Home  Vacuum Tube

Sep 7 2019

What is Vacuum Tube? Basic Structure and Types

 Catalog 

Article Core IntroductionHistory1) Based on Specific Requirement
Vacuum TubeNoteAdvantages and Disadvantages2) Based on Specific Circuit Parameters
TerminologyVacuum Tube Gate (Grid)TypesApplication Attentions
Vacuum Tube CathodeVacuum Tube AnodeSelectionExpand Service Life

  Terminology 

This video explains how electronic vacuum tubes flow electrons to amplify guitar signals and rectify AC to DC.

Vacuum tube is one of the earliest electrical signal amplifier components. It consists of an outer glass housing, several internal electrodes, and pins that connect the electrodes. In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. There is a filament, a cathode (a metal plate close to the filament or the filament itself, usually indicated by K, in addition, the tube directly using the filament as the cathode is called the directly-heated type, and having independent cathode is called the heater-type. ), and a plate (a metal located on the outermost place, usually indicated by P). And the diode is the simplest vacuum tube.

A structure of vacuum tune is that an 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. An electric field is used to inject an electronic modulation signal into the control grid in the vacuum, and different signal data is obtained at the anode through signal amplification or feedback oscillation. In an actual working situation of an electron tube, when the filament having current, it will generate heat. When the heated cathode reaches a certain temperature, the electrons will get enough energy to be emitted, and the electrons will be absorbed by the plate. Due to the filament or the cathode cant absorb electrons, so the direction can't be reversed. This one-way conductivity is the basis of the working principle of the tube.

In early stage, vaccum tubes were used in electronic products such as televisions and radio amplifiers, but it has been gradually replaced by amplifiers and integrated circuits made of semiconductor materials in recent years. However, in some high-fidelity audio equipment, electron tubes with low noise and high stability are still used as audio power amplifier components.

vacuum tubes

  Vacuum Tube Cathode 

Cathode is a part for emitting electrons, and is divided into an oxide cathode and a carbonized thoriated tungsten cathode. In general, the oxide cathode is the heater-type, which uses a special filament to heat a cathode body coated with ruthenium oxide for thermal electron emission, and its life expectancy is generally between 1000 and 3000 hours. The carbonized thoriated tungsten cathode is generally the directly-heated type, and generates heat electron radiation by heating, so it is both a filament and cathode. Theoretically, the carbonized thoriated tungsten cathode has a much longer life than the oxide cathode, and is generally in the range of 2,000 to 10,000 hours. In addition, the most widely used high-power launch tube is the carbonized thoriated tungsten cathode, which is generally used in launch tubes with an output of less than 1 kW.

In recent years, there have been many high-power transmitting tubes using mesh cathodes. The mesh cathode is made of a thin tantalum tungsten wire, and its advantages are:

1) Since it is braided with a large number of twisted tungsten wires, it has good current circulation.

2) It is easy to reduce the cathode-grid distance, which is beneficial to improve transconductance.

3) Because the filament is in a mesh structure, the current of the single filament is small, and the local magnetic field is weak, so that the ac vibration interference generated by the cathode current is also small.

kt-88 vacuum tubes

Note: 

A. Directly-heated cathode and heater-type cathode

There are two ways of cathode heating: directly-heated cathode and heater-type cathode. The heated-type cathode tube is designed for battery power supply, and the filament is the cathode. Therefore, the filament heating is only suitable for DC power supply. If AC power is used, a large hum can be generated under normal conditions.

The heater-type cathode is separated from the filament, the cathode is formed into a cylindrical shape, and the filament is placed in the center of the glass cylinder. The cathode is in a high temperature heated by the filament to cause the cathode to reach a sufficient temperature to emit electrons. The way is uniform and efficient. Since the cathode is separated from the filament, hum noise will not generated by the AC power supply. In practice, he heater-type cathode can also be powered by DC.

B. Cathode types

Among the large family of electron tubes, there are three types becoming the mainstream, such as a tungsten cathode, a carbonized cathode, and an oxide cathode.

