A Quick And Dirty Musician's Glossary of Vacuum Tubes

There were moments in the 1980s and '90s when musicians who favored tube amplification were hesitant to commit their money to purchasing vacuum tube amplifiers, out of fear that the limited supply of tubes, and the dwindling number of manufacturers would cause their equipment to become obsolete and unserviceable. This fear turned out to be unfounded. In 2010, the supply of vacuum tubes is healthy, and there has in fact been an increase in the number of vacuum tube manufacturers due to the strong demand for tube-powered equipment.

This guide is not intended as a complete resource to tube electronics, but should serve to clarify most of the terms used by tube electronics manufacturers in their marketing materials and technical specifications.

The single most common preamp tube in modern audio electronics - especially guitar amplifiers. A dual-triode design, the 12AX7 has a high gain factor, and is robust. A miniature, noval design. The dual triodes can be used independently for two gain stages, or tied together to increase frequency response, gain, and amplification before clipping.

A high powered octal vacuum tube which provides a large amount of output. Used by Fender in the Bassman, Twin, and Showman amplifiers.

A common lower-powered octal vacuum tube developed to drain less power from the batteries powering automotive radios than the 6L6 types. Used by Fender in the Champ, Princeton, and Deluxe amplifiers.

Anode of a vacuum tube. Collects electrons emitted by the cathode. The Anode maintains a negative electrical charge, and the regulation of this charge is an essential function of the power supply of the device.

From early days of electronics, when all power was delivered from batteries, "B+" refers to the high-voltage supply applied to a tube's plate. This is most often in the hundreds of volts, but at relatively low current.

A DC (direct current) voltage applied to the grid of a vacuum tube to set the middle point of the voltage swing to equal the middle point of the incoming signal. Bias is either fixed (in that the bias setting remains at a consistent point) or dynamic, through a process known as cathode-biasing, in which the tube's electrical characteristics combined with the design of the circuit maintain a bias point. Preamp tubes are typically cathode biased, while power tubes may be cathode or fixed-biased.

Emits electrons (current) when heated directly or indirectly. The source of current within a vacuum tube.

The point at which a vacuum tube receives a signal at a level that it lacks the ability to further amplify, and cannot reproduce the peaks. At this point, the signal becomes "clipped." Vacuum tubes clip differently than solid-state devices, which are thought to be tonally "harsher." This is a primary reason for the favored-status of tube audio electronics. Clipping is often confused in marketing literature with the concept of gain (see "gain").

A two-electrode device. Allows current to flow in only one direction. Diodes were the first electrical devices developed within glass tubes.

A high powered British vacuum tube most identified with Marshall Amplifiers, but also used to great effect by Hiwatt, Orange, Traynor, and others. American manufactured versions are often labelled 6CA7.

A low powered noval vacuum tube which most famously powers Vox's AC15 and AC30 amplifiers, as used by The Beatles, Tom Petty, and Brian May of Queen.

An element withing a vacuum tube which applies heat to the cathode, causing thermonic emission. Some tubes are built with the heating element and cathode as separate devices. Some are constructed in such a way that the heater actually acts as the cathode. Filaments may also be referred to as "heaters."

Filtering, Filter Capacitor
Tube power supplies almost universally employ filtering elements designed to compensate for the residual "ripple" in the DC (direct current) power supply that is generated by even the most efficient rectifiers in the process of converting AC (alternating current) power into DC. In the case of instrument amplification, these capacitors can act as a bottleneck when the power section of the amplifier attempts to draw more current than is available within the capacitors themselves. This effect is experienced as "sag" and is audible as a low-frequency response that is sluggish to develop as the capacitors struggle to supply enough current. Amplifiers with higher levels of filtering experience this effect less severely. After a sufficient level of filtering for noise-reduction has been achieved, designers may choose to include additional filtering components to combat this "sag." Musicians prefer high and low levels of filtering for differing styles of music, but there is no rule on what is most appropriate for any given application.

A measurement of the amplification ability of a tube. Gain is dependant on the circuit that runs the tube, as well as the electronic and mechanical characteristics of the tube itself. The term "gain" is also used to refer to the amount of distortion produced by the preamp sections of amplifiers equipped with a master-volume. Such amplifiers are typically marketed as "high gain." This is something of a misnomer, as such distortion is more accurately the result of clipping, though the level of clipping is increased as the level of gain feeding the element which is clipping increases.

