The intention of this is to give a basic explanation of the vacuum tube. It is not intended to explain various tube circuits, including those used for audio amplification.
The predecessor to the vacuum tube was the incandescent light bulb, patented by Thomas A. Edison in 1879. In 1883, Edison realized he could detect electrons flowing from the heated filament to a metal plate located in the vacuum with the filament, but he pursued these findings no further. The effect is known as the Edison effect.
Around 1900, John Ambrose Fleming, of Lancaster England, invented what he called the "thermionic valve", now known as the diode vacuum tube, and patented his invention in 1904. In 1906, American inventor Lee de Forest added a third electrode and the triode vacuum tube was born.
The outstanding difference between the vacuum tube and most other electrical devices is that the electric current does not flow through a conductor, but through empty space-a vacuum.
If a piece of metal is heated to incandescence in a vacuum, electrons near the surface are given enough energy of motion to fly off into the surrounding space. The higher the temperature, the greater the number of electrons emitted. This is called "thermionic emission", the emitting metal is called a "cathode". These free electrons form a "cloud" of negative charge within the vacuum bottle, which is called the "space charge".
If a second conductor is introduced into the vacuum, and given a positive charge by connecting a voltage source between it and the cathode, the free electrons will be attracted to it. This second conductor is usually a cylinder or metal plate, and is called an "anode" or "plate".
Since electrons are negatively charged, and are attracted to the plate only when it is positively charged with respect to the cathode, the vacuum tube conducts in only one direction.
The Filament or Heater
It is not essential that the heating current flow directly through the cathode, just that it is heated. The "filament", or "heater", can be separate electrically from the cathode, which makes it possible to use material that heats to high temperature under much lower current than the cathode material, so the cathode can be directly heated (current actually running through the cathode) or indirectly heated (current running through a separate filament that heats the actual cathode by induction).
A small positive voltage on the plate will only attract a small number of electrons, because the space charge prevents those electrons nearest the cathode from being attracted to the plate. As the cathode voltage is increased, the effect of the space charge is increasingly overcome, and more electrons are attracted to the plate. Thus, the "plate current" is directly related to the "plate voltage".
This two element tube is called a "diode", and has limited uses. It is commonly used for rectification - changing alternating current into direct current. Common diode tubes are 5AR4 (European designation GZ34), 5Y3, and 6C4.
If a third element is inserted between the cathode and plate, it can be used to control the effect of the space charge. This element is called a "grid". If the grid is positively charged with respect to the cathode, it will help neutralize the negative charge of the space charge, and more electrons will flow to the plate. If the grid is negatively charged, it will reduce the number of electrons that can reach the plate at any selected plate voltage. Since the grid is intended to control the space charge, and not to attract electrons to itself, it is usually made in the form of a wire mesh, hence the name, and electrons pass through the open spaces in the grid to reach the plate. The grid can also be in the form of a spiral.
This tube element arrangement of cathode, single grid, and anode is called a "triode". It is very common to see two triodes in the same vacuum bottle, and those tubes are called "twin triodes". Common twin triode tubes are the 12AX7 (ECC83), the 12AT7 (ECC81), and the 12AU7 (ECC82), and these tubes are often seen as preamp tubes in audio circuits.
The grid acts as a valve to control the flow of plate current. It actually has a much greater effect on plate current flow than does the plate voltage - a small change in grid voltage is just as effective in changing plate current as is a large change in plate voltage. This allows for the possibility of "amplification" in the triode tube. The many uses of the vacuum tube, besides rectification, nearly are all based on this amplifying feature. The amplified output is not obtained from the tube itself, but from the voltage source connected between the plate and cathode...the tube simply controls the power from this source, changing it to the desired form.
Each pair of elements in a tube creates capacitance...there are three such instances within a triode, between the cathode and grid, between the grid and plate, and between the plate and cathode. These capacitances are very small, but they can have a very pronounced effect on the operation of an amplifier circuit.
Inserting a second grid between the grid and the plate can reduce their capacitance to a negligible value. This is called a "screen grid" or "screen" and serves as an electrostatic shield to prevent capacitive coupling between the "control grid" (our original grid) and the plate.
The shielding action of the screen grid prevents the plate from attracting electrons as it did in the triode, so it is necessary to apply a positive charge to the screen grid. The electrons are now attracted towards the screen grid, much as the plate, but most of the electrons acquire such velocity that they shoot between the screen wires and then are attracted to the plate.
A tube with four elements, anode, control grid, screen grid, and plate, is called a "tetrode".
The electrons, after flowing through the control grid and towards the plate, are now traveling at such a velocity that when it strikes the plate, it dislodges other electrons from the plate surface, and they "splash" into the space between the tube elements. This is called secondary emission. The problem is that the positively charged screen attracts these secondary electrons, causing a reverse current to flow between the screen and plate. This creates a huge problem in the tetrode tube. Two methods have been used to overcome this reverse current.
In a "beam tetrode", the screen and control grids are carefully lined up as so the openings in the screens make a direct line between the cathode and plate, and secondary plates are placed on the sides to direct the electrons towards the plate. The electrons now flow in "beams" towards the plate, and form a stationary "cloud" near the plate surface that acts to repel the secondary emission.
Another method of overcoming this is inserting a third grid, called the "suppressor grid" between the screen and plate. This grid acts as a shield between the screen grid and plate so that the secondary electrons cannot be attracted back to the screen grid. A five element tube is called a "pentode". Common pentode tubes are 6L6, 6V6, EL34, and EL84, and are commonly seen in the power amplification section of audio amplifiers.
It is also possible to combine triodes and pentodes in the same vacuum bottle. These tubes are not as common as the others, probably due to problems caused by the extra heat created in the same bottle. The 6GW8 (ECL86), the 6BM8 (ECL82), the 11BM8, and the 11MS8 are the more commonly seen triode/pentode tubes. There are also triple triodes, dual pentodes, diode/pentodes, etc.
Vacuum tubes have many characteristics that determine what they are used for, how they will react in various situations, how much amplification they are capable of, etc. I will limit explanation of these factors to just a few.
The "plate resistance" of a tube is the resistance of the path from cathode to plate.
The "amplification factor" of a tube, usually designated by the Greek letter mu, is the ratio of the change in plate voltage to the change in grid voltage to effect equal changes in plate current. If, for example, an increase of 10 plate volts raised the plate current 1.0 mA, and an increase in (-) grid voltage of 0.1 volt were required to required to return the plate current to its original value, the amplification factor for that tube would be 100. Triodes generally have an mu value of 3 to 100.
The amplification factor of screen grid tubes is usually over 1000, but, as these tubes have a high plate resistance, the actual voltage amplification is much less than the large amplification factor would indicate. A voltage gain from 35 to 200 is typical of a pentode.
The "saturation point" is the point where the plate voltage has overcome the space charge, and almost all of the electrons are going to the plate. At plate voltages above this point, the plate current stays nearly the same, and the output signal normally distorts. Which, for guitarists, is not a bad thing.
Reference: "The Radio Amateur's Handbook 1972 edition", ARRL: Newington Conn, 1972