Don Corleone
10-29-2005, 12:17 PM
Amp Glossary
A - the symbol for amps, or amperes, which is a unit of current flow. Common prefixes are "m", for mA (10-3 amps or mili amps), and "u", for uA (10-6 amps or micro 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. "Hz" stands for "Hertz", which is the name of the unit for frequency, and means "cycles per second", indicating how many cycles, or changes from positive to negative, the AC waveform goes through each second. In some older literature, you may see the term "CPS", which stands for "cycles per second", used in place of "Hz". Alternating current does not have to be sinusoidal in shape; the square wave of a distorted guitar amplifier output is also AC, because it changes polarity periodically.
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.
Admittance - the reciprocal of impedance. Y = 1/Z = G + jB, where G = conductance, and B = susceptance.. The unit of admittance is the "mho", same as conductance.
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 WeberVST Blue Dog and the 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 greyish metal piece that encloses most of the other elements.
Attenuator - (a) a passive network that is used to reduce the voltage or power 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 network attenuates the signal by an amount dependent upon the ratio of the two resistor values, and is sometimes called a "resistive divider" when used with small signals, such as interstage coupling between tube stages in an amplifier. When used to reduce power levels instead of small-signal applications, it is commonly called an "L-pad", or an "L-pad matched in the direction of the series arm" (because it matches impedances in one direction only). The L-pad can also be built with a shunt resistor to ground from the output and a series resistor in series with the output after the shunt to ground (this is commonly called an "L-pad matched in the direction of the shunt arm"). There are other variations of attenuators, including the "T", "PI", and "ladder" configurations and others.
(b) A passive device used to reduce the volume of an amplifier. It goes between the amplifier and the speakers, allowing a non-master volume amplifier to be cranked up to full power without being overly loud, in order to get the desired overdrive tone from the amplifier. Note: there is another class of device on the market now, which uses a passive load followed by an active amplifier (which has a maximum gain of one). This device functions in a manner similar to an attenuator in that it reduces the signal level to the speaker while allowing the amplifier to run at full power. However, it is really a "reamp" device, with the main discernable difference being that it requires a source of power to operate (via the AC mains). These devices should be referred to as "active attenuators" to avoid confusion with the standard passive attenuators on the market.
B - the symbol for susceptance, also the symbol for magnetic flux density.
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.
Back bias - a method of obtaining a negative bias voltage by means of a resistor or zener diode in the center tap of a full-wave rectifier circuit. The current in the center tap flows in the same direction for both half-cycles, so the voltage drop is the same for both. This full-wave-rectified negative voltage can be filtered and used as a negative bias supply. The downside is that all the plate current of the output stage flows through the back bias circuit, so it can be impractical for higher-powered amplifiers. Also, the resistive drop method should only be used for true class A amplifiers, because there can be a large difference between the idle and full-power current draw of a class AB or class B amplifier. The zener method is much more suitable, and in fact, creates a regulated bias voltage that is relatively independent of the current draw of the amplifier, provided it is above the minimum current necessary to keep the zener in the normal reverse breakdown region.
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. The distortion is lowest 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 actually just cathode-biased class AB amplifiers.
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 AB - an amplifier operating with the grid bias adjusted so plate current flows for greater than 180 degrees, but less than 360 degrees of the input waveform, by biasing the tube above cutoff, but below the point required for class A operation. The distortion is higher at low signal levels than true class A, but the efficiency is higher, although not as high as class B, allowing more output power than class A for a given plate dissipation.
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.
Common cathode - the "standard" tube circuit where the cathode is connected to the "common" point on the circuit, usually ground, and usually through a resistor, which is often bypassed with a capacitor, placing it at "AC" ground potential.
Common grid - a tube stage which has the grid connected to the "common" point on the circuit, usually ground. This doesn't have to be a physical DC connection, it can be an AC ground, i.e. grounded through a capacitor.
