This page is dedicated to explaining the fundamentals of electron tube technology.

Electron Tubes are electro-mechanical devices that use electrons to perform work.


Note: The following text may be heresy to modern scientists, however this was the state of the art in 1959.

The RCA Receiving Tube Manual RC-19 (1959) describes "electrons" in this way:

"All matter exists in the solid, liquid, or gaseous state.

These three forms consist entirely of minute divisions known as molecules, which, in turn, are composed of atoms.

Atoms have a nucleus which is a positive charge of electricity, around which revolve tiny charges of  negative electricty known as electrons.

Scientists have estimated that electrons weigh only 1/30-billion, billion, billion, billionth of an ounce, and that they may travel at speeds of thousands of miles per second.

Electron movements may be accelerated by the addition of energy.

Heat is one form of energy, which can be conveniently used to speed up the electron. For example, if the temperature of a metal is gradually raised, the electrons in the metal gain velocity. When the metal becomes hot enough, some electrons may acquire sufficient speed to break away from the surface of the metal. This action, which is accelerated when the metal is heated in a vacuum, is utilised in most electron tubes to produce the necessary electron supply."


The RCA Receiving Tube Manual RC-19 (1959) and the RCA Transmitting Tube Manual TT-4 (1956) propose that:

"Electrons are of no value in an electron tube unless they can be put to work.

Therefore, a tube is designed with the parts necessary to utilise electrons, as well as those required to produce them.

Now, the transfer of electrical energy through a circuit involves control of two factors - rate and direction.

The rate of energy transfer is determined by the number of individual electron charges moving unidirectionally through the circuit in a given interval of time and is proportional to the applied voltage.

The direction in which the electron charges move is determined by the polarity of the applied voltage.

In electrical circuits, control of the direction of current flow is necessary when the power source produces ac (alternating current) voltages and currents and the load requires a uni-directional current.

Devices that are used primarily to control the direction of current flow are known as "rectifiers".

All such devices (rectifiers) however are also rate control or rate limiting devices in the sense that they have a finite current carrying capacity.

Now, electric charges may be transferred through a circuit by several methods.

In one method, kinetic energy is transferred between adjacent electrons within the molecular structure of a conductor. This method is employed in switches, rheostats, potentiometers and other devices that utilise conductive materials as control electrodes.

Because the currents through such devices are controlled by mechanical means, the speed with which the amount or direction of current can be changed is limited by friction or inertia.

In a second method, individual electrons are transferred in one direction through semi-conducting materials such as in silicon diodes and transistors. This method has the advantage that the rate of current flow may be controlled by electric fields.

In a third method, individual electrons are transferred through a low-density, non-conductive medium, such as a vacuum or low pressure gas. This method is used in electron tubes and has the advantage that both rate and direction of current flow may be controlled by electric fields.

Because these fields, as well as the electrons, have negligible inertia, electron tubes can effect changes in the value and direction of electric current at speeds considerably higher than those obtainable with mechanically operated devices.



The ELECTRON TUBE is an electro-mechanical device that enables an electrical current to be controlled be external electrical means.

The Vacuum Tube is commonly known around the world by the names "Vacuum Tube, Electron Tube, Radio Tube or Thermionic Valve."

The RCA Receiving Tube Manual RC-19 (1959) describes it so:

"The Electron Tube is a marvellous device.

It makes possible the performing of operations, amazing in conception, with a precision and a certainty that are astounding.

It is an exceedingly sensitive and accurate instrument - the product of co-ordinated efforts of engineers and craftsmen.

Its construction requires materials from every corner of the earth.

Its use is world-wide.

Its future possibilities, even in the light of present day accomplishments, are but dimly forseen; for each development opens new fields of design and application.

The importance of the Electron Tube lies in its ability to control almost instantly the flight of the millions of electrons supplied by the cathode. It accomplishes this control with a minimum of energy.

Because it is almost instantaneous in its action, the electron tube can operate efficiently and accurately at electrical frequencies much higher tha those attainable with rotating machines.

An Electron Tube consists of a cathode, which supplies electrons, and one or more additional electrodes, which control and collect these electrons, mounted in an evacuated envelope.

The envelope may be made of glass, metal, ceramic, or a combination of these materials."

3.2    DESIGN

In its simplest form, an electron tube consists of a cathode (the negative electrode) and an anode or plate (the positive electrode) in an evacuated sealed envelope.

The envelope may be made from glass, metal, ceramic, or a combination of these materials.

More complex types may also contain one or more additional electrodes which control and collect electrons. The electrodes are encased in the evacuated envelope and have the necessary connections brought out through air-tight seals.

The air is completely removed from the envelope to allow free movement of the electrons, and to prevent injury to the emitting surface of the cathode.

The purpose of the cathode is to furnish a continuous supply of free electrons; the plate collects these free electrons.

Now, rate control requirements in electrical circuits range from an occasional on-off switching to continuous variations occuring several billion times per second.

The rate at which electrons are collected by the plate (the plate current) is determined by the the number of free electrons available and by the polarity and the strength of the electric field between the plate and cathode.

Tubes that provide this form of control are known generically as "amplifiers".

Power tube amplifiers are capable of controlling relatively large amounts of energy.

(Note: All triode and multi-grid power tubes are inherently rectifiers as well as amplifiers, because they deliver uni-directional current regardless of the kind of energy provided by the power source.)

Power tubes and rectifiers are usually operated so that the number of electrons available is constant. Consequently, the rate of collection or current flow is determined by the characteristics of the internal electric field.




When the cathode is heated, electrons leave the cathode surface and form an invisible cloud in the space around it.

