|
A Glossary of Common Amplifier Terms (courtesy of Randall Aiken, Aiken Amplification) There are many terms commonly seen in literature describing
guitar amplifier circuitry which can be somewhat confusing to a person
with no prior background in electronics. This page will attempt
to shed some light on these mysteries, and provide descriptions of common
electronic terms, components, and circuitry in somewhat easy-to-understand
form, although some terms represent concepts that are difficult to explain
in simple language, so you may have to do some additional side reading
to fully comprehend them. The material is presented in an alphabetical,
glossary-style form, so there is some overlap in the definitions. A - The symbol for amps,
or amperes, which is a unit of current flow. Common prefixes are
"m", for mA (10-3 amps), and "u", for
uA (10-6 amps). AC - Alternating Current.
This is electric current that periodically changes the direction in
which it flows. The most common form of an alternating current
supply is the sinusoidal current that comes out of a wall outlet.
It has no positive or negative terminals, because AC has no polarity,
other than an instantaneous polarity that changes at a rate equal to
the frequency of the current. Common household AC current is supplied
at a frequency of 60Hz in the Active - a component
that needs a power source to function, as opposed to a passive component.
Examples of active components are tubes, transistors, opamps, etc.
Also commonly used to refer to guitar pickups that have built-in preamps,
which require batteries to operate. Ali - the name
given to the Alnico - an alloy
of aluminum, nickel, and cobalt which was commonly used in vintage speakers.
It was replaced by cheaper and stronger ceramic (strontium ferrite)
materials, but is making a comeback in "modern vintage" style
speakers, such as the VOX Celestion Alnico Blue, among others. Amplifier - the
other half of rock n' roll (thanks to Ritchie Fliegler for that one).
Anode - the "current
collecting" element of an electron tube, also called the "plate".
The anode usually has a large positive voltage connected to it in order
to attract the negatively-charged electrons from the cathode element
of the tube. If you look at a tube, this is the large grayish metal
piece that encloses most of the other elements. Attenuator - (a)
a network that is used to reduce the amplitude of a signal. Typically,
this is accomplished with two resistors, one in series with the signal
and another from the output of the first resistor to ground. This
attenuates the signal by an amount dependent upon the ratio of the resistor
values. B+ - the high voltage
supply in a tube amplifier. The name is a holdover from the old
days of battery-powered radios, which had an "A" supply for
the filaments, a "B" supply for the high voltage, a
"C" supply for the bias, and a "D" supply for the
screen grids, if a separate supply was used. The conventions held
when radios switched over to rectified AC supplies. Bias - the amount
of negative voltage applied to the grid of a tube with respect to the
cathode, or the amount of idle current flowing in the tube when no AC
signal is present on the grid pin. Biasing - the term
commonly used for the practice of setting the idle current in an output
tube. Preamp tubes are biased as well, but they are biased only
during the initial design of the amplifier and use what is known as
"cathode biasing", and don't require rebiasing as part of
general amplifier maintenance. Blackface - the
term given to older Fenders which had a black metal control panel.
This era of Fender amps transitioned into the "silverface"
amps, which had a silver metal control panel. The transition occurred
at the time CBS bought the company, and some "improvements"
were made to the circuitry of most of the amplifiers. These "improvements"
are generally regarded as detrimental to the tone of the amplifier,
which led to a practice known as "blackfacing"
a Fender amp, which means converting the circuit back to the pre-CBS
schematic. Bridge rectifier
- a set of four rectifiers arranged in a "square" or
"diamond" shape (depending on how you look at it). The
four diodes allow full-wave rectification without the need for a center-tap
on the transformer. Bypass cap - a
capacitor that is connected from the power supply to ground. It
"bypasses" the AC signals to ground, while passing the DC
supply through. This is used to make the DC supply rail "clean",
or free from AC noise. Usually bypass caps are relatively small,
on the order of 0.1uF or so. Larger caps connected in the same
manner are usually called "filter caps". This term is
also used to refer to a capacitor connected across the cathode resistor
on a tube. It bypasses the AC signal to ground without affecting
the DC bias of the tube. This increases the gain of the amplifier
stage. This capacitor can also be used to tailor the frequency
response of the stage. C - the symbol
for capacitance Cap - short for
capacitor. Capacitor - a device
consisting of two parallel plates separated by an insulator, called
the "dielectric". The capacitance is proportional to
the area of the plates, and inversely proportional to the distance between
them. Capacitors are used to block DC while passing AC.
They are frequency-dependent devices, which means that their capacitive
reactance, or "effective resistance" to AC increases as the
frequency gets lower. This makes capacitors useful for tone controls,
where different frequency bands must be passed, or for bypassing AC
signals to ground while passing DC through for filtering purposes. Capacitance - the
"size" of a capacitor. The unit of capacitance is the
Farad, but a one Farad capacitor would be quite large, indeed!
The most common capacitors are sized in microfarads (uF, or mfd in very
old texts - 10-6 farads), nanofarads (nF - 10-9
farads), and picofarads (pF - 10-12 farads). Cathode - the "current
generating" element of an electron tube. The heater heats
the cathode to a very high temperature, causing it to emit electrons,
which are then collected by the anode, or plate, which has a high positive
voltage, which attracts the negatively charged electrons from the cathode.
