A Quick And Dirty Musician's Glossary of Vacuum Tubes |
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Cached from: www.zzounds.com/edu--vacuumtubes
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.
12AX7
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.
6L6
A high powered octal vacuum tube which provides a
large amount of output. Used by Fender in the Bassman, Twin, and Showman
amplifiers.
6V6
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
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.
B+
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.
Bias
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.
Cathode
Emits electrons (current) when
heated directly or indirectly. The source of current within a vacuum tube.
Clipping
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").
Diode
A two-electrode device. Allows current to flow in
only one direction. Diodes were the first electrical devices developed within
glass tubes.
EL34
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.
EL84
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.
Filament
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.
Gain
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.
Grid
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.
Headroom
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.
Heater
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.
NOS
"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.
Noval
A vacuum tube with a nine-pin base. Most frequently
of a miniature size, and with a glass-base.
Octal
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.
Pentode
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.
Plate
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.
Preamp
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.
Rectifier
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."
Screen
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.
Triode
A three-electrode device. Signal at the grid
modulates a current flowing from the cathode to
anode.
Valve
Term used interchangeably with "vacuum tube."
Preferred terminology in Europe.
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