How tubes work

Tubes, also known as vacuum tubes, or valves, are electronic elements generally made of glass and metal. They use the thermionic emission and electrostatic attraction to work. They are composed of metal plates and grids.

The physics of thermionic emission

Tubes are based on two effects : the thermionic emission and the attraction and repulsion of electrical charges. Metals are conductive materials due to the presence of free electrons in the surface. Hence, they cannot escape the metal without enough energy. The energy can be supplied in a number of ways. One of them is heating the material above a critical temperature, giving enough energy to the electrons to escape the surface. This is done with the help of a filament in wich an electrical current flows. The filament has to reach sufficient temperature, typilcally above 500°C. To whistand that temperature, they are made of special material, including tungsten and thoriated tungsen.

The first illustration shows a metal plate (1) at low temperature, the electrons (2) stays on the surface. The next illustration shows the metal above the critical temperature, a majority of electrons are escaping.

Attraction and repelling

The electron's movement is influenced by electrical fields created by charges around it. As an image, he is always attracted toward the most positive charge, and repelled from the negative charge.

The following illustration shows electrons lefting the metal, repelled by the negative plate, but attracted by the positive one.A metal plate (1) emitting electrons, the electrons (2) repelled from the negatively-charged plate (3) and attracted toward the positively-charged plate (4).
We will see in the following sections that these effects are used by elements called grids to control the electron flux, and then the current flowing through the tube.

  1. A Metal plate The metal plate is heated and emits electrons.

  2. Electrons The electrons are lefting the metal plate due to their energy.

  3. Negatively-charged plate This plate repels electrons

  4. Positively-charged plate This plate attracts electrons

 
 
The diode

The diode is the simplest vacuum tube. The filament, heated with an electrical current, heats the cathode.  As every metal heated to a sufficient temperature, the cathode emits electrons because they have enough energy to left the cathode’s surface.  When the potential of the anode is positive regarding to that of the cathode, electrons are attracted: an anode current builds up. When the anode’s potential is negative, no electrons are attracted, there is no current. This tube then let current flows only in one direction, it’s a diode used in rectifiers.

 

  1. A filament Generally made of tungsten.

  2. A cathode Covered with a special material to enhance electron emission.

  3. An anode To collect emitted electrons.

 
The triode

The triode is an evolution of the diode and permits to control the anode current with a voltage. It is made of a diode to which a grid is added between the anode and the cathode. This grid is held at a negative potential regarding to the cathode and influences the electrons that left the cathode. These electrons are then under the influence of the anode voltage and of the grid voltage. This grid, when sufficiently positive let electrons left the cathode to the anode, or forces them to stay at the cathode when sufficiently negative. Then the anode current is controlled by the grid voltage. The more negative is the voltage on the grid regarding to the cathode, the less the current is in the tube.

  1. A Filament Heating the cathode.

  2. A Cathode Heated by the filament and emitting electrons.

  3. An anode Toward which electrons are going.

  4. A control grid Controlling the flux of electrons from the cathode to the anode.

 

The tetrode

The triode has a low internal resistance, meaning that the anode current not only depends on the grid voltage, but also to the anode voltage. When the grid voltage is more negative, the anode current tends to lower. This increases the anode voltage which tends to increase the anode current. The two effects are opposed. To counter this effect, another grid is added, called « screen grid », between the first grid (control grid) and the anode. Held to a constant positive potential, it counters the influence of the anode voltage on the anode current. Thus, the internal resistance of the tube is greatly increased.

  1. A Filament Heating the cathode.

  2. A Cathode Heated by the filament and emitting electrons.

  3. An anode Toward which electrons are going.

  4. A control grid Controlling the flux of electrons from the cathode to the anode.

  5. A screen grid To counter the effects of anode voltage on anode current.

 
The pentode

The tetrode has a major drawback: the secondary emission effect. The electrons reaching the anode have a high velocity. The collision between the electrons and the anode releases many other electrons called secondary electrons, which are attracted to the screen grid when the anode voltage is low. This effect lowers the anode current, depends on the anode voltage and distorts signals. A new tube has then been made, the pentode, with a third grid: the suppressor grid. This grid is directly connected to the cathode and repels the secondary electrons because of its negative potential regarding to the anode.  An evolution is the “beam power tetrode” using beam-forming plates instead of the suppressor grid, achieving a higher efficiency.

  1. A Filament Heating the cathode.

  2. A Cathode Heated by the filament and emitting electrons.

  3. An anode Toward which electrons are going.

  4. A control grid Controlling the flux of electrons from the cathode to the anode.

  5. A screen grid To counter the effects of anode voltage on anode current.

  6. A suppressor grid To repell the secondary electrons emitted from the anode.

Other tubes

Many other tubes exist. Here is an example of an ECF80, which is a tube including a triode and a pentode. It was used in radio equipment. We can distinguish the two elements : the bigger one is the pentode, and the smaller one is the triode.

 
Filament heating

The thermionic emission of the cathode depends largely on temperature. For a tungsten cathode, it rises quickly at about 2500°K. It could then look like a good idea to use the tube with a higher filament voltage, but the life expectancy depends largely on the filament’s temperature: for some types of tungsten filaments, the life expectancy is about 1500 hours at 200mA/cm² (high emissivity value) and 5000 hours for 50 mA/cm². Some consequences are that a small variation in the filament’s voltage, or an overload of the tube, can reduce the life expectancy of the tube (we will see implications in the section “importance of bias”). The optimal temperature is a compromise between performances and life expectancy.

 
The getter

During the process of making a tube, vacuum is initially created and the glass envelope is sealed. Any small amount of gases will ionize and cause bad insulations values or arcing, leading to a major malfunction. This gas can come from internal materials that releases absorbed gases for a long time after vacuum is made, or from a broken glass envelope.
A silver-colored material is then placed on the tube, which reacts with gas molecules and absorbs them. This removes all small amounts of gases left inside the tube.
When the getter has absorbed too much gases, his color changes from silver to white, indicating that the tube cannot be used anymore. This usually indicates a broken glass envelope.
Here are two same tubes, one in working condition and the other with a broken glass envelope.