Cathode biasing
- a method of biasing a tube where the bias is generated by the voltage
drop across a resistor in the cathode. The grid is referred to
ground through a resistor, and the current flow through the cathode
resistor produces a positive cathode voltage with respect to the grid,
which is effectively the same as making the grid negative with respect
to the cathode. Chassis - the metal
box that encloses the amplifier parts. It is usually made of steel,
but occasionally aluminum is used. The transformers and
choke are usually mounted on top, while the passive components are usually
mounted inside the chassis. Choke - another
term used for an inductor, most commonly an inductor used as a power
supply filter. Class A - an amplifier
operating with the grid bias adjusted so plate current flows for the
entire 360 degrees of the input waveform, by biasing the tube halfway
between cutoff and saturation, in the most linear portion of the operating
curves. Distortion is low in class A operation, but the efficiency
is also very low. With the exception of single-ended amplifiers,
the amplifiers most manufacturers call "class A" are class
AB amplifiers when a strong signal is present. Class A1 - class
A operation where grid current does not flow for any portion of the
input cycle. Class A2 - class
A operation where grid current flows for some portion of the input cycle.
Class AB1 - class
AB operation where grid current does not flow for any portion of the
input cycle. Class AB2 - class
AB operation where grid current flows for some portion of the input
cycle. Class B - an amplifier
operating with the grid bias adjusted so plate current flows for right
at 180 degrees, by biasing the tube right at cutoff. The distortion
is higher than class A or class AB, and there is usually a large amount
of crossover distortion, but the efficiency is higher than class AB,
allowing more output power for a given plate dissipation. Class B1 - class
B operation where grid current does not flow for any portion of the
input cycle. Class B2 - class
B operation where grid current flows for some portion of the input cycle.
Combo - a guitar
amplifier that has a built-in speaker. Control grid -
a wire mesh element located between the cathode and plate of an electron
tube which controls the flow of electrons between the two elements.
The control grid draws no current, and as such, presents a high impedance
to the driving circuit. Voltage variations on the control grid,
with respect to the cathode, cause variations in plate current, which
is the basis of amplification within the tube. Coupling capacitors - capacitors which are used between stages in a guitar amplifier.
They block the DC plate voltage of the previous stage, while passing
the AC guitar signal on through. Concertina phase splitter - the name given to the single-tube phase inverter in which
the in-phase signal is taken off the cathode and the out-of-phase signal
is taken off the plate, with equal-value plate and cathode resistors.
This phase splitter configuration has excellent balance, but only unity
gain. Also called a "split-load" phase inverter. Crossover distortion - Crossover distortion is the term given to a type of distortion
that occurs in push-pull class AB or class B amplifiers. It happens
during the time that one side of the output stage shuts off, and the
other turns on. Depending upon the bias point, there is a small
amount of time where both tubes are in very non-linear portions of their
operating curves, or even cut off entirely, and this "kink"
in the transfer curves results in a distortion, or notch, at the zero
crossing point of the reconstructed waveform. Current - The term
given to electron flow. The unit of current is the "amp",
or "ampere", and indicates a current flow of one coulomb per
second. A coulomb is a unit of electron charge. Cutoff frequency
- The "corner point" of a filter, usually the point where
the response is down -3dB compared to the midband signal level. dB - decibels. DC - Direct Current. This is electric current that flows
in one direction only. The most common form of a direct
current supply is a battery. The battery will have positive and
negative terminals. If a circuit is connected between the two
terminals, a current will flow in one direction only. The actual
electron flow is from negative to positive, but "conventional"
current flow is indicated as a current flow from positive to negative.
This has been a source of confusion since the early days of electricity,
and you will see both conventional and electron flow used in literature.
Decoupling
- the process of isolating one stage of an amplifier from
another. This is usually done by adding a resistor in series with
the power supply to a gain stage and a large value electrolytic capacitor
from the supply to ground after the resistor. Decoupling
prevents oscillations and other noises that may occur due to unwanted
feedback through the power supply connections. It also provides further
filtering of the power supply to reduce ripple, producing a cleaner
DC supply for the low-level preamp stages. Decoupling capacitor - the large electrolytic
capacitor used to filter the power supply after the decoupling resistor.
Decoupling resistor - the series resistor
used to isolate one stage of an amplifier from another. Dielectric
- the insulating material used in a capacitor. Typical
dielectric types used in amplifiers are: polystyrene, polypropylene,
polycarbonate, polyester, and ceramic. Diode - a two-element device which passes a signal in one direction
only. They are used most commonly to convert AC to DC, because
they pass the positive part of the wave, and block the negative part
of the AC signal, or, if they are reversed, they pass only the negative
part and not the positive part. This allows them to be used to
generate a positive or negative DC supply. There are both solid-state
and tube diodes. Since a diode will pass current in only one direction,
they can also be used to "clip" the top or bottom part of
a signal. Diodes are also commonly called "rectifiers"
because they rectify the AC voltage, however, the term "rectifier"
is usually reserved for diodes used in the power supply section of an
amplifier, while "diode" is generally used in small signal,
or low power applications, such as clippers. Direct box - a
device that allows a guitar or amplifier to be connected directly into
a mixing board without the use of a microphone. There are two
basic types of direct boxes, those that go between the guitar and the
amp, feeding a clean guitar signal to the board, and those that go between
the amplifier output and the speakers, feeding the amp signal to the
board. The latter usually contain some type of frequency compensation,
or "speaker emulation" to give a sound similar to a miked
speaker. E - the symbol
for electromotive force, or voltage Effects loop -
a circuit that allows insertion of external effects devices in the signal
path of an amplifier. Noise performance is usually improved by
using the effects loop rather than putting the effects in series with
the guitar input. Electron Tube- the
device used to make guitar amplifiers sound good! Actually, this
is the name given to the amplifying devices in some guitar amplifiers.
