Basic Vacuum Tube Glossary

This is a basic glossary of terms used in electron tube equipment. The definitions are brief, and intended only to give the barest of descriptions. The descriptions given are intended to be intuitive, rather than strictly accurate. The author is aware that protons and neutrons are not truely fundamental particles.


Synonym for alternating current. Informally used to refer to alternating voltage.

Active Component

Active components are components which have a connection to a source of electrical power, and a control electrode which is used to control the flow of current through them. Typical active components are electron tubes, transistors, and integrated circuits.

Alternating Current

Also called AC, electric current which reverses direction periodically. This term is also sometimes used informally for alternating voltage.

Alternating Voltage

Also called AC informally, voltage which reverses sign between (+) and (-) periodically.


Synonym for amplitude modulation.


Abbreviation for ampere.


The unit of electric current (after André-Marie Ampère 1775-1836), representing a flow of electric charge amounting to one coulomb per second, often abbreviated to amp.


An electronic circuit which takes an electronic signal as an input, and produces a replica with increased amplitude, (voltage amplifier), current (current amplifier), or power (power amplifier).


One of the four distinguishing characteristics of periodic signals.

Amplitude Modulation

See modulation.


The basic unit of which pure elements are composed. They have a compact and massive nucleus at the center which contains protons bearing a (+) charge, and neutrons which bear no charge. Electrons, which bear a (-) charge, surround the nucleus. For more information see Basic Tube Theory.

Audio Frequency

A frequency which lies within a range considered audible to human beings. This is nominally the range of 20 Hz to 20,000 Hz, though few human beings can actually hear sounds over that entire range.


Capacitance is the property of storing energy in an electric field.


A capacitor is a passive component used to introduce a controlled amount of capacitance into an electronic circuit.


A cathode is an electrode in an electron tube used to emit electrons into the highly evacuated or gas filled interior of the tube. Cathodes may be either heated or cold. Heated cathodes emit electrons only when they are hot, and may be either directly or indirectly heated. Cold cathodes emit electrons without the necessity of being heated

A directly heated cathode is one which is heated by passing an electric current through it, and is called a filament. Any heated cathode which is not directly heated is termed indirectly heated. Indirectly heated cathodes may be either heated by a separate heater, in which case they are called unipotential cathodes, or may be heated by other means, e.g. by ion bombardment and eventual electronic current as used in the 0Z4 tube.

A cold cathode emits electrons, obtaining the energy necessary to do so by means other than heat. Phototubes, for example, use light energy to cause their cathodes to emit electrons.

For more information see Basic Tube Theory.


As a noun, an abbreviation for electric charge. As a verb, it means to remove or introduce charged particles, thus producing a net electric charge.

Charged Particle

Charged particles are particle which bear electric charge and are subject to the forces exerted by electric fields. The two most important charged particles are the proton and the electron. These two, along with the neutron, comprise atoms.


Abbreviation for electronic circuit.

Cold Cathode

A cold cathode is a cathode which is not heated.


Components are basic single units from which electronic circuits, are constructed. Components may be active or passive.


Synonym for capacitor.


A substance or thing in which electrically charged particles are relatively free to move, and which therefore can carry an electric current.

Control Electrode

A control electrode is an electrode in an active component used to control the flow of current through the component.


A type of circuit which accepts as an input a modulated periodic signal and produces as output a periodic signal at another frequency having the same modulation characteristics as the input signal.


The unit of electric charge (after Charles-Augustin de Coulomb 1736-1806), equivalent to the amount of charge carried by 6.28 x 10^28 (62,800,000,000,000,000,000,000,000,000) electrons.


Abbreviation for electric current.


Synonym for direct current. Also used informally to refer to a relatively constant voltage which does not change sign between (+) and (-).


A circuit which recovers the impressed intelligence from a modulated signal.


Synonym for demodulator.


An electron tube with two elements, a cathode, and a plate. most commonly used as rectifiers and detectors.

Direct Current

Also called DC, a relatively constant current which flows in one direction only.

Electric Charge

Unfortunately, we do not know what electic charge is. We do know that it comes in two forms, (+) positive and (-) negative. The unit of charge is the coulomb. For more information, see Basic Tube Theory.

