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Tube Amp Dictionary

By Rob Robinette

2203 & 2204 Marshall's first high gain amplifiers. Almost identical to the later Marshall JCM800.

5E3  Internal model number of the 1950's Fender narrow panel tweed Deluxe tube guitar amplifier. The 5E3 features two push-pull power tubes. It is the most popular tube guitar amp kit.

5F1 Internal model number of the 1950's Fender narrow panel tweed Champ tube guitar amplifier. The 5F1 is a very simple, single-ended amp with a single power tube. It is the second most popular tube guitar amp kit.

5F6-A  Internal model number of the Fender 1950's narrow panel tweed Bassman tube guitar amplifier. Although it was originally designed as a bass amplifier the Bassman makes a fantastic guitar amp with an amazing overdrive tone. It is considered the grandfather of most great Rock-and-Roll amplifiers. The 5F6-A circuit was copied almost verbatim by Marshall in the early 60's to create the JTM45 amplifier.

AB763  Internal model number for the July 63 (763) blackface amplifier circuit and considered by most as the Fender high water mark for guitar amplifier tone. The blackface Deluxe Reverb was the most popular AB763 amp.

AC  Alternating Current. In an AC circuit voltage and current move back and forth--the direction of movement alternates. A guitar audio signal is AC. The power from the wall receptacle or mains is also AC.

Anode  The tube electrode often called the "plate".

B+  DC voltage at the rectifier output. From old "Battery +" notation.

B+1 The first power node (often the same voltage as B+) which usually powers the output transformer and power tube plates. Many Fender amps use A B C D on their schematics instead of B+1, B+2, . . .

B+2 The second power node which usually powers the power tube screens.

B+3  The third power node which usually powers the phase inverter or a section of the preamp. Some amps have more power nodes for a B+4 or B+5.

Bias Voltage The idle (no signal) voltage difference between a tube's cathode and grid. Bias Voltage controls the Bias Current.

Bias Current is the current flowing between a tube's plate and cathode at idle (no signal). See Plate Dissipation below.

Blackface & BF  Mid 60's Fender classic tube amps with a black control panel. Considered by most as the Fender high water mark of amplifier tone.

Bypass Cap  Cathode bypass capacitor supplies the AC audio signal with a bypass around the cathode resistor to boost gain.

Cathode Follower  A tube circuit where the guitar signal output is via the cathode instead of the plate. A cathode follower puts out a "thicker" low impedance signal.

Cathodyne Phase Inverter  A common phase inverter used in the 5E3 Deluxe and Princeton Reverb. It only uses one triode but produces little gain. See Phase Inverter below.

Cone Cry  I like Sweetwater's definition: "A term most commonly associated with guitar amp speakers, "cone cry" refers to an audible frequency generated by a speaker that is unrelated to the input (guitar) signal. Cone cry is caused by the speaker cone itself resonating at a particular frequency, which may appear higher or lower than the notes played by the guitarist. It results from the voice coil being driven to where it exceeds the amp's damping." A speaker that cries may work fine in a different amp with more negative feedback and damping.

Cone or Coil Rub When a speaker voice coil rubs against its surrounding speaker magnet causing noise and distortion. It can sometimes be corrected by spinning the speaker 180 degrees in its cab. More valuable speakers can be refurbished by re-coning.

DC  Direct Current. In a DC circuit electrical current flows in one direction. A rectifier transforms AC into DC to power an amp's tubes.

Death Cap  A capacitor used to connect an amplifier's chassis to the power cord neutral wire to provide RFI shielding for the amplifier chassis and guitar cable. It's called a death cap because if it fails as a short it can put full wall receptacle (mains) voltage on the chassis and guitar strings. Amplifiers with death caps should be retrofitted with a three prong power cord and have the death cap removed.

Diode  A diode has two electrodes, a plate (anode) and cathode. Many rectifier tubes are diodes. "Diode" can also refer to a solid state diode.

ESR  Equivalent Series Resistance  Internal resistance in capacitors. It can reduce the effectiveness and lifespan of electrolytic capacitors. Lower ESR is better.

FMV  Fender/Marshall/VOX. See TMB Tone Stack below.

Grid  The Control Grid of a tube. The grid is the input of a tube and controls the flow of electrons through the tube.

Grid Bias  A method of generating bias voltage by grounding the cathode and using a high value (5M or higher) grid leak resistor. Grid current flowing through the large grid leak resistor creates a negative bias voltage on the grid.

