Tubes: Common Terms

The following is a list of terms and characteristics commonly used in the world of vacuum tubes. This is not meant to be exhaustive or authoritative, just a quick reference list. For more information, consult a good tube reference manual.

Amplification: the reproduction of variations in voltage or current on a larger scale.

Amplification Factor: if a grid voltage change of 5 volts had the same affect on the plate current as a 100 volt change on the plate (both with respect to the cathode), the ratio would be 20 to 1; the amplification factor would be simply 20.

Anode: traditionally, an element positively-charged with respect to ground. In most tubes the anode is called the plate. Anodes are usually found in ballast tubes.

Audion: Lee DeForest's trade-name for his first three-element vacuum tube; the first triode.

Ballast: a current regulating tube; current remains constant by internal changes of resistance to voltage fluctuations.

Base: on older tubes, this is the lower portion of the tube where the pins are located and the envelope (or shell) is mounted. Early tube bases were typically make of some kind of metal, later tubes were typically bakelite.

It also refers to the pin-arrangement and the way the tube mounts to the socket, especially on later tubes where the envelope (or shell) is the entire outer structure.

Beam Power Tube: a method of focusing the electron beam onto the plate in such a way as to replace the supressor grid. Typically found in power audio tubes like the 6L6.

Bias: a term for a voltage offset —usually applied to the control grid. E.g. a control grid with a voltage of -4 volts with respect to the cathode is said to be biased -4 volts.

Cathode: an electrode that provides the electrons for the tube. Electrons come from the cathode being heated, either directly or indirectly, and this heat boils off electrons from the molecules that make up the cathode surface. This action is called thermionic emission.

Cathode-Ray Tube : a more precise term for a television picture tube.

Class A, B, C Amplifiers: the class of amplifier determines how much of the wave form is amplified without distortion (changing the wave form). Electrically, it means how much time plate current flows. For the examples below, assume a standard sinusoidal wave shape:

Cold Cathode: a tube without a heating element, so the cathode is cold. Typically used as voltage regulators.

Control Grid: an electrode that is usually biased negative with respect to the cathode. The control grid is the element placed closest to the cathode with respect to the plate. When biased positive with respect to the cathode, there's a slight current that flows, called a grid leak. When biased negative with respect to the cathode, it reduces the current-to-plate current; the current reduction varies with the bias voltage on the grid. Varying the grid voltage therefore varies the plate voltage and current, and the tube acts as a voltage or current amplifier depending on the tube design. Grid bias can also cut the the plate current almost entirely, so that the tube can be used as an electronically controlled switch.

Cutoff: a condition where the electron current between the cathode and plate is interrupted; usually due to a control-grid negative-voltage that is high enough to overcome the positive-voltage of the plate.

Diode: a two-element tube, or a tube with elements that function as a two-element tube; i.e. a cathode and plate. A tube may have two plates and one cathode, but only one plate works at a time.

Edison Effect: an early term for Thermionic Emission, as described by Thomas Edison. The term "effect" is typically given to an observable phenomenon whose cause cannot be adequately explained.

Envelope: the outer covering on a vacuum tube, usually made of glass but sometimes made of metal. Also known as a shell. Terminology largely depends on the source of the literature. RCA preferred envelope; Sylvania preferred shell.

Filament: this is the heating element of a tube. In directly-heated tubes, the filament gets hot and supplies electrons, thus serving as the cathode as well. In indirectly-heated tubes, the filament only provides heat for a separate element acting as a cathode.

Fleming Valve : an early diode, invented by Ambrose Fleming.

Full-wave Rectifier : a tube with two plates, each connected such that when one plate is negative with respect to the cathode, the other is positive; this passes both half-waves of an AC cycle, even though it inverts one so that both waves are above ground potential. Full-wave rectifiers are typically used in power supplies.

Gain: an engineering term for amplification.

Gas: may describe the presence of a gas in a tube, such as mercury-vapor in certain rectifier tubes. Most often gas is a condition where unwanted ionized gasses exist inside the tube. All tubes have some level of ionized gas, but too much ionized gas is a fault that makes the tube act erratically in use; such a tube is called gassy. The test for gassiness is usually a grid-leak test.

Grid: used for any electrode placed between the plate and the cathode, as in control grid, screen grid, and supressor grid. But grid by itself always refers to the control grid.