1) A tungsten cathode is a cathode made of a tungsten wire. Tungsten cathode is one of the earliest used cathodes in electron tubes. It has the advantages of good stability and long life. It does not reduce the emission performance due to the impact of positive ions generated by residual gas ionization in the tube, so it is suitable for tubes with high anode voltage. The disadvantage is that enough electrons can be emitted when the temperature reaches to 2300°C. To achieve such a temperature, the heating current is large and the energy consumption is high, so it is mostly used in high-power emission tubes.

2) The carbonized cathode is coated with a cathode made of carbon and thorium oxide on the tungsten wire, and emits sufficient electrons at around 1700 °C. Compared with the tungsten cathode, the heating current is small and the energy consumption is low, but the stability is not as good as that of the tungsten cathode. Moreover, the working plate pressure is low, so it is mostly used for the medium power launch tube.

3) The oxide cathode is made of a cathode made of lanthanum, calcium, lanthanum or the like coated on a nickel wire, and emits enough electrons at a temperature of about 800 °C, and the emission current is much larger than that of the tungsten cathode and the carbonized cathode. One of the most efficient emission in the cathode is used for small power tubes. In addition, it also has powerful pulse emission characteristics. When a pulse voltage having a large duty factor is applied between the plate and the cathode, a large pulse emission current can be obtained, which is unique. Therefore, this cathode is also widely used in electron tubes (thyristors) in which pulse current is operated.

The oxide cathode has the disadvantages of poor stability, low working plate voltage, and is easily damaged by positive ions impact, commonly known as cathode poisoning. The so-called poisoning refers to the phenomenon that the cathodes work function is increased under the action of harmful gases, and the emission performance is lowered. When the filament voltage is insufficient and the cathode temperature is too low, the oxide cathode is more susceptible to damage. The oxide cathode is easily to be affected by positive ions, so when the cathode is operated in a continuous emission state, it is only suitable for use in an electron tube having an anode voltage lower than 3000V.

6650 EH vacuum tube

  Vacuum Tube Gate(Grid) 

The gates of the tubes are divided into first gate and second gate according to their roles in the tube, sometimes referred to as control grid or screen. The main function of the first gate is to control the cathode current, and the function of the second grid is to shield the influence of the plate pole on the first grid. The structure of the gate is related to its mechanical performance and heat dissipation effect, and it is also related the tube stability. In order to reduce the transit time of electrons, the cathode-grid spacing less than 1mm. Therefore, manufacturers use materials with high mechanical strength, high thermal conductivity, good emissivity and high melting point to make the gate. Also it has the function to avoid thermal bumps occurring at very small intervals. The first gate and the second gate should be strictly meet with the alignment of grids, so that the screen has small electron interception, which can reduce the screen consumption and improve the current distribution.

 

  Vacuum Tube Anode 

The anode is an electrode that collects the electrons emitted by the cathode. When the electron tube works, with the electron tube bombards the surface of the plate and the heat radiation of other electrodes, a large amount of heat energy is generated in the plate. Because the power dissipation density of the plate is tens of watts to several hundred watts per square centimeter, such a large power density can no longer be addressed by traditional cooling method, like the natural radiation. Therefore, forced cooling is required, and air cooling, water cooling and evaporative cooling are commonly used.

 vacuum tubes image

  History 

In 1883, Thomas Edison was doing a small experiment to find the best filament material for light bulbs. He installed a small piece of copper wire near the carbon wire inside the vacuum bulb, using the copper wire to prevent the carbon filament from evaporating, and he failed. But he accidentally discovered that there was no copper wire connected to the circuit, but it generated a weak current due to the reception of the hot electrons emitted by the carbon wire. At the time, Edison was concentrating on studying the urban power system and did not pay attention to this phenomenon. But he applied for a patent for this discovery and named it the Edison Effect”. Later, it was proved that the current was generated because the hot metal could emit electrons to the surroundings. But the first to realized the practical value of this effect was the British physicist and electrical engineer Fleming.

In 1904, the worlds first electronic diode was inverted by the British physicist Fleming, and he  obtained the patent for this invention. The birth of the first electron tube, marking the world has entered the electronic age.

In 1906, the American inventor De Forest Lee invented the first vacuum transistor by adding a grid between the filament and the plate of the diode. This small change had brought unexpected results. It not only responded more sensitively, but also emitted vibrations of music or sound. Moreover, it combined the functions of detection, amplification and oscillation. Therefore, many people regard the invention of triodes as the real birth point of the electronics industry. De Forest Lee himself was also very surprised.