Impedance  whose symbol is Z, is only a factor in alternating current (AC) circuits. Impedance is made up of three things that impede or restrict AC current flow: resistance, inductance and capacitance but its easier if you just think of impedance as "AC resistance". The term "impedance" can apply to both signals and circuits. A high impedance signal has relatively high voltage but low current so the signal is "thin". A low impedance signal has relatively low voltage but lots of current so the signal is "thick" and there's more current backing up the signal voltage. A high impedance circuit is a low load circuit that restricts AC current flow. A low impedance circuit is a heavy load circuit that allows AC current to flow easily.

JTM45  Marshall "Plexi" amp copied from the Fender 5F6A Bassman.

JCM800 Marshall high gain amplifier. Almost identical to the earlier 2203 and 2204 "Master Volume Lead" amps.

LTP  Long Tail Pair phase inverter. The most common phase inverter in tube guitar amps. It uses two triodes and puts out the gain roughly equal to half of one triode. See Phase Inverter below.

Mains  Power from your wall receptacle. 125 volts at 15 amps in the USA and 100 to 250v in other countries around the world.

NFB  Negative FeedBack reduces gain, distortion, noise (especially hiss), flattens frequency response and adds stability to the amp circuit. It also tightens the transition from clean to dirty which affects the amp's playing and picking dynamics.

OT  Output Transformer

Pentode  A tube with five electrodes, the control grid, cathode, screen grid, suppressor grid and plate.

Phase Inverter  (PI) A circuit that creates two audio signals that are 180 degrees out of phase. The two signals are used to drive two or more push-pull power tubes.

PI  See Phase Inverter above.

Plate  Slang for a tube's anode. High voltage DC is applied to the plate to pull electrons from the cathode.

Plate Dissipation  The power (voltage x current = watts) dissipated by the plate at idle (no signal). It is equal to: (Plate voltage - Cathode voltage) * Plate current = Plate Dissipation in watts. Plate Dissipation is given in watts or as a percentage of maximum plate dissipation. Plate dissipation is adjusted to set a tube's bias.

PT  Power Transformer

Push-Pull  A power amp topology that uses two or more power tubes to push the guitar signal through the output transformer while another tube pulls the signal through the transformer.

Rectifier  Transforms AC into DC. Rectification can be done with a rectifier tube or solid state diodes.

Red Plate  Too much current and voltage through a tube can overheat its plate (anode) and cause it to glow red.

Screen  The Screen Grid of a tetrode or pentode tube. The screen boosts gain and decreases tube Miller capacitance.

Silverface & SF  Late 60's and early 70's Fender classic tube amps with a silver control panel. This amp lineup is not as highly regarded by most as the earlier blackface amps but they still sound great and most can be easily "blackfaced" to the earlier circuit.

Single-Ended  A power amp topology that is not push-pull. Push-pull could be referred to as "double-ended". A single-ended power amp uses a single power tube to drive the output transformer and does not need a phase inverter. A single-ended output transformer must handle much more DC current than a push-pull transformer so single-ended output transformers are larger and more expensive than a comparably rated push-pull transformer.

Tetrode  A tube with 4 electrodes: a control grid, cathode, screen grid and plate. Beam Tetrodes also have beam forming plates.

TMB  Treble/Mid/Bass tone stack or circuit is used in many amps including the Fender 5F6A Bassman, blackface, silverface and many Marshall amps. Also called the FMV for Fender, Marshall & VOX which used this tone stack. It's called a "stack" because when drawn as a schematic the treble, bass and mid circuits are stacked vertically. Some amps replace the Mid control with a fixed resistor so it is called a TB (Treble/Bass) tone stack.

Triode  A triode has three electrodes, a grid, plate (anode) and cathode. A 12AX7 tube has two triodes inside.

Tweed  1950's Fender amps covered in tweed fabric. The 5E3 Deluxe is the most popular tweed amp.

Valve  European term for vacuum tube.

Widowmaker Amp  An amplifier without a main power transformer which uses mains voltage to power the amp. These amps should be fitted with an isolation transformer and a three prong power plug. Their circuit should also be modified to remove their death cap and V1 cathode current return resistors--both of these components connect the amp chassis to the power cord hot/neutral.


To learn how all this ties together see How Tube Amps Work.

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