Vacuum Tubes--Vacuum Tubes

Ambient Temperature. The temperature of the surrounding air, as in a room or inside a cabinet where electronic equipment is housed.

Back Emission. Emission from an electrode, occurring only when the electrode is polarized in a manner opposite to that required for normal conduction; a form of primary emission common to rectifiers during the inverse portion of their cycle.

Ballast Tube. A device used in series heater radios to limit the cur rent to normal. Not really a tube because it is not evacuated; it contains a resistor and resembles a metal tube.

Bogey. The average, or published, value for a tube characteristic.

A bogey tube would be one having all characteristics on bogey.

Button Stem. The glass base of a tube on which the mount structure is assembled. The pins may be sealed into the glass, in which case no base is needed. In some large tubes, the stiff wires are passed directly into the base pins to give added strength. (See "Pressed Stem.") Catastrophic Failures. Those failures which occur suddenly and with out warning; they usually render the equipment unusable.

Cathode Activity. A measure of the efficiency of an emitter; the mathematical relationship between the two values of emission current measured under two conditions of cathode temperature.

Cathode-Current Density. The current per square centimeter of cathode area, expressed as amperes or as milliamperes per centimeter squared.

Cathode Interface. A layer formed between the nickel sleeve and the oxide coating of an indirectly-heated cathode. The layer exhibits resistance and capacitance; it can be largely nullified by raising the cathode temperature.

Characteristic Spread. The range between the minimum and maxi mum values for a given characteristic that is considered normal in any large group of tubes.

Class-A Amplifier. An amplifier in which the grid is never driven positive nor beyond cutoff. Plate current flows throughout the entire 360° of signal swing.

Class-B Tubes. Tubes designed especially for use in Class-B amplifiers. The tube, essentially cut off at zero bias, conducts when its grid is driven positive. Used only in pairs to form a Class-B amplifier. Plate current flows for only 180° of the full cycle for each tube.

Class-C Tubes. Tubes designed to be operated in Class-C amplifiers, where the tube is biased well beyond cutoff and is driven into conduction by short excursions of the grid, which goes far into the positive-grid region.

Contact Potential. An electrical force which develops between dissimilar metals when heated. Specifically, a voltage developed at the grid of a tube as a result of metallic differences in the grid and cathode structures. Often used as a misnomer for all grid currents.

Correlated Characteristic. A characteristic known to be reciprocally related to some other characteristic.

Critical Characteristic. A characteristic not having the normal tolerance to variables.

Cutoff. That condition in a vacuum tube where the grid has been made sufficiently negative to reduce the plate current or the transconductance virtually to zero. Often specified as the 50 microampere or 50-micromho point on the 1.-E, curve.

Differential Cooling. Cooling which takes place at a different rate at various points on an object or surface.

Filament Sag. The bending of a filament as a result of slack caused by heating and expansion.

Fixed Bias. The application of a potential to a tube grid which is unaffected by other operating conditions of the tube.

Fixed Screen. The application of a potential to a screen grid which is unaffected by other operating conditions within the tube.

Folded Heater. A type of heater made from a single strand of coated wire folded and inserted into a cathode sleeve. The insulation is removed at each fold to make it possible for the wire to be folded.

Frame Grid. A type of grid construction which stretches the individual grid wires across a rigid frame. They are not wound like conventional grids.

Gas Cleanup. The process by which many gas-filled tubes tend to lose their gas pressure and hence become inoperable. Gas, in the form of ions, is driven at high velocity into the metal parts or the glass envelope of the tube, where they form stable compounds and are lost as far as the tube is concerned.

Gas Current. The current which flows in the grid circuit when gas ions are present within the tube and the grid is polarized favorably in order to attract them.

Getters. Substances which have a strong affinity for gas evolved within a tube and which, by their action, bind these gas atoms into themselves, where they remain inactive and harmless.

Glow Discharge. Within a vacuum tube, that form of ionized discharge which precedes the arc discharge and is characterized by a glow which covers the cathode surface.

Grid-Circuit Testers. Testers designed to measure the grid resistance of vacuum tubes without discrimination as to the type or polarity of impedance measured.

Grid Current. Any current which flows in the grid-to-cathode circuit of a vacuum tube; it is usually a complex current made up of several individual currents having a variety of polarities and impedances.

Heat Gradient. The difference in temperature between two parts of the same solid object.

Heat Sink. The object used to absorb heat from some other object.

The ultimate heat sink is our atmosphere, since all heat must eventually be transferred to it. Most heat sinks seek to transfer the heat directly to the surrounding air.

Heater Biasing. The application of a DC potential to the heaters of vacuum tubes for the purpose of eliminating diode conduction between the heater and some other element within the tube.

Hum-Balancing Pot. A potentiometer usually placed across the heater circuit. The arm of the potentiometer is grounded, thereby permitting the heater voltage to be balanced with respect to ground.

Hum Bucking. The introduction of a small amount of voltage, at the power-line frequency, into a circuit to cancel unwanted power line interference.

Initial Velocity Current. A current which flows between an electrode, such as the grid of a vacuum tube, and its cathode as a result of electrons thrown off from the cathode because of heat alone.

Their velocity is sufficient to allow them to reach the grid with out the need for an accelerating field.

Interelectrode Leakage. The current which is not the result of normal conduction and which flows between elements not normally connected in any way.