Grid Bias: a term for the voltage offset of the control grid with respect to the cathode. E.g. a control grid with a voltage of -4 volts is said to be biased -4 volts.

Grid Cap: a cap located somewhere on the envelope (or shell), usually on top, that connects to an internal element. Common on early tubes where the control grid's lead needed to be physically separated from the cathode and plate pins to reduce interference.

Grid Leak: a current in the control grid circuit. Sometimes used as a detector circuit, or as a test for gas in a tube.

Grounded Grid: many tube circuit designs assume that the cathode is at ground potential and all other elements are rated with repect to it; thus a control grid might have a bias of -5 volts with respect to the cathode. Grounded-grid circuits assume that the control grid is ground potential, and that the cathode and all other elements float above it.

Half-wave Rectifier : a tube with one plate, which passes one half wave per cycle.

Hard Vacuum : a tube with a high vacuum internally, as opposed to tubes that may have a higher internal gas content inside, or are gas-filled.

Heater: another name for the filment. Used where there is a separate cathode and the filament's sole role is to provide heat to the cathode.

IEEE: the Institute of Electrical and Electronics Engineers; a body that recommended standards in terms, formulas, schematic symbols, etc.

Lock-in, Loctal and Loktal: three terms for the same type of tube, Sylvania's version of an octal type. Lock-in was Sylvania's term; Loctal and Loktal appear in other literature.

Loctals are very similar to octal tubes and came out soon after; they have eight pins in a circular configuration around a non-conductive center pin. The primary difference is that the center pin has a channel near the tip that can be gripped by spring-loaded jaws that hold the tube in place, preventing it from wiggling out of the base.

Despite their similarities, Loctals are incompatible with octal tubes. The pins are a different size and the socket base is different.

Kinescope: RCA's early term for television picture tubes. This term generally disappeared in the late 1950s.

Magic Eye: RCA called them "electron-ray" tubes, but the rest of the world calls them magic eye tubes. Electrons strike a plate that flouresces and emits light. Magic eyes compare two voltages and visually show the difference. They are commonly used as tuning indicators (showing grid-bias voltage) on radios or as recording level indicators on tape recorders.

Mho: Ohm spelled backward; in the USA, it was the unit of measure for conductance before the metric Siemens unit was adopted in the 1980s. It is typically used to describe transconductance, with the most common form being the microMho.

Miniature tube: a series of 7- and 9-pin, all-glass envelope tubes which appeared during and after World War II. They are physically much slimmer and a bit shorter than octal tubes. The last major tube configuration before solid-state components made tubes obsolete.

Mu: another way of expressing amplification factor. In tube literature, amplification tubes are typically described as low-mu (amplification factor less than 10), medium-mu (between 10 and 50) and high-mu (greater than 50).

Mutual Conductance : an early term for transconductance, though still often seen in literature well after transconductance became the preferred term. In formulas its designation is Gm. For more detail, see transconductance.

Nixie: a tube with a series of stacked neon tubes inside that could be lit to show numerals.

Noval: RCA's term for a 9-pin miniature tube.

Octal: a standardized tube configuration which appeared in the mid-1930s. Octals used a standard base with 8, same-sized pins in a circular configuration, and a non-conductive pin in the center. The center pin was keyed so that it would fit into a socket only one way.

Pentode: a five-element tube, typically an amplifier; a tetrode with the addition of a supressor grid.

Picture Tube: also known as a Cathode-Ray Tube: a tube used to display an image in televisions.

Pin: an electrical contact that protrudes from the base of the tube and is connected to an element or elements inside.

Plate: electrically, it is the anode of a tube; plate is the common term in most tubes, and gets its name from the early versions when the element resembled a large, rectangular target for the electrons to hit.

Power: typically anything where "high" current is handled by a tube; high being relative to tubes that do not need much current to fulfill their roles. Power rectifiers provide a lot of current for power supplies; power output tubes provide a lot of current to drive speakers, etc. Voltage-amplifier tubes are typically very low power.

Power Pentode : a pentode type tube designed to handle relatively large current; typically used as an audio amplifier.

Quiescent Mode: the state of the tube when there is 0 volts bias on the grid; full current flow from cathode to plate.

RCA: Radio Corporation of America. Among other things, a major manufacturer and developer of vacuum tubes in the United States.

