Many types of vacuum tubes can be recognized from their appearance. A considerable amount of heat is produced when tubes operate. In most circuits the tube is about 30-60% efficient dependent on the class of operation (classes A, B, or C), which means that 40-70 % of input power to the stage is lost as heat. The requirements for heat removal significantly change the appearance of high-power vacuum tubes.
Most tubes contain two sources of heat when operating. The first one of these is the filament or heater. Some types contain a directly heated cathode. This is a filament similar to an incandescent electric lamp and some types glow brightly like a lamp, but most glow dimly. (The "bright emitter" type possess a tungsten filament alloyed with 1-3 % thorium which reduces the work function of the metal, giving it the ability to emit sufficient electrons at about 2000 degrees Celsius. The "dull emitter" types also possess a tungsten filament but it is coated in a mixture of calcium, strontium and barium oxides, which emit electrons easily at much lower temperatures due to a monolayer of mixed alkali earth metals coating the tungsten when the cathode is heated to about 800-1000 degrees Celsius.)
The second form of cathode is the indirectly heated form which usually consists of a nickel tube, coated on the outside with the same strontium, calcium, barium oxide mix used in the "dull emitter" directly heated types, and fitted with a tungsten filament inside the tube to heat it. This tungsten filament is usually uncoiled and coated in a layer of alumina, (aluminium oxide), to insulate it from the nickel tube of the actual cathode. This form of construction allows for a much greater electron emitting area and, because the heater is insulated from the cathode, the cathode can be positioned in a circuit at up to 150 volts more positive than the heater or 50 volts more negative than the heater for most common types. It also allows all the heaters to be simply wired in series or parallel rather than some requiring special isolated power supplies such as specially insulated windings on power transformers or separate batteries. For small-signal tubes such as used in radio receivers, heaters are rated from 50 mW to 5 watts, (directly heated), and about 500 mW to 8 watts for indirectly heated types. Once filament/heater power is included in total power consumption, small tubes have very poor efficiencies. A 6BM8/ECL82 audio stage consumes a total power of some 15 watts for 3.5 watts of useful audio power, giving an efficiency of around 23%. Some signal amplifiers, particularly high-frequency amplifiers such as the 6BA6, consume some 5.9 watts of power in normal operation and deliver only 1.1 watts of power at the plate.[6]
The second source of heat is generated at the anode, when electrons, accelerated by the voltage applied to the anode, strike the anode and impart a considerable fraction of their energy to it, raising its temperature. In tubes used in power amplifier or transmitting circuits, this source of heat will exceed the power dissipated in the cathode heater. (The plates or anodes of 6L6 devices used in guitar amplipfiers can sometimes be seen to reach red heat if the bias is set too high, they should not emit any visible radiation when driven at maximum ratings.) No tubes in domestic, music, or studio equipment should operate with glowing anodes.
This heat usually escapes the device by black body radiation from the anode/plate as infra red light. Some is conducted away through the connecting wires going to the base but none is convected in most types of tube because of the vacuum and the absence of any gas inside the bulb to convect.
It is the way tubes get rid of heat which most affects their overall appearance, next to the type of unit, (triode, pentode etc.), they contain, or whether they contain more than one of these basic units. For devices required to radiate more than 500 mW or so, usually indirectly heated cathode types, the anode or plate is often treated to make its surface less shiny, (see black body radiator), and to make it darker, either gray or black. This helps it radiate the generated heat and maintain the anode or plate at a temperature significantly lower than the cathode, a requirement for proper operation. Types 6BQ5/EL84 and 6BM8/ECL82 are examples of indirectly heated types with gray anodes.
Other internal elements of high-power tubes, such as control grids and screen grids, may also dissipate heat if carrying large currents. Limits to grid dissipation are listed for such devices, to prevent distortion and failure of the grids.
Tubes used as power amplifier stages for radio transmitters may have additional heat exchangers, cooling fans, radiator fins, or other measures to improve heat transfer at the anode. Broadcast transmitters may use water-cooling or evaporative cooling for tube anodes. The water cooling system must withstand the high voltages present on the anode.
Low power rated tubes, such as the 1.4 volt filament, directly heated tubes, designed for use in battery powered equipment, often retain shiny metal anodes as they produce so little heat. 1T4, 1R5 and 1A7 are examples of devices with shiny untreated anodes. Gas filled tubes, such as thyratrons, although they possess a greater plate dissipation than a "1 volt battery type", still often possess a shiny metal anode finish as the gas filling conducts and convects the heat to the bulb wall. Types 884 and 2D21 are typical examples.
The outer electrode in most tubes is usually the anode. Some small signal types, such as sharp and remote cut-off R.F. and A.F. pentodes and some pentagrid converters have a shield fitted around all the electrodes enclosing the anode. This shield is sometimes a solid metal sheet, treated to make it dull and gray like an anode or plate, and sometimes it is fabricated from expanded metal mesh, acting as a Faraday cage but allowing sufficient heat from the anode beneath to escape. Types 6BX6/EF80 and 6BK8/EF86 are typical examples of this shielded type commonly using expanded mesh. Types 6AU6/EF94 and 6BE6/EK90 are examples using a gray sheet metal cylindrical shield giving them a very similar overall appearance.
Indicators such as some "magic eye" tubes and the type 6977 fluorescent-anode type have glowing electrodes.
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