They consist of a glass tube containing several elements which are brought
out to pins on the base of the tube. All of the air inside the
tube is evacuated at time of manufacture, which keeps the filament from
rapidly burning up. Feedback - a circuit
that allows a portion of the signal from a later stage in an amplifier
to be "fed back" to an earlier stage, or within the same stage.
Feedback can be voltage or current, negative or positive. Negative voltage
feedback decreases gain, and is used to reduce distortion, flatten frequency
response, increase input impedance, decrease output impedance.
Negative current feedback increases output impedance, and is used in
some solid-state amplifiers to obtain a more "tubelike"
response. Positive feedback will increase gain, but can make a
circuit oscillate if too much is applied. Sometimes a small amount
of positive feedback is used to offset the reduction in gain caused
by application of negative feedback. Filament - the
heating element in an electron tube, also called the "heater".
The filament heats the cathode to a very high temperature, which "boils
off" electrons, which are then collected by the plate. The
filament can be seen as the glowing element through the holes in the
plate of most tubes. Filter - a circuit
which is used to either block or reduce a range of frequencies.
There are lowpass filters, which pass frequencies below a certain point,
called the "cutoff frequency", highpass filters, which pass
frequencies above the cutoff frequency, bandpass filters which pass
frequencies above a lower cutoff frequency and below an upper cutoff
frequency, bandstop filters, which pass frequencies below a lower
cutoff frequency and above an upper cutoff frequency, and allpass filters,
which pass all frequencies at the same amplitude, but which have certain
phase or delay characteristics. Filter caps - Filter
capacitors. The term used for the large capacitors used to filter
out the residual AC ripple in the power supply. The rectifier
converts AC to pulsating DC, since it just allows current to flow in
one direction. The output of the rectifier is a series of "humps",
which must be "smoothed out" to become flat, ripple-free direct
current. The filter caps store up the voltage on the positive
rise of the pulsating rectified AC waveform, and hold it there while
the rectified waveform goes down to zero. This charge, hold, charge,
hold, etc. behavior is what smoothes out the ripple. In general,
the larger the capacitor, the less residual ripple there will be. Fixed biasing -
a method of biasing a tube or output stage by using a negative DC voltage
on the grid with respect to the cathode. This name is sometimes
confusing, because an amplifier may have a bias adjustment pot to adjust
the negative grid voltage, but it is still called "fixed"
biasing to differentiate it from "cathode biasing". Frequency response
- a measure of how "wide" a set of frequencies an amplifier
will pass. Typically, this is specified as the frequency span
between the lower and upper points where the amplitude of the signal
has fallen off -3dB, or 0.707 times the midband voltage level.
Closely related is the term "flatness", which specifies the
deviation from center in the passband. Full-wave rectifier - a rectifier that conducts on both positive and negative halves of the
incoming sinusoidal signal. It produces a "pulsating"
DC composed of single-polarity "humps" at twice the incoming
AC frequency. The full-wave rectifier requires less filtering
than a half-wave rectifier to produce the same degree of ripple in the
output DC waveform. Fuse - a component
designed to protect electronic circuits, usually made of a thin piece
of metal mounted in a glass or ceramic tube with metal end caps, that
is designed to safely burn in two if the current passing through it
exceeds the rated maximum. Global negative feedback - negative feedback that is applied over several amplifier
stages, as opposed to local negative feedback, which is applied on one
stage only. An example of global negative feedback is the feedback
loop in a Marshall or Fender amplifier, where there is a feedback path
from the speaker output back to the phase inverter, through an attenuator
composed of the "feedback resistor" and a resistor to ground
on one side of the phase inverter. Grid - the "control
element" in a vacuum tube. The grid is normally biased negative
with respect to the cathode. As the grid is made less negative
with respect to the cathode, more current will flow from the cathode
to the plate. As the grid is made more negative with respect to
the cathode, less current will flow from the cathode to the plate.
It usually only takes a relatively small grid voltage swing to control
the plate current over it's entire range. Since the grid element
controls of the current flow in the tube, it allows the tube to be used
as an amplifier to take a relatively small input signal on the grid
and generate a relatively large signal swing at the plate. The
amount of signal voltage at the plate is equal to the current flowing
through the tube multiplied by the resistance connected to the plate.
Grid resistor -
the term usually given to a series resistor connected to the grid of
a tube, also called a "grid stopper", but sometimes used to
refer to the resistor connected from the grid of a tube to ground, which
is also sometimes called a "grid leak" resistor. Grid stopper -
a resistor connected in series with the grid of a tube, usually right
at the pin of the tube. It is used to prevent parasitic oscillations
and reduce the chance of radio station interference by forming a lowpass
filter in conjunction with the input capacitance of the tube. Ground - The common
"reference" point for the circuit. This is usually also
connected to the chassis, but there can be independent circuit grounds
and chassis grounds. H - the symbol
for magnetizing force, also the symbol for the unit of inductance, the
Henry. Half-wave rectifier - a rectifier that conducts on only the positive or only the negative
half of the incoming sinusoidal signal. It produces a "pulsating"
DC composed of single-polarity "humps" at the incoming AC
frequency, with a flat "dead time" during the time the input
signal goes to the opposite polarity . The half-wave rectifier
requires more filtering than a full-wave rectifier to produce the same
degree of ripple in the output DC waveform. Heater - the heating element in an electron tube, also called the
"filament". HT - stands
for "high-tension", meaning high voltage. Occasionally
the B+ fuse on an amplifier will be labeled "HT Fuse". Hz- stands for
"Hertz", which is the name given to the frequency of an alternating
current. The units are in cycles per second. In some older
literature, you may see this represented as "CPS", which,
of course, stands for "cycles per second". A prefix
of "k" or "M" is used to indicated kilohertz, or
kHz, and megahertz, or MHz, which indicate thousands and millions of
cycles per second, respectively. I - the symbol
for current Impedance - a complex
quantity containing both a resistance and a reactance. The symbol
for impedance is "Z", and the unit of impedance is the ohm.