The advent of electron tubes has promoted the booming of radio electronics. Around 1960s, the radio industry in the west had an annual output of 1 billion radio tubes. It was applied to telephony amplifiers, marine and air communications, and also widely used in home entertainment, broadcasting news, educational programs, literature and music to thousands of households. Even with the invention and further development of aircraft, radar, and rockets, electron tubes have also played an important role.

Soon after the introduction of the tube, people were trying to find new electronic devices. In the Second World War, the shortcomings of the electron tube were more exposed. Ordinary tubes used in the radar operating band are extremely unstable. The tubes used in mobile military equipment and devices are more awkward and prone to failure. Therefore, the inherent weaknesses and urgent wartime needs of the tubes had prompted many research institutes and scientists to concentrate on developing new solid components to replace the clumsy tubes.

 

 Advantages and Disadvantages 

Advantages

1) good load capacity

2) Excellent linear performance

3)High working frequency

4)The operating characteristics of the high-frequency and high-power fields are better than those of the transistors, so they continue to play an irreplaceable role in some places (such as high-power radio transmitting equipment and high-frequency medium heating equipment).

Disadvantages

The tube has large size, large power consumption, large heat generation, short life, low power utilization efficiency, fragile structure and needs high-voltage power supply.

 preamp vacuum tube

 Types 

According to its different functions, electron tube can be divided into voltage amplifier, power amplifier, gas tube, thyristor, ignition rectifier, converter tube, rectifier tube, detector tube, resonance indicating tube (tuning eye), voltage regulator and so on.

According to the number of electrodes, the electron tube can be divided into voltage amplifier, triode tube, quadrupole tube, pentode tube, hexapole tube, hexapole tube, octopole tube, ninth pole tube and composite tube according to the number of electrodes. An electron tube excess three electrodes are also called multielectrode tube or multigate tube.

According to its shape and housing material, the tube can be divided into a glass tube (ST tube), an acorn tube, a cylindrical glass tube (GT tube), a large glass tube (G-type tube), a metal porcelain tube, a small tube ( also known as peanut tube or finger tube, MT tube), tower tube (light tube), ultra-small tube (pencil tube) and so on.

According to its internal structure, the electron tube can be divided into a single diode, a diode, a duplex diode triode, a dual diode, bipolar transistor, bipolar-pentode compound transistor, beam quadrupole, triode-pentode compound transistor, triode-hexapole compound transistor, triode-hexapole compound transistor and other types.

According to heating way, the electron tube can be divided into a directly-heated type cathode tube (current directly passes through the cathode to reach a thermal electron emission state) and a heated-type cathode tube (heating the cathode through a filament beside the cathode). 

According to shielding way, The electron tube can be divided into a sharp cut-off shield tube and a remote cut-off shield tube. 

According to cooling way, the tubes can be divided into water-cooling tubes, air-cooling tubes and naturally cooling tubes.

 

 Selection 

1) Based on specific requirement

There are many types of tubes with different functions. When selecting, the appropriate type and model should be selected according to the specific requirements of the applied circuit (for example, whether it is a voltage amplifier tube or a power amplifier tube).

2) Based on specific circuit parameters

The tube should be used in strict accordance with the voltage values (including filament voltage, screen voltage and curtain grid voltage, etc.) specified in the product manual. The operating voltage and current values of the application circuit also should be considered, otherwise the service life of the tube will be shortened.

 electron tubes

  Application Attentions 

1) The voltage of each electrode of the electron tube should be connected in strict order, that is, filament-bias-anode-screen-excitation signal, and in reverse order when switch off. The filament of the tube, especially the tungsten carbide filament, is very fragile, thus the frequent switch has a fatal effect on it. The cold and hot resistances of the filament are different, which will produce a certain electromagnetic attraction. Therefore, it is better to add voltage to the filament gradually, which is very beneficial for extending the service life of the vacuum tube.