Electric Current

The motion of charged particles, by convention from (+) to (-). Unfortunately, in electronic circuits the moving particles are electrons which bear a (-) charge, and hence move in the direction opposite to conventional current. The unit of current is the ampere. For more information see Basic Tube Theory.

Electric Energy

Energy in the form of an electric field, magnetic field, or electric current.

Electric Field

An effect on the space surrounding charged particles exerting forces on the particles, either attractive or repulsive, depending upon their respective charges. We say that like charges repel, and unlike charges attract. Creating a net electric field in otherwise neutral matter requires moving charged particles contrary to the electric force between them, thus expending energy. This energy gets stored in the field as electric energy. For more information, see Basic Tube Theory.

Electric Force

The force charged particles experience when immersed in the electric field of other charged particles.

Electric Potential

The effective height of the electric field surrounding a charged particle. By analogy with the gravitational force, we can picture electric fields as similar to a landscape with hills (regions of positive potential) and dales (regions of negative potential). The gravitational force propels water away from hills and causes it to flow toward dales.

Likewise positively charged particles experience a force which propels them away from regions of positive potential and attracts them to regions of negative potential, causing an electric current to flow. Unfortunately for this analogy, negatively charged particles, which make up common electric current must be imagined as flowing uphill, due to a mistake by Benjamin Franklin.


An electrical conductor used to make contact with the vacuum or low pressure gas in an electron tube.


One of the three fundamental particles comprising atoms.

Electron Tube

An electron tube is an evacuated chamber in which electrons flow from an emitting electrode called the cathode to one or more other electrodes. Electron tubes may either be highly evacuated vacuum tubes, or filled with a gas at low pressure. For a brief description of how most vacuum tubes work, see Basic Tube Theory.

Electronic Circuit

An arrangement of one or more electronic components through which an electric current flows.

Electronic Signal

An electric current or voltage which is varied in time for the purpose of conveying information, such as voice, music, digital data, etc.


The ability to do work. It takes various forms, as the energy of motion (kinetic energy), the energy of heat (thermal energy) or electric energy.


A condition in a region of space created by particles in or near that region, and affecting some or all of the other particles within that region. We think of forces as originating in fields which emanate from the particles. The most commonly experienced fields are the gravitational field, the electric field, and the magnetic field.


A filament is a directly heated cathode.

First Detector

Synonym for converter.

Fleming Valve

A high vacuum electron tube diode used as a radio detector invented in 1897 by John Fleming in England, and initiating the electron tube era. They were called valves because electric current could flow through them in only one direction.

early Fleming Valves

Fleming's Experimental Valves


Synonym for frequency modulation.


One of the four distinguishing characteristics of periodic signals.

Frequency Modulation

See modulation.


Short for control grid, a control electrode in triodes or in multigrid tubes used to control the current flowing between the cathode and plate. Multigrid tubes have other grids in addition to the control grid.

For more information see Basic Tube Theory.


A heater is an element of an electron tube which is used to heat an indirectly heated cathode.


Inductance is the property of storing energy in a magnetic field.


An inductor is a passive component used to introduce a controlled amount of inductance into an electronic circuit.


A substance or thing in which electrically charged particles are tightly bound to a location, and which therefore cannot carry an electric current.


An atom which has become charged by losing or gaining one or more electrons. See Basic Tube Theory.

Magnetic Field

The magnetic effect on the space surrounding magnetic materials and electric currents. Creating a magnetic field using electric currents requires moving electrons against the magnetic force they experience exerted by the increasing magnetic field, thus requiring energy. Hence magnetic fields store electric energy. This effect is used in inductors.


A circuit which modulates a periodic signal.


The act of modifying the amplitude, frequency, or phase of a periodic signal, producing a modulated signal. While the resulting signal is no longer periodic, it may be nearly so, and one still may speak of its amplitude, frequency, and phase, in an average sense.

If the modulation affects the amplitude of the periodic signal, we speak of amplitude modulation, or AM. If the modulation affects the frequency of the periodic signal, we speak of frequency modulation, or FM. If the modulation affects the phase of the periodic signal, we speak of phase modulation, or PM.