Z = R + jX, where R is the resistance,
and X is the reactance of the circuit, and j is the complex, or imaginary,
operator, indicating multiplication by the square root of -1.
Inductive reactances have positive imaginary components, and capacitive
reactances have negative imaginary components. For example, an inductor
of 1mH with a resistance of 8 ohms would have an impedance of (8 + j6.3)
ohms at 1000 Hz. Since an impedance is a complex number, it has
both a magnitude and a phase. Typically, when discussing amplifiers
or speakers, impedances are referred to as the magnitude of the complex
number, instead of the rectangular form as given in the definition.
The magnitude of the (8+j6.3) example is 10.2 ohms, as calculated by
the square root of the sum of the squares of the real and imaginary
parts (the "length" of the resulting vector).
The concept of imaginary numbers can be a bit confusing to those who
haven't encountered it before. If you are interested in finding
out more about this, check out a textbook on introductory circuit analysis,
as they usually have a good treatment of the subject. Inductance - the
"size" of an inductor, not the actual physical size, but the
"electrical" size. The unit of inductance is the Henry,
or "H". Most power supply inductors, or chokes, are
measured in henries, typically 2-20H. The inductance of a transformer
primary may also be several henries. Smaller inductors are measured
in millihenries (mH - 10^-3 henries) or microhenries (uH - 10^-6 henries).
Inductor - a circuit
element consisting of a coil of wire would on a core material made of
ferrous or non-ferrous material. An inductor resists changes in
the flow of electric current through it, because it generates a magnetic
field that acts to oppose the flow of current through it, which means
that the current cannot change instantaneously in the inductor.
This property makes inductors very useful for filtering out residual
ripple in a power supply, or for use in signal shaping filters.
They are frequency-dependent devices, which means that their inductive
reactance, or "effective resistance" to AC decreases as the
frequency gets lower, and increases as the frequency gets higher.
This property makes them useful in tone controls and other filters.
Jack - the
input or speaker output connector on a guitar amplifier. k - the prefix
indicating "kilo" or thousands, as in a 10k resistor, which
means ten thousand ohms. K - the symbol
for the cathode of an electron tube L - the symbol
for inductance. LDR - light dependent
resistor. Often used in referring to an optocoupler in which
the active element is a photoresistor, whose resistance changes as current
is passed through the lighting element, which is usually an LED or neon
bulb. LED - light
emitting diode. These are semiconductor devices that emit light
of various colors when an electric current is passed through them. They
are typically used as indicators, but occasionally are used as clipping
diodes because of their larger forward voltage drop when compared to
a standard silicon diode. Local negative feedback - feedback that is applied over one stage only, as opposed
to global negative feedback, which is applied over several stages of
amplification. An example of local negative feedback is a cathode
follower, where the feedback signal is not so apparently derived by
the current flowing through the cathode resistor, or a common-cathode
stage with an unbypassed cathode resistor. Long tail pair
- a phase inverter topology that has a single resistor connected
as a pseudo-current source from the junction of two tube cathodes, with
the outputs taken off the individual tube plates, one in phase with
the input signal, and the other out of phase with the input signal.
The circuit gets its name from the "tail" resistor connected
to the cathodes. M - the prefix
for mega, or millions, as in a 1M resistor, which means one million
ohms. Mains - the
AC line voltage input. Occasionally the fuse on the AC input will
be labeled "Mains Fuse". Master volume - a
second volume control, located at the end of the preamp section of a
guitar amplifier, which allows the guitarist to turn the preamplifier
up to the point of distortion, while keeping the overall volume low.
Various Master Volume circuit designs exist. They can also be placed
before or after the phase inverter. Microphonics - the tendency for a component to induce audible noise into
the amplifier circuit when mechanically disturbed. Tubes are the
most common microphonic component, and they will usually make an audible
"thump" or "ring" when tapped. Occasionally,
the problem is severe enough in combo amplifiers to cause uncontrollable
feedback from the speaker to the tube, resulting in a "squealing"
or "howling" noise when the volume is turned up loud.
Although it is not commonly known, capacitors can also be quite microphonic.
Different types have different levels of microphony, with ceramic types
usually being the worst. Modeling amp –
an amplifier that uses digital technology to replicate the sound of
traditional amps. Negative feedback
- feedback in which a portion of the signal from a later amplifier stage
is fed back to an earlier stage (or to the same stage) in such a manner
as to subtract from the input signal. Ohm - the unit
of resistance or impedance. Ohm's law - the
fundamental relationship between voltage, current, and resistance.
It is usually stated as: E = I*R, or V=I*R, where E or V = voltage (in
volts. E stands for "electromotive force" which is the same
thing as voltage), and I = current (in amps), and R = resistance (in
ohms). The equation can be manipulated to find any one of the
three if the other two are known. For instance, if you know the
voltage across a resistor, and the current through it, you can calculate
the resistance by rearranging the equation to solve for R as follows:
R = E/I. Likewise, if you know the resistance and the voltage
drop across it, you can calculate the current through the resistor as
I = E/R. A related equation is used to calculate power in a circuit:
P = E*I, where P = power (in watts), E = voltage (in volts), and I =
current (in amps). For example, if you measure 20V RMS and 2.5A
into a load, the power delivered to the load is: P = 20*2.5 = 50W.