Rectifier: a device that only passes one half of a cycle of alternating current. All tubes only pass one half of a cycle, but a rectifier does this as its primary role, rather than for amplification or some other purpose. The term rectifier is mostly commonly used regarding power supplies, as the first step in changing AC current to DC current.

Remote Cutoff Tube: a tube where even at full operating grid bias, cathode-to-plate current is never completely cut. Remote cutoff tubes are typically Class A amplifiers. Semi-remote cutoff tubes function as Class AB amplifiers. Early RCA literature called them super-control amplifiers. Also see sharp-cutoff tubes.

RMA / RETMA: RMA stands for the Radio Manufacturer's Association, which was later renamed the Radio & Electronic Television Manufacturer's Association. It is the entity that provided the standard tube designation system in the United States beginning in the early 1930s. I.e. a 12AX7 tube is the RMA or RETMA number. Both terms are used interchangably, and preference depends on the source of the literature. RMA is more commonly used.

Saturation Current: the maximum current that can flow between cathode and plate at a given plate voltage.

Screen Grid: an electrode located between the plate and the control grid; it is typically coupled to the plate with a resistor to drop the voltage, usually to around 70% of the plate voltage value. The screen grid primarily acts to reduce internal capacitance between the cathode and the plate, which provides better performance at RF freqencies.

Screen Grid Tube : an early marketing term for a tetrode. Very common in the late 1920s when this was a selling point.

Secondary Emission: a source of electrons due to impact by electrons or ionized particles. For our purposes, electrons ejected from the plate. Secondary emission usually occurs because of the high energy bombardment of electrons hitting the plate causes other electrons to be ejected, which sets up a negative space charge around the plate, which reduces the apparent plate voltage with respect to the cathode. Secondary emission is typically prevented (or minimized) by the action of asupressor grid.

Sharp-Cutoff Tube: a tube designed to easily cut the cathode-to-plate current with a small change in bias. Sharp-cutoff tubes typically function as Class B or C amplifiers. Also see remote-cutoff tubes.

Shell: the outer covering on a vacuum tube, usually made of glass but sometimes made of metal. Also known as an envelope. Terminology largely depends on the source of the literature. RCA preferred envelope; Sylvania preferred shell.

Siemens: modern unit of conductance, including transconductance. Its formula representative is the letter G.

Space Charge: a cloud of free electrons ejected from an element. In tubes, this is typically found next to the cathode. Under normal operating conditions, a tube should be able to draw all of the electrical current it needs from the space charge, not directly from the surface of the cathode. The size of the space charge varies with the composition and temperature of the donor element.

Supressor Grid: an electrode located between the screen grid and the plate, typically as near the plate as possible without causing an electrical short at the tube's highest operating voltage. The supressor performs two functions: it's primary role is to supress secondary emission from the plate; and it further reduces internal capacitance between the cathode and the plate, which provides better performance at RF frequencies.

Sylvania: aka Hytron-Sylvania. A major manufacturer of vacuum tubes in the United States.

Tetrode: a tube with two grid elements, the control grid and screen grid, between the cathode and the plate. Sometimes called screen grid tubs.

Thermionic Emission: this is where electrons are ejected from the surface of a heated element, typically the cathode. The ejected electrons form a space charge in the area around the element, and are then drawn toward a positively-charged element, typically the plate. Originally known as the Edison Effect.

Triode: a tube with one grid element (the control grid) between the cathode and the plate.

Transconductance: formerly known as mutual conductance, its name was changed by the IEEE in the 1930s. Transconductance is the conductance between the cathode and plate under a given condition (i.e. plate voltage and grid bias). In formulas it is given the letter Gt, though Gm is commonly used even when transconductance appears in the literature.

Other modern literature, or outside the US, the unit of measure is typically Siemens. Inside the USA, especially in older literature but it still persists among people who work on vacuum-tube based equipment, the unit of measure is the mho (which is Ohm spelled backward)—most typically the microMho.

Transconductance is calculated as the amplification factor divided by the plate resitance. I.e. a 12AX7 tube has an amplification factor of 100 and a plate resistance of 62,500 ohms (at 250V); so the transconductance is 0.0016 mhos, or 1,600 micromhos (or microSiemens if you prefer).

Valve: UK term for a vacuum tube.