Interface Resistance. See "Cathode Interface." Law of Normal Distribution. The Gaussian law of the frequency distribution of any normal, repetitive function. It describes the probability of the occurrence of deviants from the average.

Leakage Reactance. The reactance represented by the uncoupled inductance of two mutually coupled inductances. When two inductances are mutually coupled and their value is measured, aiding as well as bucking, the difference between these two values is the uncoupled inductance, and its reactance is the leakage reactance.

Microphonics. The mechanical translation of vibration or shock into an electrical signal by a vacuum tube.

Mount Structure. That portion of a vacuum tube which consists of all of the essential elements except the glass envelope. It is the "works," minus the enclosure.

Negative Ions (Gas). Atoms of some gas which have taken on additional electrons and so have an excess negative charge.

Negative Resistance. A resistance which exhibits characteristics contrary to normal resistors; namely, when the voltage is in creased across such a resistor, the current will decrease.

Negative Temperature Coefficient. A device having characteristics which respond to a change in temperature in a manner opposite to other so-called "normal" devices. For example, most resistors increase in value when heated. A resistor which drops in value when heated has a negative temperature coefficient.

Peak Current. The maximum current which flows during the complete operational cycle.

Performance Characteristic. A characteristic measurable in terms of some useful denominator, such as gain, power output, etc.

Pin Holes. Small punctures in the glass envelope of a vacuum tube.

Positive Ions (Gas). Atoms of some gas which have lost an electron and so have an excess positive charge.

Pre-burning. The process of stabilization which calls for the continuous operation of tube heaters for a given number of hours.

Cathode current may or may not be drawn at the same time, and the tubes may be vibrated all or part of the time.

Pressed Stem. That type of vacuum-tube construction which forms all support wires into a flattened piece of glass tubing, similar to and a relic from the lamp-maker's art. Now considered an obsolete method of tube construction.

Primary Grid Emission. Grid emission which results from contamination of the grid wires by cathode coating material or from excessive grid temperatures.

Pulse Emission. Emission drawn for short periods; it may or may not follow a regular repetition rate.

Reactivation. The restoration of an emitter to useful life by means of some process which usually consists of elevating its temperature and momentarily drawing large currents from it.

Reliability. The degree to which a piece of apparatus can be expected to perform its normal function without interruption, whenever called upon to do so.

Ruggedization. The redesign of a piece of equipment or its components to make them able to withstand operation in environments where vibration and mechanical shock are commonplace.

Runaway. Any condition, additive in nature, under which continued exposure to them will result in eventual destruction of the device.

Saturable Reactor. A transformer or an inductor designed to take advantage of the core-saturation principle.

Secondary-Emission Tube. A tube which makes use of secondary emission to achieve a useful end. The photomultiplier tube is an example.

Secondary Grid Emission. Emission from the grid of a tube as a result of high-velocity electrons being driven against it and thereby knocking off additional electrons. The effect is the same as for primary grid emission.

Selected Tubes. Tubes which, though bearing a standard designation, do not contain all the characteristics of other tubes bearing the same designation, because they have been subjected to special tests which may or may not be registered or carried out by other tube manufacturers.

Skewed Distribution. A frequency distribution of any natural phenomenon which has zero or infinity for one of its limits.

"Sleeping Sickness." Slang for cathode-interface resistance.

Space Charge. The electron cloud which surrounds the hot cathode of a vacuum tube.

Space Charge Tube. A tube which makes use of the space charge in a unique manner to greatly increase its transconductance.

A positively charged grid is placed next to the cathode, before the control grid. This enlarges the space charge, moving it out to where the control grid can have a greater effect on it, and hence, on the plate current.

Spurious Emission. Any emission not controlled by the designated control grid (in other words, unwanted emission). Standing Waves. When a condition of resonance exists in a linear electrical circuit or in a mechanical system, standing waves will usually be present. This means the location of the energy maxima and minima along the system will remain fixed with reference to the ends of the system.

Star Cracks. Also known as "pin holes," these are small fractures in the glass envelope with pointed, star-like radials emanating from them.

Starting Voltage. The voltage necessary to cause a gaseous voltage regulator to ionize or start conducting. As soon as this point is reached, the voltage drops to the operating voltage.

Temperature Limited. A cathode is said to be temperature limited when all the electrons emitted from it are drawn away by a strong positive field. The only way to increase the flow of electrons is to raise the cathode temperature.

Thermal Lag. The time taken to raise the entire mass of a cathode structure to the temperature of the heater.

Thermocouple. A device for converting heat into electrical energy.

It consists usually of two dissimilar metals connected to a sensitive current indicator.

Tipoff. That portion of a vacuum-tube bulb which is the last to be melted and sealed after evacuation of the bulb.

Transconductance. The ratio of the amount of grid voltage needed to cause a given change in plate current.

Tube Bridge. An instrument used in the precise measurement of vacuum-tube characteristics. It contains one or more bridge type measuring circuits, plus power supplies and signal sources for all possible electrode combinations.

Tube Drop. The voltage measured across a tube, from plate to cathode, when the tube is conducting at its maximum current rating.

Waiting Time. The time that must elapse between the turning on of heaters and the application of plate voltage to certain tubes, like thyratrons.

Warm-up Time. The time which elapses, after the heater is turned on, before the cathode reaches its optimum operating temperature in an indirectly-heated type of tube.

Zener Diode. A particular type of semiconductor diode operated in a reverse-biased manner and exhibiting voltage-regulator characteristics at the Zener breakdown point .

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