Vacuum tubes, which can also be referred to as electron tubes, or thermionic valves are a type of active device that uses thermionic emission to generate electrons. All tubes will have at least two elements, an anode and a cathode. The entire assembly is sealed inside a glass or metal envelope under a hard vacuum. Metal tubes may have the envelope electrically connected to one of the external pins, so that it can be used as a shield.

The cathode may either be a bare tungsten wire (a directly heated cathode), or more commonly a metal oxide coated metal tube containing NiCr heating elements electrically isolated from the cathode with mica fibers. Directly heated cathodes are usually used when the filament will be running from DC, or in some types of rectifiers. Since indirectly heated cathodes aren't electrically coupled to the heater, a separate voltage is used, which may be AC or DC. The combination of heat and a high-vacuum environment allows the cathode to generate a cloud of free electrons, which are attracted to the positively charged anode. Depending on the type of tube, there may be one or more elements between the cathode and anode to control the beam of electrons.

Some tubes have gain, and can be used as an amplifier. The level of gain of triodes and other tubes with gain is referred to in technical literature as mutual conductance, expressed as mu or μ. High mu tubes are tubes with a high level of voltage gain, which are used to increase the voltage of low level signals like those output from microphones and phono cartridges. An example of a high mu tube is the 12AX7, which has enough voltage gain to increase the 4.7mV output from a phono cartridge to almost line level.

Medium mu tubes have slightly less gain and are used further along in the signal chain, as a gain buffer in a preamp, for example. A common medium mu tube that is frequently used as a preamp output tube is the 6DJ8.

Low mu tubes have low voltage gain and high current gain, with a lower output impedance. They're used as power output tubes to drive things like loudspeakers and headphones.

A tube is generally plugged into a device via a standard type of tube socket. The three most common types of tube sockets are 1" 8-pin sockets (octal), four pin sockets used on a few directly heated triodes like the 300B, and miniature 9-pin (noval) sockets used for smaller all-glass tubes. All tubes generate some heat, how much is dependent on the type of tube. Small preamp tubes only become warm to the touch, while larger output tubes and rectifiers can get quite hot under load, although not as hot as a lightbulb. Since heat rises, heat sensitive components like electrolytic capacitors shouldn't be mounted directly above a tube.

Types of Tubes

In audio engineering there are four common types of tubes, diodes, triodes, tetrodes and pentodes, named after the number of electrodes in the tube. In each type, there are hundreds of different models, each with their own operating characteristics and pinouts.

Diodes

Diodes are the simplest form of tube with only two elements, anode and cathode. These work the same way as their solid state counterparts, allowing current to flow in only one direction, like an electrical version of a check valve. The usual use for this type of tube is in an AC-to-DC rectifier. Rectifiers with one or more diodes in the same envelope are available with a wide range of operating voltages and currents. The common 5AR4 rectifier is composed of two diodes with a common cathode in the same envelope, and provides full wave rectification when used with a center-tapped transformer, with the center tap as "ground".

Smaller diodes are used for signals, such as separating the positive and negative component of an audio signal, or as a detector in radio sets. For the most part, tube based diodes and solid state diodes are interchangable, but a lot of new audio equipment designs still use tube rectifiers, mostly because it allows for a soft start with a slow rise in voltage that prolongs the life of the other tubes in the circuit, and it's a little easier than going through the trouble of adding timers and relays.

Triodes

A triode is the basic tube with gain, it performs the same general function as a transistor, it is an electrical gate valve. As in a diode, the triode has a cathode generating electrons and a positively charged anode that attracts the electrons. In between the anode and cathode is a third element, usually made from a spiral of fine wire, called the grid. Applying a slightly negative voltage to the grid completely stops the flow of electrons. As the positive voltage on the grid is increased, more electrons are passed to the anode, increasing the anode current. Because the flow of electrons can be controlled with a small voltage to produce large voltage swings at the anode and cathode, a triode can be used as an amplifier.

Tetrodes

Tetrodes are similar to triodes, but with an extra element between the anode and grid. This screen grid lowers the capacitance between the grid and anode. Since the grid-anode capacitance repels some of the electrons, reducing it allows for higher gain.

Pentodes

Pentodes are similar to tetrodes, but with an extra element between the screen grid and anode, called a suppressor. Under certain operating conditions, electrons striking the anode dislodge other electrons in a process called secondary emission. Since this is undesirable, the suppressor was added to direct the secondary electrons back towards the anode. In practice, it is normally connected to the cathode. Pentodes are commonly used as output tubes, since they can handle larger voltage swings with less distortion.

Beam Power Tubes

This type of tube, which may be a tetrode or pentode has a set of beam confining electrodes to shape the stream of electrons leaving the cathode. Once again, this type is often used for power amplifiers because of their ability to handle large current swings with relatively low distortion. The 6V6 is an example of a beam power tetrode, the 6L6 is an example of a beam power pentode.