Common plate - a tube stage which has the plate connected to the "common" point on the circuit, usually ground This doesn't have to be a physical DC connection, it can be an AC ground, i.e. grounded through a capacitor. This is the most often seen method of making a common plate stage, where the plate is connected directly to the power supply (the AC ground connection is through the power supply capacitors, which are essentially a short to ground for AC signals). This stage is commonly called a "cathode follower".
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.
Eyelet board - a method of construction that uses a phenolic or epoxy-glass board (printed-circuit board material, usually FR-4/G-10, or Garolite, but sometimes wax-impregnated fiberboard), which has rows of metal eyelets crimped into holes in the board. The components are layed on top of the board and soldered into the eyelets, along with wires that connect from the eyelets to the tube sockets and other chassis-mounted components. This type of construction is sometimes called "point-to-point", but that is a term traditionally reserved for "true" point-to-point wiring where the components are mounted on the sockets and jacks themselves, sometimes with the use of phenolic terminal lug strips to support component junctions that don't go directly to socket pins. A similar method of construction is the turret board, which is basically the same as an eyelet board, except turret terminals are staked into the board instead of eyelets, and the components and connecting wires are soldered to the turret terminals. The most common example of eyelet board construction is old Fender amplifiers, which used wax-impregnated fiberboard's with eyelets.
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".
Flatness - the peak-to-peak deviation from the nominal voltage in the passband of an amplifier. Flatness is typically measured in dB. For example, if an amplifier has a passband "ripple" of + 0.5dB, it is said to have a "flatness" of + 0.5dB.
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.
G - the symbol for conductance.
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 leak biasing - The small amount of grid current in the tube generates a negative bias voltage across this resistor, which biases the tube to the proper operating point with respect to the cathode, which is grounded. This method of biasing is not very stable, and fell out of favor early on in the development of tube amplifiers. Most preamp stages now use cathode biasing as opposed to grid leak biasing.
Grid leak resistor - a very large resistor from the grid of a tube to ground, which is used to generate the bias voltage for the tube. See "grid leak biasing" for an explanation of how this works. This term is sometimes incorrectly used when referring to the grid-to-ground resistor in a cathode biased configuration, which is used to provide a DC ground reference for the grid circuit.
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.
IT - interstage transformer.
Jack - the input or or speaker output connector on a guitar amplifier.
Jewel - the term commonly used to refer to the screw-on pilot light lens on Fender guitar amplifiers. These usually were red or green, but purple ones are purported to have real "mojo".
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 it's 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.
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.
Miller capacitance - the effective multiplication of the plate-to-grid capacitance in a triode tube (or transistor) by the gain of the amplifying stage. Miller capacitance can decrease the frequency response of an amplifier stage by acting as a lowpass filter in conjunction with the source resistance of the preceding stage.
Modeling Amp (or Amp Modeling)- a computer that is passed off as a guitar amplifier. See "solid-state".
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 "pica", 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. Most modern amplifiers use this type of construction., however, many manufacturers use cheap, single-sided PC boards without plated-through holes, which tend to pull up pads when a component is desoldered. Some even go so far as to not use a soldermask or silkscreen. This type of construction should be avoided, and is a good indication of a cheaply made amplifier.
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 (Especially for Panamark) - the name given to early Marshall amplifiers that had a gold Plexiglas control panel on the front and rear of the chassis. This was later changed in mid 1969 to gold aluminum front and rear panels, commonly referred to as an "ali-panel" Marshall. (Webbie note: US terminology for these post Plexi amps is "metal faced." BrownSound1)
Point-to-point (also called "PTP") - 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 mainly because of ease of maintenance and durability. The most common modern example of PTP wiring is a Matchless amplifier. However, many "purists" will insist this is not a "true" point-to-point amp, because it uses terminal strips for some of the connections instead of wiring all parts using only socket and other component lugs. These people are generally deluded and have too much time on their hands.
