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A - The symbol for amps, or amperes, which is the unit of current flow. 

AC - Alternating Current. Current that changes direction at a set Frequency.

Active - a component or system that needs a power source to function.

Amplifier – The thing that lets you (and the neighbors) hear just how nice your ’59 Les Paul and ’62 Strat sound (and just how much more practice you need!).

Anode - also often called the "plate". This usually has a large positive voltage connected to it in order to attract the negatively-charged electrons coming from the cathode element of the valve.

Attenuator – a device used to reduce the voltage or power of a signal. 

B+ - the high voltage supply in a valve amplifier.

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 a valve.

Blackzone™ - a gain structure rotary control found only on Sheldon® amps and pedals.

Bypass cap - a capacitor that is connected from the power supply to ground.  It "bypasses" the AC signals to ground.

Capacitor – Capacitors are used to block DC while passing AC.

Capacitance - the "size" of a capacitor. 

Cathode - the "current generating" element of an electron tube. 

Cathode biasing - a method of biasing a tube where the bias is generated by the voltage drop across a resistor in the cathode. 

Chassis - the metal box that encloses and holds the amplifier parts. 

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.

Class A/B - 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.

Class B - an amplifier operating with the grid bias adjusted so plate current flows at 180 degrees.

Compass™ – a variable tone network found only on Sheldon® amps and pedals.

Coupling capacitors - capacitors that 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-valve 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.

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.  Sounds awful if excessive!!

Current - The term given to electron flow. 

dB - decibels.

DC - Direct Current.  This is electric current that flows in one direction only.  

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.  

Diode - a two-element device which passes a signal in one direction only. 

DI box - a device that allows a guitar or amplifier to be connected directly into a mixing board without the use of a microphone. 

Effects loop - a circuit that allows insertion of external effects devices in the signal path of an amplifier. 

Electron - Negatively charged sub-atomic particle.

Eyelet board - a method of construction that uses a phenolic or epoxy-glass board (printed-circuit board material, which has rows of metal eyelets crimped into holes in the board. A most common example of eyelet board construction is old Fender amplifiers, which used wax-impregnated fiberboard's with eyelets, and of course Sheldon® TrueTone™ amps.

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.

Filament - the heating element in an electron tube, also called the "heater". 

Filter - a circuit which is used to either block or reduce a range of frequencies. Used for tone controls.

Filter caps - Filter capacitors.  The term used for the large capacitors used to filter out the residual AC ripple in the power supply.  

Fixed biasing - a method of biasing a tube or output stage by using a negative DC voltage on the grid

Freeze™ – a Class A – AB power structure rotary control found only on a Sheldon® amp.

Frequency response - a measure of how "wide" a set of frequencies an amplifier will pass. 

Full-wave rectifier - a rectifier which conducts on both positive and negative halves of the incoming signal. 

Fuse - a component designed to protect electronic circuits.

Global negative feedback - negative feedback that is applied over several amplifier stages.

Grid - the "control element" in a vacuum tube. 

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. 

Grid leak resistor - a very large resistor from the grid of a tube to ground.

Grid resistor - the term usually given to a series resistor connected to the grid of a tube, also called a "grid stopper".

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.

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.

Half-wave rectifier - a rectifier which conducts on only the positive or only the negative half of the incoming signal. 

Heater - the heating element in an electron tube, also called the "filament".

HT - stands for "high-tension", meaning high voltage. 

Hz - stands for "Hertz", the units are in cycles per second. 

Impedance - a complex quantity containing both a resistance and a reactance. 

Inductance - the "size” of an inductor, not the actual physical size, but the "electrical" size.  The unit of inductance is the Henry, or "H". 

Inductor - a circuit element consisting of a coil of wire would on a core material made of ferrous or non-ferrous material. 

IT - inter-stage transformer.

Jack - the input or speaker output connector on a guitar amplifier.

KillerKorners™ - handed corner protectors found only on Sheldon® amps.