This equation can also be rearranged to solve for the other two quantities
as follows: P = E*I, E = P/I, and I = P/E. You can also combine
the power equation with the first Ohm's law equation to derive a set
of new equations. Since E = I*R, you can substitute I*R for E
in the power equation to obtain: P = (I*R)*I, or P = I2R.
You can also find P if you know only E and R by substituting I=E/R into
the power equation to obtain: P = E*(E/R), or P = E2/R.
These two equations can also be rearranged to solve for any one of the
three variables if the other two are known. For example, if you
have an amplifier putting out 50W into an 8 ohm load, the voltage across
the load will be: E = sqrt(P*R) = sqrt(50*8) = 20V RMS.
Optocoupler - another
name for optoisolator. Optoisolator -
a device which contains a optical emitter, such as an LED, neon bulb,
or incandescent bulb, and an optical receiving element, such as a resistor
that changes resistance with variations in light intensity, or a transistor,
diode, or other device that conducts differently when in the presence
of light. These devices are used to isolate the control voltage
from the controlled circuit. Typical optoisolators are the Vactec
and photoFET devices used in channel-switching
amplifiers, as well as the "trem-roach"
neon bulb/photoresistor package used in the tremolo circuit in some
Fender amplifiers. Oscillator - a
circuit that produces a sustained AC waveform with no external input
signal. Oscillators can be designed to produce sine waves, square
waves, or other wave shapes. They are typically used as variable
speed generators in tremolo circuits in guitar amplifiers. OT - output
transformer. Output transformer
- a transformer used to match the low impedance of a speaker voice
coil to the high impedance of a tube output stage. Output transformers
consist of at least two windings, a primary and a secondary. Some
output transformers have multiple impedance taps on the secondary side,
to allow matching to different speaker cabinets, typically 4, 8, and
16 ohms. p - the prefix
for "pico", or 1*10-12, as in a 100pF capacitor,
which means 100x10-12 Farads. Originally the term "uuF"
or "micro-micro Farads" was used. Parasitic oscillation - an unwanted oscillation in a tube amplifier, often at supersonic,
inaudible frequencies. Parasitic oscillations can cause all sorts
of problems, including overheating output tubes and bad tone. Passive - a component
that doesn't need a power source to function. Examples of passive
components are: resistors, capacitors, inductors, transformers, etc.
Also used to refer to guitar pickups that don't have built-in preamps,
and don't require batteries to operate. PCB - printed circuit
board, or PC board. A piece of phenolic or glass-epoxy board with
copper clad on one or both sides. The portions of copper that
aren't needed are etched off, leaving "printed" circuits which
connect the components. Pentode - A five-element electron tube, containing a control grid,
screen grid, suppressor grid, cathode, and plate as active elements,
in addition to the filament. Phase - the
instantaneous "polarity" of an AC signal, or more correctly,
the point in the rotation of the vector, measured in degrees, from 0
to 360 degrees total. Phase inverter
- a circuit that generates two output signals, each 180 degrees out
of phase with the other. This is a bit of a misnomer, since it
does more than just invert the phase of a signal, it actually generates
two out of phase signals. Phase splitter
- another name for a phase inverter. PhotoFET - an optoisolator in which an LED controls the turn on/off
of a bilateral MOSFET device. These devices are commonly
used as channel-switching devices. Plate - the "current collecting" element in a vacuum tube.
Also called the "anode". This is also the term
used for each of the two terminals of a capacitor, which are on either
side of the dielectric. Plate dissipation
- the amount of power dissipated in
the plate element of a vacuum tube. At idle, or quiescent conditions,
it is equal to the DC plate current multiplied by the DC voltage difference
between the plate and cathode elements. When the tube is amplifying
a signal, the average plate dissipation depends on several things, including
the quiescent bias point, the amount of signal voltage between the plate
and cathode, and the class of operation. Average plate dissipation
can either increase, decrease, or remain the same at full power, depending
on these things. In a class AB or class B amplifier, the power
dissipation increases, because the signal swing above and below the
quiescent point is not the same (the tube is in cutoff for a portion
of the cycle) and in a true class A amplifier the plate dissipation
decreases at full power, because the plate current and plate voltage
are 180 degrees out of phase, so the product of the two is zero when
one is at max and the other at zero, and is maximum at idle. Plexi - the name
given to early Positive feedback
- feedback in which a portion of the signal from a later amplifier stage
is fed back to an earlier stage (or to the same stage) in such a manner
as to add to the input signal. Pot - short for
"potentiometer". Potentiometer -
a variable resistor. It usually has three terminals: the two end
terminals, across which the entire resistance appears, and a third terminal,
the "wiper", which moves to a different spot on the resistor
as the shaft is turned. In this manner, the resistance between
the wiper and one end terminal gets smaller while, at the same time,
the resistance between the wiper and the other end gets larger.
This allows the potentiometer to be used as a variable voltage divider,
for use in attenuators, such as volume controls or tone controls. Power - the rate
of doing work, equal to the voltage multiplied by the current in a circuit.
In an amplifier, this work results in either heat or mechanical energy,
such as moving the loudspeaker coil to produce sound. Power amp - the
high-level amplifying stage in a guitar amplifier. This is where
the smaller preamp signal is converted into a high power signal necessary
to drive the speakers to the desired output level. Power transformer
- a transformer used to convert the incoming line (or mains) voltage
to a higher or lower value for use in the guitar amplifier. Typically,
the power transformer will have at least one primary, but sometimes
two or more, to allow use at 120V/240V/etc. mains voltages.