Any positive electric potential within the evacuated envelope offers a strong attraction to the electrons (unlike electric charges attract; like charges repel).

Such a positive electric potential can be supplied by an anode (plate) (positive electrode) located within the tube in proximity to the cathode."

The Vacuum tube is called a "Valve" is some countries because it performs the traditional function of the valve - ie a valve controls or regulates the flow of fluid in a device, usually a pipe or tube, by limiting the volume of flow of fluid to a pre-determined proportion, or percentage, of the maximum possible rate.

A "valve" is also a "regulator". The function of a "regulator" is to limit the rate, pressure or volume of flow to within a range controlled within pre-determined limits.

The current flowing through a vacuum tube is regulated by the external circuitry - to control the current flow between pre-determined limits.

As we will see in this paper, electrical current behaves just like a fluid and this analogy will be referred to several times.


The Vacuum Tube is an extremely simple device.

In its simplest form, the 'diode", the vacuum tube comprises a cathode and an anode mounted in a vacuum chamber.

A high direct current voltage is applied between the anode and cathode. The cathode is heated to stimulate it to emit electrons. The electrons are then free in space in the vacuum tube, and are attracted to the anode, setting up a stream of electrons, just like water in a pipe.

In a diode the volume of electrons flowing is controlled by the load in the circuit. Diodes are therefore not "valves".

The maximum current that can flow in the circuit ("prospective current") is determined by the capacity of the mains, battery or generator supply limited by the sum of the internal impedances (reactance) of the vacuum tube, circuit components (eg transformers) and the load.

Diodes have limited application and are therefore mainly used to rectify (convert) alternating current to direct current, in which application they offer outstanding performance and reliability if used correctly.

Vacuum tubes are not very efficient, giving up about half the input energy as heat, transferred to the atmosphere and mounting components by convection and conduction respectively. Electrons generated by the hot cathode but not used by the load are also released as heat, described as "plate dissipation".

In directly heated tubes the cathode is heated by itself and is called a "filament" because the electrons are emitted directly from it, just as in a light bulb. Filament warm-up time is quick so current flows soon after the tube is switched on.

In indirectly heated tubes - ie where the cathode is physically separate to the heater, the cathode is called a "cathode", to describe its true function. Cathode warm-up time is slow, so current flows some time after switch on - usually controlled by tube design to about 11 seconds from cold to full warm up.

For a comprehensive explanation of how tubes work see 1952 Text US Army TM11-662 and Airforce Text TO16-1-255

Electron Tubes are designated by the number of electrodes used in their design and construction.

In directly heated tubes the "filament" is classed and counted as an electrode.
In indirectly heated tubes, the "heater" is not classed or counted as an electrode.
In most cases except the "monode" the Electron Tube contains an ANODE ( the "plate") and a CATHODE (the directly heated "filament" or indirectly heated "cathode"). The remaining electrodes are called "grids" and are used to control the flow of electrons between cathode and plate.
Certain types of gas filled tubes may vary slightly from the above configuration.

a)    MONODE  - Single Filament Electrode eg LIGHT GLOBE or LAMP. (Not usually classed as an "Electron Tube" even though they are typically a vacuum tube.)

b)    DIODE - Two electrodes

c)    TRIODE - Three electrodes

d)    TETRODE - Four electrodes

e)    PENTODE - Five electrodes

f)    HEXODE - Six Electrodes

g)   HEPTODE - Seven electrodes

h)   OCTODE - Eight electrodes

i)    BEAM POWER TUBE - Further refinements produced the Beam Power Tube, which may be a tetrode or pentode whose performance is enhanced by mechanical manipulation of the electron beam, to produce substantially greater efficiency, power output and reduced distortion.

Examples of  the Beam Power Tube are demonstrated in the 6L6 family of designs, which include the 6L6GT, 807, 1614, 1625, 5881, 7027A, 7581A/6L6GC and KT66.

Other Beam Power Tubes include 6AQ5, 6CZ5, 6DZ7 (2 x 6BQ5), 6V6GT, 6005, 6550/KT88, 6973, 7581A/KT66, 8417, KT66 and KT88.

These tubes evolved from the 42 and 6F6 family which were pentodes. Comparison of their performance shows reduced heater power requirements and substantially improved performance.

Please note the 6BQ5/6DZ7/EL84/7189, 6CA7/EL34, and 7591/7868/6GM5 families are not Beam Power Tubes but are Pentodes.

Beam Power Tubes may generally be identified by a box-like structure on top of the plate assembly, which extends down inside the tube between the plate and the control grids to direct electron flow.

For an overview of Beam Power Tube design and application see RCA Beam Power Tubes

Electron Tubes may be used in a very wide range of applications.

In audio amplifier and modulator applications the primary functions of Electron Tubes are:

a)    Rectifier
b)    Voltage Stabiliser or Regulator
c)    Voltage Amplifier
d)    Phase Splitter or Phase Invertor (push-pull circuits only)
e)    Power Output

Tube signal output is controlled by the concurrent application of direct current (DC) and alternating current (AC) voltages to the electrodes of the tube.

Most electron  tubes are provided with pins to enable insertion into or removal from a socket, to which external control circuit wiring is attached, however some more modern types have wires instead of pins, to enable direct wiring into apparatus.

Despite the socket/pin system suggesting unreliability, electron tubes can provide reliable service for many, many years - depending upon a range of factors including circuit design.

It is common for electron tubes that have been in storage for 60 years to reliably operate first time when energised.

The electron tube is truly one of the most important devices to benefit mankind ever invented.

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Page last revised 07 April 2012

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