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 (see definition above).
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.
Reflected impedance - the impedance seen "looking into" the primary of a transformer when it the secondary is loaded with a specific impedance. The impedance on the secondary side is transformed by the square of the turns ratio of the transformer. For example, if a 2:1 turns ratio transformer has a 10 ohm load on the secondary, the impedance measured across the primary terminals will be 40 ohms, because the secondary impedance of 10 ohms is multiplied by 22, or 4.
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.
Scaling - the process of shifting an electronic parameter up or down. For instance, a tone circuit that has a midrange boost/cut centered around 1kHz might be scaled to 800Hz to better suit the application. This would be an example of frequency scaling. Impedances may also be scaled up or down.
Schmitt phase inverter - a phase inverter configuration using two cathode-coupled tubes, with the first tube acting as a common cathode stage providing an out-of-phase signal at its plate, while the second tube operates as a common-grid stage, providing an in-phase signal at its plate. This type of inverter has moderately good balance, providing the plate resistor of the out-of-phase side is made slightly smaller than the in-phase plate resistor to compensate for differences in the amplification between the two stages. This phase inverter provides high gain.
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 - a component that has been specifically designed to make a guitar amplifier sound bad. Compared to tubes, these devices can have a very long lifespan, which guarantees that your amplifier will retain it's thin, lifeless, and buzzy sound for a long time to come.
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.
Susceptance - the reciprocal of reactance, measured in mhos.
Swirl - A dynamically-changing, slightly "phasey" sound as a note or chord decays, which is common to some tube amps. Typically, "swirl" is caused by a midrange "dip" or varying duty-cycle change in a clipped square wave that changes position as the note decays, giving a sort of mild phase shifter effect.
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 most guitar amplifiers, such as Marshall or Fender style vinyl.
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 Marshall style, the Fender style, the Vox style, and the lesser used Baxandall, or James style. These tone stacks vary in their construction, consisting of either a bass and treble control, or bass, mid, and treble controls. Some amplifiers have a tone stack consisting only of one control, usually a treble cut control, but sometimes it will be a single control that cuts treble at one end of the rotation, and cuts bass at the other end. These types of control are usually labeled "tone", or "cut".
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".
Turret board - a method of construction that uses a phenolic or epoxy-glass board (printed-circuit board material, usually FR-4/G-10, or Garolite), which has rows of metal terminals press fit and swaged into holes in the board. These terminals are sometimes shaped like little "turrets", which is where the name came from. The components are either wrapped around the turret terminals or stuck through holes in the top of the turret, and then wires are connected from the turrets to the tube sockets, pots, and other parts of the amp. This type of construction is sometimes called "point-to-point", but that is a term that is traditionally reserved for "true" point-to-point wiring where the components are mounted on the sockets and jacks themselves, sometimes with the use of phenolic terminal lug strips to support component junctions that don't go directly to socket pins. A similar method of construction is the eyelet board, which is basically the same as a turret board, but eyelets are staked into the board instead of turret terminals, and the components and connecting are soldered into holes in the eyelets. The most common example of turret board construction is an old Hiwatt amplifier.
Tweed - the name given to the covering on old Fender amplifiers which preceded the introduction of the Tolex vinyl covering.
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).
Vactec - the common name given to the Vactrol optoisolator device used for channel switching. The name is printed on the Vactrol because the company that invented them was named Vactec, later EG&G Vactec.
Vactrol - an optoisolator device used for channel switching in many modern amplifiers, such as Soldano and Mesa. It is a single package combining an LED and a photoresistor, which changes resistance from very high (essentially an open circuit) to very low (essentially a short circuit) as the current through the LED is turned on and off. It is used as a substitute for relays, to avoid the "clicks" and "pops" that can occur when they are used for channel switching.
Vacuum tube - Another name for "electron tube".
Valve - Another 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"
Y - the symbol for "admittance"
Z - the symbol for "impedance"
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