There will also usually be a 6.3V filament winding, sometimes center-tapped
to allow balancing the filament string symmetrically around ground for
hum reduction. There is sometimes a 5V winding for use with a
tube rectifier. This winding is eliminated when using a solid-state
rectifier. There is also a third winding for generating the high
voltage, or "B+", as it is commonly called. This winding
may be center-tapped, unless a bridge rectifier is used. PP - push-pull.
PPP - parallel
push-pull. Preamp - the low-level
amplifying stages in a guitar amplifier. This is where the tiny
signal from the guitar pickup is amplified and shaped for the desired
tonality before being sent to the power amplifier, which generates the
high power signal needed to drive the speakers. Presence - a control
on a guitar amplifier that boosts the upper frequencies above the normal
treble control range for added high-end. This control is usually
a shelving type of equalizer, and is normally implemented as a lowpass
filter inside the global negative feedback loop. By decreasing
the amount of high frequencies that are fed back, the high frequencies
at the output of the amplifier are boosted. PSE - parallel
single ended. PT - power
transformer. PTP - point-to-point.
A method of wiring an amplifier without using a PC board, where the
components are mounted on terminal strips or tube sockets lugs, and
the wiring is put in by hand to make the circuit connections.
Widely regarded as "sounding better" than PCB because of supposedly
higher bandwidth, but this is a myth, as PCB's are regularly used into
the MHz region. PTP wiring is generally better than PCB for guitar amps
because of ease of maintenance and durability. Push-pull - In
a push-pull amplifier, the power supply is connected to the center-tap
of the transformer and a tube is connected to both the upper and lower
end of the center-tapped primary. This allows the tubes to conduct on
alternate cycles of the input waveform. A push-pull stage can
be biased class A, where current flows in both tubes for the entire
input cycle (but in opposite directions), or class AB, where current
flows alternately in both halves, but less than a full cycle in each,
or class B, where current flows only half the time in each tube.
Most designs are biased class AB for best efficiency and power output
with minimal crossover distortion (but not necessarily best "tone",
although this is subjective). A push-pull stage requires at least
two tubes to operate, but can have more connected in parallel with each
side, resulting in an amp with four, six, or even eight output tubes
for higher-power amps. This is called "parallel push-pull"
operation, or PPP. Q - the symbol
for the "quality factor" or figure of merit for a reactive
component, such as a capacitor or coil. Low reactive element Q's
can affect the response of filters near the cutoff frequency. Also the
symbol for "quality factor", or selectivity of a filter network,
used to denote the relative "sharpness" of a filter.
For instance, a high Q bandpass filter would be one that has a very
narrow width and steep slopes on the sides. It is a measure of
the ratio between the center frequency and the bandwidth of a bandpass
filter. R - the symbol
for resistance. RDH4 - Radiotron
Designer's Handbook, 4th edition - the legendary "bible" of
tube amplification, also known as "the big red book". Reactance - the
"imaginary" component of impedance, or the resistance to AC
signals at a certain frequency. Capacitive reactance is equal
to 1/(2*pi*f*C), and inductive reactance is equal to 2*pi*f*L.
The unit of reactance is the ohm. Reactive load -
a load that contains inductance or capacitance, either with or without
resistance as well. An example of a reactive load is a loudspeaker
which has an impedance that varies with frequency, unlike a purely resistive
load, whose impedance is flat for all frequencies in the range of a
guitar amplifier. Rectifier - this
is the same thing as a diode, but the term is usually reserved for diodes
used in the power supply section of an amplifier. Relay - an electromechanical
switch, operated by passing current through a coil of wire wound around
a steel core, which acts as an electromagnet, pulling the switch contact
down to make or break a circuit. These are available in several
types, including SPST (single-pole, single-throw), SPDT (single-pole,
double throw), DPST (double-pole, single throw), and DPDT (double-pole,
double-throw), and not as commonly, in multi-circuit configurations
such as 3PDT or 4PDT (three and four poles, double-throw). Resistance - the
"size" of a resistor. The unit of resistance is the
ohm. Resistors vary in size from fractions of an ohm to several
million ohms. The prefix "k" is used for kilohms, or
thousands of ohms, and the prefix "M" is used for megohms,
or millions of ohms. Resistive load
- a load that contains no inductance or capacitance, just pure resistance.
An example of a resistive load is a dummy test
load consisting of a single resistance equal to the output impedance
of the amplifier under test. The resistive load has an impedance
that is flat for all frequencies in the range of a guitar amplifier.
Resistor - a circuit
element that presents a resistance to the flow of electric current.
A current flowing through a resistance will create a voltage drop across
that resistance in accordance with Ohm's law. Resonance - a control
on a guitar amplifier that boosts the lower frequencies at or below
the normal bass control range for added low-end, also called "depth"
or other names. This control is usually a shelving type of equalizer,
and is normally implemented as a highpass filter inside the global negative
feedback loop. By decreasing the amount of low frequencies that
are fed back, the low frequencies at the output of the amplifier are
boosted. Resonance is also the term given to an electronic circuit
that contains both capacitive and inductive elements - there is a "resonant"
point where the capacitive reactance equals the inductive reactance.
Depending upon whether the elements are in series or parallel, this
will result in a maximum voltage and maximum impedance across the elements
(parallel resonance) or maximum current and minimum impedance through
the elements (series resonance). If the circuit has resistance,
either across the parallel resonant circuit or in series with the series
resonant circuit, the maximum peak will be limited, and the bandwidth
of the resonance will be broader. The relative "sharpness"
of the resonant circuit is called the "Q", or "quality"
factor. See the definition of "Q" for more details.