2) The filament voltage should within the allowable error range of the rated value, usually 5%, and it is advantageous to maintain the lifetime within ±1%. When the voltage of the oxide cathode tube filament is too high, the decomposition of barium oxide is accelerated and the life of the cathode is shortened; when the filament voltage is low, the barium atom cannot be rapidly diffused to the surface of the cathode, and the emission capability of the cathode electron tube cannot be recovered. When the filament voltage of thecarbonized thoriated tungsten cathode is high, the thorium atoms in the filament will evaporate quickly, shortening the life of the cathode, and when the filament voltage is low, the cathode will be destroyed by positive ion bombardment.

It has been proved that the transmitting tube of the DC filament power supply should change the positive and negative polarity of the filament once after working for a period of time, so that the entire cathode can be uniformly used, and the polarity of the electrolytic capacitor connected to the ground of the filament input end is also changed. In addition, the power is reduced due to insufficient cathode emission, and the filament voltage can be appropriately increased to increase the filament current to expand the service life.

3) The warehouse where the electron tubes are stored requires dry, dust-free, moisture-proof, shock-proof and well-ventilated, and do not place volatile chemicals. The room temperature is between 5 °C ~35 ° C, and the relative humidity is not more than 80%. The tube should be placed vertically with the anode down, and do not place heavy objects on the tube. The storage time is generally not more than 3 years to prevent the vacuum from weakening. In addition, the tube inside will release some gas after long-term storage, causing vacuum reduced, so it should be aging when used. First, add the 50% filament voltage to the tube for 10 min to 15 min; then add the rated filament voltage for 30 min; third add a bias voltage; four, add 50% of the rated anode voltage for 20 min to 30 min, and finally increase the anode voltage when it is equal to the rated value. This aging process can extend the life of the pipe.

4) Before installation, the appearance of the electron tube shall be checked for bubbles, oil, cracks and any mechanical damage. Metal parts shall not be rusted. The stain on the ceramic may be wiped off with alcohol; the rust on the metal parts shall be erased with sandpaper before use, then wipe clean with alcohol. Be careful and slow during installation process, avoiding damage due to vibration. The tube should be placed vertically in the axial direction to maintain the vertical concentricity of the anode. The electrodes should be in good contact with the cavity reeds, otherwise high voltage ignition or unstable operation may occur.

5) In the work, all the poles of the launch tube should be well ventilated, if the cooling temperature is too high, which will reduce the life of the tube, or cause damage. For air cooling, pay attention to the smoothness of the air vent; for water cooling system, evaporative cooling must use soft water or distilled water.

Check the working status of the transmitter regularly, do not allow the transmitter to appear high frequency ignition, machine detuning, parasitic oscillations, etc., and try to avoid excessive gate flow, in addition, too short grid flow may also damage the tube.

6) Although it can keep working normally, the life will be shortened when used in the extreme parameters.

7)When using a heated-type vacuum tube, the potential difference between the cathode and the filament cannot exceed the specified limit.

8)Try to avoid using a filament series circuit.

 

  Expand Service Life 

Long-life tubes must not only have good quality, but also have properly designed circuits, sufficient heat dissipation, and thoughtful suspension.

When using, the vaccum tubes should have good ventilation and heat dissipation. Overheating of the temperature must shorten the life of the tube, so it is necessary to keep the tube at a lower temperature as much as possible. The tube is afraid of vibration, so it is also important to take measures to avoid vibration as much as possible. To do this, there should be a suitable space around the tube device, especially above it for good convection ventilation, and if possible, a fan can be used to help dissipate heat.

The cathode of the tube is applied a high-voltage power source when it is not yet at the required temperature, its cathode will be damaged, which will also shorten the life of the tube. Therefore, if the electronic tube equipment has a preheating device, it must be used. For example, the filament low voltage power supply is preheated first, and the high voltage power supply is slowly applied after the opening. If there is no preheating device, then you should not connect the input signal immediately. If a high-temperature rectifier is used to supply the high voltage of the whole machine, it provides a simple and effective high-voltage delay. Also, do not switch the power supply frequently during normal use.

Of course, if the tube circuit is properly designed to avoid misuse, the life of the tube can be extended. The most common errors in circuit design are the large potential difference between the tube filament and the cathode, the tube control gate is suspended, filament voltage is too low or too high, and the tube installation is improper to cause electrode overheating and high voltage power supply.

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