When a vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 circuit malfunctions and draws excessive current, the anode

Anode

An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

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

Red

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 or orange glow. In consumer electronics

Consumer electronics

Consumer electronics are electronic equipment intended for everyday use, most often in entertainment, communications and office productivity. Radio broadcasting in the early 20th century brought the first major consumer product, the broadcast receiver...

, this is universally indicative that the tube is experiencing an overload

Overcurrent

In electricity supply, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load,...

 condition, though the reasons for the overload may vary.

Some high-powered vacuum tubes (e.g. large transmitter tubes) have graphite

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...

 or zirconium

Zirconium

Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

 coated tantalum

Tantalum

Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...

 plates which are intended to operate at glowing temperatures. The zirconium plate coating acts as a getter

Getter

A getter is a deposit of reactive material that is deliberately placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption...

, 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

Eimac is a trade mark of Eimac Products, part of the Microwave Power Products Division of Communications & Power Industries. It produces vacuum tubes for radio frequency applications such as broadcast and radar transmitters.-History:...

's "pyrovac".

Causes

In consumer equipment, it is usually a sign of a short

Short circuit

A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

ed or badly mistuned load

External electric load

If an electric circuit has a well-defined output terminal, the circuit connected to this terminal is the load....

, 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

Cathode

A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 will be consumed very quickly. The equipment's power supply and the tube's load (output transformers, flyback transformer

Flyback transformer

A flyback transformer , also called a line output transformer , is a special transformer, which is used for conversion of energy in electronic circuits. It was initially designed to generate high current sawtooth signals at a relatively high frequency...

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
  Rectifier
  A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
  
   tubes (5U4, 80, etc.), usually due to shorted capacitor
  Capacitor
  A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
  
  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 operating well out of its design range. Deflection yokes are sometimes faulty.
  • Horizontal output tubes may also be overloaded by a flyback transformer with shorted turns, a defective damper (ie. 6W4), high voltage rectifier (ie. 1B3) or high voltage regulator (ie. 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.