Reverb - a short,
recirculating delay effect used on some guitar
amplifiers. It is similar to echo, but instead of discrete, long
delay repeats, it is a series of very short delays that add up to create
a sense of spaciousness in the tone. A spring unit with a sending
transducer at one end and a receiving transducer at the other end is
usually used as the delay unit, although some amplifiers use an analog
or digital delay line. RMS - stands for
"root mean square". It is a term used with AC voltages
or currents to indicate the equivalent DC voltage or current.
For a sine wave, the RMS value is equal to the peak-to-peak value divided
by 2*sqrt(2), or 2.282, or the peak value divided by sqrt(2), or 1.414. You can also multiply the peak value
by 0.707, which is the same as dividing by 1.414. The RMS value
of the signal depends on the shape of the waveform. For instance,
the RMS value of a square wave is not the peak value multiplied by 0.707,
rather, it is equal to the peak value of the square wave. Sag - a "drooping"
of the power supply voltage in a guitar amplifier as a note or chord
is played. This "drooping" causes a slight drop in volume,
for an effect similar to a compressor. It adds "touch sensitivity"
to the amplifier, and is one of the reasons tube guitar amplifiers sound
subjectively better than solid-state guitar amplifiers. Screen grid - a
second grid element interposed between the control grid and the plate,
to act as an electrostatic shield between them. This shielding
action greatly reduces the input capacitance of the tube, which increases
it's frequency response, and makes the plate current virtually independent
of plate voltage. There is no screen grid in a triode, only in
a tetrode or pentode. Secondary emission
- electrons in a vacuum tube may be moving at a sufficient speed to
dislodge additional electrons when they strike the plate of the tube.
These electrons emitted from the plate can reduce the current flow in
the tube. A third grid element, called the "suppressor grid",
is used to reduce the effects of secondary emission. SE - single-ended.
Silverface - the
name given to Fender amplifiers that have a silver control panel.
The panel was changed from black to silver at the time CBS bought Fender.
In addition, certain "improvements" were made to the circuitry
at the same time. The general consensus is that these amplifiers don't
sound as good as the blackface amplifiers, which has led to a practice
known as "blackfacing" the amp, which means converting the circuitry
back to match the blackface schematic. Silkscreen - the
name given to the "component identification" ink layer
screened onto a printed circuit board. Also the name given to
the lettering screened on the front and back of a guitar amp control
panel. Single-ended -
The term "single-ended", or SE, is given to an amplifier output
stage configuration whose output transformer primary is not center-tapped.
It has only two connections, one of which goes to the power supply,
the other to the plate of the power tube. Tubes can also be paralleled
for more power as in a push-pull stage, resulting in what is called
"parallel single-ended" operation, or PSE. A single
ended stage for guitar amplification is always biased class A.
Old Fender Champs are a good example of a single-ended guitar amplifier.
Higher power amplifiers are usually push-pull instead of single-ended,
which allows higher efficiency and better frequency response with a
smaller output transformer. Output transformers for single-ended
amplifiers require an air gap to avoid saturation of the core due to
the offset DC current in the transformer. This air gap greatly
reduces the primary inductance. so the core must be made larger and
the number of turns must be increased to obtain good low frequency response.
A push-pull output transformer has no offset DC current flowing in the
primary, because the DC bias current flows in opposite directions on
each side of the primary, so it doesn't need an air gap, and can be
made smaller. Single-ended output stages do not have the inherent
even-harmonic cancellation and power supply rejection that push-pull
output stages have, so the output tone is quite different, and the DC
plate supply must be better filtered in order to keep the hum to a low
level. Solid-state – consisting
of semiconductor materials and components and related devices. Solder mask - a
coating on a PC board, usually a dark green or dark blue, but occasionally
a yellowish color, which is designed to insulate and protect the copper
traces and keep them from shorting together during the wave soldering
process. The soldermask is "masked
out" at solder pads, to allow for soldering component leads. Speaker - a transducer
designed to reproduce audio frequencies. There are many different
models of guitar speakers, each with its own particular power handling
capability and tone. Speaker emulator
- a device composed of filters that are designed to emulate the response
of a loudspeaker, commonly used for direct recording applications. Split-load phase inverter - the name given to the single-tube phase inverter in which
the in-phase signal is taken off the cathode and the out-of-phase signal
is taken off the plate, with equal-value plate and cathode resistors.
This phase splitter configuration has excellent balance, but only unity
gain. Also called a "Concertina" phase splitter. Star ground - a
preferred amplifier circuit grounding system, where all the local grounds
for each stage are connected together, and a wire is run from that
point to a single ground point on the chassis, back at the power supply
ground. Sometimes multiple star points are used for lower hum
and noise levels in the amplifier. Suppressor grid
- a grid in a pentode vacuum tube that is used to minimize secondary
emission from the plate, by virtue of it's negative charge, which repels
electrons emitted and returns them back to the plate. It eliminates
the "kink" in the characteristic curves of a tetrode. Switch - a device
that opens and closes an electric circuit. Taper - the rate
at which the resistance of a potentiometer changes as the shaft is rotated.
There are several common tapers used in guitar amplifiers. There
is linear taper, which means that the resistance changes linearly as
the pot shaft is rotated, i.e., the resistance at midpoint is half the
total resistance from end to end. Another common taper is log
taper, short for logarithmic taper, which means that the pot changes
in a logarithmic fashion as the shaft is rotated, i.e., the resistance
at 1/10 the rotation is half the total resistance from end to end.