The usual reason for modulating a signal is to convey a message, or other form of intelligence, as music, voice, etc. The act of recovering the impressed intelligence is demodulation. The corresponding demodulator is often referred to as a detector, and we speak of AM or amplitude detectors, FM or frequency detectors, and PM or phase detectors.


One of the three fundamental particles comprising atoms.

Passive Component

A passive component is a component which has no control electrode. Typical passive components include resistors, capacitors, and inductors. For more information see Introduction to Components (warning: 30 megabytes).


The unit of resistance (after Georg Simon Ohm 1789-1854), symbolized as Ω (greek upper case omega), in some vintage literature as ω (greek lower case omega). One hundred feet of twenty gauge wire or one foot of forty gauge wire each have a resistance of about one ohm. See Ohm's Law.

Ohm's Law

A difference of potential of one volt produces an electric current with intensity of one ampere in a conductor with a resistance of one ohm.

In symbols:

V = I x R


V is the voltage in the circuit

I is the current in the circuit

R is the resistance in the circuit


One of the four distinguishing characteristics of periodic signals.

Periodic Signal

A signal which varies in time such that at regular time intervals the signal repeats itself. The smallest such time interval is called the period of the signal. The reciprocal of the period is called the frequency of the signal, the unit of which is the hertz (after Heinrich Rudolf Hertz 1857-1894), symbolized as Hz, previously by the cycle per second (cps), or even briefly as cycles. By convention, unvarying signals are also considered periodic with infinite period and zero frequency.

The overall shape of a periodic signal during one period is called the waveform. The measure of the strength of a periodic signal is called its amplitude. The difference in time between the starting points of the periods of two otherwise identical periodic signals is called the relative phase of the signals. When the waveform of the signals is sinusoidal, the phase is sometimes expressed in degrees.

Periodic signals can differ only in waveform, amplitude, period (or equivalently frequency), and phase.


One of the four distinguishing characteristics of periodic signals.

Phase Modulation

See modulation.


A type of phototube used for seeing in low illumination environments, colloquially to "see in the dark".


An electron tube in which the energy supplied to the electrons in the cathode causing them to be emitted into the chamber is supplied by light.


The most positive electrode in diodes and triodes.


Synonym for phase modulation.


The rate of expenditure of energy. The basic unit of power is the joule per second, or watt (after James Prescott Joule 1818-1889 and Scottish engineer James Watt 1736-1819, resp.). In electronic circuits with only resistors in them, the power converted to heat may be calculated as

P = I x I x R = V x V / R


P is the power dissipated (turned into heat)

I is the current in the circuit

V is the voltage in the circuit

R is the resistance in the circuit

Power Amplifier

An amplifier used to increase the power of a signal, as for driving a speaker or transmitting antenna.


One of the three fundamental particles comprising atoms.

Radio Frequency

A frequency which lies above a range considered audible to human beings, i.e. greater than 20,000 Hz.


An electron tube used to convert alternating current into pulsed unidirectional current, which may then be filtered into direct current.


An electronic circuit which stabilizes the voltage or current provided as the output from another circuit, making it constant.


The property of converting electric energy into thermal energy (heat).


An electronic component used to insert a controlled amount of resistance into an electronic circuit.

Second Detector

Synonym for demodulator.

Thermal Energy

Energy in an object due to the random motion of the particles comprising it.


An electron tube with three active electrodes, a cathode, a grid, and a plate. Used as amplifiers, detectors, modulators etc. For more information see Basic Tube Theory.


Abbreviation for electron tube.

Unipotential Cathode

A unipotential cathode is a cathode which is indirectly heated by a heater.

Vacuum Tube

A vacuum tube is a highly evacuated electron tube.


Synonym for electron tube


The unit of difference in electric potential, symbolized as V (after Alessandro Giuseppe Antonio Anastasio Volta 1745-1827). A standard flashlight cell produces a constant difference of potential of about 1.5 V. See Ohm's Law.


The difference in electric potential between two points. It is measured in volts.


One of the four distinguishing characteristics of periodic signals.