LDR - light dependent resistor.  Often used in referring to an optocoupler.

LED - light emitting diode.

Local negative feedback - feedback that is applied over one stage only.

Long tail pair - a phase inverter that has a single resistor connected as a pseudo-current source from the junction of two tube cathodes.

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.

Micro phonics - the tendency for a component to induce audible noise into the amplifier circuit when mechanically disturbed. 

Modeling amp – an architecture that digitally reproduces amp tone. 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.

Optocoupler - another name for optoisolator.

Optoisolator - a device which contains an 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.

Oscillator - a circuit that produces a sustained AC waveform with no external input signal.  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. 

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. 

PCB - printed circuit board, or PC board.  A piece of phenolic or glass-epoxy board with copper clad on one or both sides. 

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.

Phase inverter - a circuit that generates two output signals, each 180 degrees out of phase with the other.  Actually 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.

Plate - the "current collecting" element in a vacuum tube.  Also called the "anode".  

Plate dissipation - the amount of power dissipated in the plate element of a vacuum tube.

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. 

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. 

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 output amplifying stage in a guitar amplifier. 

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. 

PP - push-pull.

PPP - parallel push-pull.

Preamp - the pre amplifying stage or stages in a guitar amplifier. 

Presence - a control on a guitar amplifier that boosts the upper frequencies above the normal treble control range for added high-end. 

PSE - parallel single ended.

PT - power transformer.

PTP - point-to-point (see definition above).

Push-pull -  In a push-pull amplifier, the output side amplifies both cycles of the input AC waveform, but typically uses two devices (one for each half cycle) to do it.

Reactance - The “imaginary” component of impedance or the resistance to AC signals at a certain frequency. 

Reactive load - a load that contains inductance or capacitance, either with or without resistance as well. 

Rectifier - this is usually reserved for diodes used in the power supply section of an amplifier.

Reflected impedance - the impedance seen in the primary of a transformer when the secondary is loaded with specific impedance. 

Relay - an electromechanical switch.

Resistance - the "size" of a resistor. 

Resistive load - a load that contains no inductance or capacitance, just pure resistance. 

Resistor - a circuit element /component that resists the flow of electric current. 

Resonance - a control on a guitar amplifier that boosts lower frequencies for added low-end.

Reverb - a short, re-circulating delay effect used on some guitar amplifiers. 

RMS - stands for "root mean square".  It is a term used with AC voltages or currents to indicate the equivalent DC voltage or current. 

Sag - a drop in power supply voltage in a guitar amplifier as a note or chord is played. 

Scaling - the process of shifting an electronic parameter 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.

Screen grid - a second grid element interposed between the control grid and the plate, to act as an electrostatic shield between them. 

Single-ended - The term "single-ended", or SE, is given to an output stage which amplifies the entire AC input cycle, usually with a single output device.

Solid-state – an alternative to tubes, which were developed for longevity over tonality.

Solder mask - a coating on a PC board which is designed to insulate and protect the copper traces during the soldering process. 

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.  Also called Concertina Phase Inverter.

Star ground - a preferred amplifier circuit grounding system.

Suppressor grid - a grid in a pentode vacuum tube that is used to minimize secondary emission from the plate, by virtue of its negative charge.

Susceptance - the reciprocal of reactance.

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. 

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.

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. 

Tran conductance - the ratio of the tubes plate current to its grid voltage. 

Transformer - a device for changing levels of AC signals, or for changing impedances of circuits. 

Transient response - the response of a circuit to a step waveform. 

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 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. 

Tweed - the name given to the covering on old Fender amplifiers which preceded the introduction of the Tolex vinyl covering.

Ultra linear - the term given to an amplifier configuration which uses taps on the output transformer to provide a negative feedback signal to the screen grids of the output tubes. 

Vactrol - an optoisolator device used for channel switching in many modern amplifiers.

Vacuum tube - Another name for "electron tube".

Valve - another term for "tube". Given because this is how it works.

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.