You may hear people occasionally mistakenly call this "analog taper",
but there is no such thing. There is also a reverse log taper.
The taper is chosen for the application. A volume control, for
instance, will be a log taper, because the ear hears sound in a logarithmic
fashion, and the volume must change accordingly to be perceived as linearly
changing as the pot is turned. Depending upon the type of tone
circuit, the pot used may be log or linear. If all the "action"
occurs at one end of the pot, chances are the wrong type of pot is being
used in the circuit. Tetrode - A four-element
electron tube, containing a control grid, screen grid, cathode, and
plate as active elements, in addition to the filament. Tolex - the original
DuPont trade name given to the vinyl covering used on many guitar amplifiers. Purple
and red tolex have the best tone! Tone - the characteristic
sound of an amplifier. Tone control -
a potentiometer used for controlling the tone of an amplifier.
This may be a single control or there may be multiple tone controls,
commonly called a "tone stack". Tone stack - The
term used to describe the tone controls in a guitar amplifier.
There are four main tone stacks used in most common guitar amplifiers.
They are the Transconductance
- the ratio of the tubes plate current to its grid voltage. The
unit of transconductance is the "mho", which is measured in
amps/volt, and is not surprisingly "ohm" spelled backwards,
because one ohm is equal to one volt divided by one amp, so the unit
of resistance, the ohm, is a volt/amp. Transconductance is one
"figure of merit" for a tube. Higher transconductances mean
higher gains and greater amplification from the tube. Transformer - a
device for changing levels of AC signals, or for changing impedances
of circuits. It consists of a minimum of two coils, the primary
and the secondary, wound on the same core. The core material can
be ferrous (magnetic, such as iron), or non-ferrous (non-magnetic, such
as an air core). Transformers used in guitar amplifiers are invariably
wound on iron cores. An ideal transformer has no losses, it merely
steps a voltage up or down in proportion to the turns ratio between
the primary and the secondary. This is useful in converting the
voltage from a wall outlet, typically 120 or 240 volts, into a higher
voltage for the tube plate supply, typically 400V or more, and a lower
voltage for the tube filament, typically 6.3 or 12.6V. The transformer
will also "reflect back" to the primary the impedance which
is connected to the secondary, in proportion to the square of the turns
ratio. That is, if you have a 20:1 transformer with a 16 ohm impedance
connected to the secondary, it will "look like" a 6.4K
ohm impedance on the primary side. This is useful in matching
the plate of a tube, which is very high impedance, typically on the
order of several thousand ohms, to a speaker, which is very low impedance,
typically on the order of 4, 8, or 16 ohms. Transient response
- the response of a circuit to a step waveform. An amplifier cannot
perfectly reproduce an input step waveform because of the limited bandwidth
and non-constant phase response of the amplifier. The transient
response may indicate some "overshoot" or "undershoot"
of the signal transition, or possibly some "ringing" or damped
sinusoidal oscillations at the transition. Tremolo - a circuit
that periodically varies the amplifier output level at a rate and depth
set by controls on the amplifier. The terms vibrato and tremolo are
sometimes used interchangeably. Triode - a three-element
electron tube, containing a grid, cathode, and plate as active elements,
in addition to the filament. Tube - short for
"electron tube". u - the prefix
for "micro", meaning one millionth, as in a 1uF capacitor,
which means one millionth of a Farad (originally the symbol was
the Greek letter "mu", but a lower-case "u" is usually
used nowadays). Ultralinear -
the term given to the amplifier configuration developed by Hafler and
Keroes, which uses taps on the output transformer to provide a negative
feedback signal to the screen grids of the output tubes. This
gives an operating point somewhere between that of a pentode and a triode.
This form of operation was given a bad name due to a particularly sterile-sounding
Fender amplifier that had an ultralinear output stage and far too much
global negative feedback. A few of the misinformed amp "guru"
types immediately denounced all ultralinear operation as sounding bad,
and the stigma has endured to this day, although this is slowly changing,
with the help of amp makers like Dr. Z, who are willing to experiment
with different output topologies to produce a better sounding amplifier.
Ultralinear operation, when used without global negative feedback, can
sound quite good, as the local negative feedback provided by the screen
taps increases the damping factor, lowering output impedance, and "tightening
up" the bass, without the use of global negative feedback. V - the symbol
for voltage. Common prefixes are "m", for mV (10-3
volts), and "u", for uV (10-6 volts), and "k",
for kV (103 volts). Vacuum tube - Another
name for "electron tube". Valve - the British
term for "tube". Variac - the trade
name for a brand of variable AC transformer. There are other brands,
but the term is generically used to describe all of them. A Variac
allows adjustment of the incoming AC mains voltage. The better
ones have meters for voltage, current, or both, and fuses for protection.
Vibrato - a circuit
that periodically varies the pitch of a note. True pitch-shifting
vibrato is not usually found on a guitar amplifier. The terms
vibrato and tremolo are sometimes used interchangeably. Voltage - the term
for electric force. Voltage is the energy per unit charge created
when positive and negative charges are separated. Volume control
- a potentiometer used for controlling the volume of an amplifier.
Best setting is usually on "10" or higher. W - the symbol
for watts. Typical prefixes are m, for thousandths, as in mW,
or "milliwatts", k, for thousands, as in kW, or "kilowatts",
and M, for millions, as in MW, or "megawatts". Watt - a unit of
power. Contrary to popular belief, more is not always better.
X - the symbol
for "reactance" Z - the symbol
for "impedance" Copyright © 1999,2000,2001,2002,2003
2004 Randall Aiken. |