A component in a vacuum tube which introduces a modulation into the current flow from the cathode to anode which, at the anode, is an amplified version of the signal at the grid.

An abstract concept generally used to describe either the amount of input signal a tube can take on its input before it clips, or the amount of volume that an amplifier can produce before the power tubes, output transformer, or speaker introduce clipping and/or compression.

See "filament."

Negative Feedback
A specified portion of the signal from the output of an amplifier that is fed back into a previous amplification stage, but in reverse polarity, causing the frequency components with the greatest level to cancel out to the highest degree. This results in a more even tone with a more linear frequency response, as any overabundant frequencies on the output would incur the most cancellation when returned into the circuit.

"New Old Stock." Typically refers to vacuum tubes manufactured many years ago, and stored unused since that time. NOS tubes are often expected to perform to higher tolerances than newly manufactured tubes, though there are exceptions. They are also typically more expensive on the open market than tubes that are newly manufactured.

A vacuum tube with a nine-pin base. Most frequently of a miniature size, and with a glass-base.

A vacuum tube with an eight-pin base, typically made of plastic or bakelite

Output Transformer
A transformer used to convert the high impedance output of amplifier output tubes into the low impedance signal capable of driving a speaker. The output transformer is considered to be a major contributor to the sound of an instrument amplifier.

A five-electrode device, amplifies like a triode, but with additional control and stability provided by additional screens located between the grid and cathode, and grid and anode.

See "Anode."

Power Amplifier
Also referred to as "the power section" or the "output section," the power amplifier generates the current necessary to power speakers. It is typically intended to do this in as linear a manner as possible, but vacuum tube designs are almost universally less efficient at this than solid-state circuits. Nevertheless, this inefficiency, and tendency to distort when driven hard has over time become the desirable quality in tube-powered instrument amplification. Similar arguments hold true in home (hifi) applications, which are generally more efficiently designed, but employed by users who value the perceived "warmth" and "roundness" of tube amplification.

Power Supply
The power supply of a tube-powered device is the source of the AC and DC charges that operate the device. It typically involves a power transformer, a rectifier, one or more filter capacitors, (for removing residual "ripple" from the rectifier) and may include one or more voltage dropping resistors, and a choke (an inductive device which provides additional filtering).

Power Transformer
A transformer used to convert the mains (wall) current into the voltages required by the different parts of an amplifier. Multiple voltages can be produced by the inclusion of multiple windings with multiple voltage taps.

The first section of an amplifier fed by the input. It involves one or more stages of voltage amplification. It may also be the section of the amp where tone controls, reverb, and other effects are included. Some preamps are sold separately from other parts of an amplifier, and in the case of instrument preamps, are designed to shape and color an audio signal, while outputting a line-level signal. Other preamp applications include voltage amplification for microphones for recording and live audio reinforcement, and home audio (hifi) functions - typically for phonographs and other playback devices. These non-instrument preamps are typically designed to amplify low-level signals without coloration or distortion.

A diode that creates a DC (direct current) signal out of an AC (alternating current) signal. Tube rectifiers are less efficient than solid-state rectifiers, and that inefficiency leads to characteristics that many musicians find desirable. These characteristics however, are not always sought, and many tube guitar amplifiers are built with solid-state rectifiers. Because rectifiers were amongst the first solid-state devices to replace tubes, and because they are not in the signal path, amplifiers built with solid state rectifiers are generally still considered "tube amplifiers."

A screen within a vacuum tube, positioned between the grid and either the cathode, or the anode, or both, used to regulate and control the flow of current between the grid and cathode, or grid and anode. The use of a screen allows a tube's electrical behavior to be more regulated, and can result in more efficient operation.

Thermonic Emission
The emission of electrons (current) from the cathode when heat is applied. This is the fundamental process by which vacuum tubes operate, and the basis for all vacuum tube theory and research.

A three-electrode device. Signal at the grid modulates a current flowing from the cathode to anode. 

Term used interchangeably with "vacuum tube." Preferred terminology in Europe.

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