaped metal plates are added between the screen and the anode, this is the cluster screen. The cluster screen is connected to the cathode in the tube and is equipotential to the cathode, which forces the flow of electrons that have passed over the screen grid to be directed toward the anode only in radial state on the opening of the arcuate metal sheet.


   4) pentode

The pentode is based on a triode, and two gates are added to form an electron tube having three gates. The special structure makes the interelectrode capacitance is reduced and the amplification factor is increased.


   5) multiunit tubo

A composite tube is formed by combining two or three separate tubes in a single tube.


   6) electron ray oscilloscope tube

It is widely used in electronic oscilloscopes to display waveforms of changes in electrical quantities. It consists of an electron gun, a deflector and a fluorescent screen. In addition, the electric field that controls the trajectory of the electron flow.


   7) kinescope

It consists of electron gun, the deflection yoke, the high voltage pole and the fluorescent screen, and the electric field and magnetic field are controlled to control the trajectory of the electron flow.

 12AX7 vacuum tube

Note: gas-filled electron tube

Features: saturated and breakdown

For conduction reasons, the cathode is attracted by the anode to release the carrier, which is conductive; however, the cathode emits electrons limited, resulting in saturation. But when the voltage is too large, the movement speed of the electron from the cathode to the anode is accelerated, and the inert gas inside is hit. The electrons of the gas become carriers, and this cycle leads to a sharp increase in current, causing the breakdown.

(1) Breakdown also produces a positive charge, but it is heavier and slower to move, not the main cause of a sharp increase in current.

(2) It is best to have a current limiting resistor for breakdown, otherwise the circuit will be damaged.

(3) The voltage is relatively stable after breakdown, because a small voltage increase causes the large current change.

(4) The breakdown can be recovered and can be returned to the saturation.


Ⅳ Common Vacuum Tubes for Sale

Rectifier Diode

12F, 81, 35W4, 25M-K15, 5MK9

Rectifier Dual-diode

80, 82, 83, 5Z3, 5AR4, 5U4, 6X4, 5Y3

Detector Diode

6AL5, EAA91, 6H6

Tuning Indicator Tube

6E5, EM80

Triode Tube Amplifier


Dual Triode Tube Amplifier

12AX7, 12AU7, 12AT7, 12BH7A, 6DJ8, 6SN7

Power Triode

45, WE300B, 2A3, 211, 845, 8045G

Power Dual Triode

6336A, 6080

Power Tetrode Amplifier

UY-807, KT88, 6L6, 6V6

Power Pentode Amplifier

6AU6, 6BA6, 6BD6, 6267, 6SJ7

Frequency Inverter Heptode

6SA7, 6BE6

Emitting Triode

3-500Z, 3-1000Z

Emitting Tetrode


Emitting Pentode

6146B, S2001A

Dual Cathode Heater Small Signal Triode

6922, ECC88, 6DJ8


5.1 Question

What Types of Vacuum Tubes are still in Use?

5.2 Answer

Vacuum tube devices include the X-Ray tubes, cathode ray tubes, magnetrons, and photomultipliers. They have found uses and applications even in modern-day microwave technologies used for mobile phones, Bluetooth, Wi-Fi transmissions, and even in radar and satellite communication devices.

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