When a

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...

circuit malfunctions and draws excessive current, the

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

("plate") may overheat, sometimes causing a visible

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...

or orange glow. In

consumer electronics Consumer electronics or home electronics are electronic (analog or digital) equipment intended for everyday use, typically in private homes. Consumer electronics include devices used for entertainment, communications and recreation. Usually r ...

, this is universally indicative that the tube is experiencing an overload condition, though the reasons for the overload may vary. Some high-powered vacuum tubes (e.g. large transmitter or

induction heating Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...

tubes) have

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

envelopes and/or

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...

coated

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that is ...

plates which are intended to operate at glowing temperatures and thus employ ''radiation cooling''. The zirconium plate coating acts as a

getter A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by . Thus the getter removes small amounts of ...

, and proper getter action in these tubes depends on the plate running at high temperatures. Such tubes may develop excessive gas content if underloaded for long periods of time. One trade name for this type of plate coating is

Eimac Eimac is a trade mark of Eimac Products, part of the Microwave Power Products Division of Communications & Power Industries. It produces power vacuum tubes for radio frequency applications such as broadcast and radar transmitters. The company name i ...

's "pyrovac".

Causes

In consumer equipment, it is usually a sign of a

short Short may refer to: Places * Short (crater), a lunar impact crater on the near side of the Moon * Short, Mississippi, an unincorporated community * Short, Oklahoma, a census-designated place People * Short (surname) * List of people known as ...

ed or badly mistuned

load Load or LOAD may refer to: Aeronautics and transportation *Load factor (aeronautics), the ratio of the lift of an aircraft to its weight *Passenger load factor, the ratio of revenue passenger miles to available seat miles of a particular transpo ...

, or a badly out-of-bias condition. When testing, repairing or restoring vacuum tube-based equipment, it is wise to watch the plates of all the tubes for this condition.

Dangers

If the tube is overloaded, not only can the plate warp, causing a short to outer grids or beam-shaping elements, but the emissive layer on the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

will be consumed very quickly. The equipment's power supply and the tube's load (

output transformer A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional part ...

flyback transformer A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high voltage sawtooth signals at a relatively high frequency. In modern applications, ...

s, etc.) are likely to be damaged by a sustained overload condition, so power should be immediately disconnected when a glowing plate is found.

Common Occurrences

In modern-day repair and maintenance of early tube-based consumer electronics devices, a glowing plate will be rarely encountered. As a visible symptom of a destructive failure, checking for an overheated tube is essential to ensure the safe and reliable operation of the equipment undergoing maintenance. Most frequently, a glowing plate overload will be found in rectifiers and output tubes. In particular: *

Rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...

tubes (5U4, 80, etc.), usually due to shorted

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s or other substantial short circuits within the device. The power supply's output may be easily disconnected from the rest of the circuit in order to determine whether the overload is occurring in the supply or elsewhere. *Horizontal and vertical output tubes in television sets (6CD6, 21LU8, etc.), usually due to a shorted coupling capacitor upsetting the bias or the driving

oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

operating well out of its design range.

Deflection yoke A deflection yoke is a kind of magnetic lens, used in cathode ray tubes to scan the electron beam both vertically and horizontally over the whole screen. In a CRT television, the electron beam is moved in a raster scan on the screen. By adjusti ...

s are sometimes faulty. **Horizontal output tubes may also be overloaded by a flyback transformer with shorted turns, a defective damper (i.e. 6W4), high voltage rectifier (i.e. 1B3) or high voltage regulator (i.e. 6BK4) tube. **Early color television receivers also used a complex dynamic convergence system, which was driven by both the horizontal and vertical output stages. Failure of part of this system could overload either tube, though this author has never seen such a failure in person. *Audio output tubes in radios, stereos, public address systems and guitar amplifiers (50C5, 6L6, etc.), usually due to a bad coupling capacitor upsetting the bias or a shorted load. If any tube presents a glowing plate, the equipment should be shut down immediately to avoid further damage. Vacuum tubes Electrodes {{electronics-stub