Vacuum Tube - Heat Generation and Transfer

Heat Generation and Transfer

A considerable amount of heat is produced when tubes operate, both from the filament (heater) but also from the stream of electrons bombarding the plate. The requirements for heat removal can significantly change the appearance of high-power vacuum tubes. Although the miniature tube style became predominant in consumer equipment, high power audio amplifiers and rectifiers would still require the larger "octal" style of enclosure. Transmitting tubes could be much larger still.

Most tubes produce heat from two sources during operation. The first source is the filament or heater. Some tubes contain a directly-heated cathode. This is a filament similar to an incandescent electric lamp; some types glow brightly like a lamp, but most glow a dim orange-red. The "bright emitter" types 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 emits electrons easily at much lower temperatures due to a monolayer of mixed alkali earth metals coating the tungsten; these only reach 800–1000 degrees Celsius.

The second form of cathode is the indirectly-heated form which usually consists of a nickel cylinder, coated on the outside with the same strontium, calcium, barium oxide mix used in the "dull emitter" directly heated types; inside the cylinder is a tungsten filament to heat it. This filament is usually uncoiled and coated in a layer of alumina (aluminium oxide) that insulates it from the actual cathode. This form of construction allows for a much greater electron-emitting area and allows the cathode to be held at a potential difference, typically 150 volts more positive than the heater or 50 volts more negative than the heater. For small-signal tubes such as those used in radio receivers, heaters consume between 50 mW and 5 watts, (directly heated), or between 500 mW and 8 watts for indirectly-heated types. Thus, even a small signal amplifier might consume a watt of power just to warm its heater, compared to the milliwatts (or less) that a modern semiconductor amplifier would require for the same function. Even in power amplifiers the filament power may be responsible for an appreciable reduction in efficiency.

The second source of heat generated is at the plate (anode), as electrons accelerated by its high voltage strike it, depositing their kinetic energy there and raising its temperature. In tubes used in power amplifiers or transmitter output stages, this source of heat will far exceed the power due to the cathode heater. The plates of improperly operated or overloaded beam power tubes can sometimes become visibly red hot; this should never occur under normal operation of consumer electronics and is a precursor to tube failure.

Heat escapes the device by black body radiation from the anode (plate) as infrared radiation. Convection is not possible in most tubes since the anode is surrounded by vacuum. Considerations of heat removal can affect the overall appearance of some tubes. The anode is often treated to make its surface less shiny and darker in the infrared (see black body radiator). The screen grid may also generate considerable heat, which is radiated toward the plate which must reradiate that additional heat along with the heat it generates itself. Limits to screen grid dissipation, in addition to plate dissipation, are listed for power devices. If these are exceeded then tube failure is likely.

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 (plate). High power transmitting tubes may have the surface of their anodes external to the tube, allowing for water cooling or evaporative cooling. Such a water cooling system must be electrically isolated to withstand the high voltage present on the anode.

Tubes which generate relatively little heat, such as the 1.4 volt filament directly-heated tubes designed for use in battery-powered equipment, often have shiny metal anodes. 1T4, 1R5 and 1A7 are examples. Gas-filled tubes such as thyratrons may also use a shiny metal anode, since the gas present inside the tube allows for heat convection from the anode to the glass enclosure.

The outer electrode in most tubes is the anode (plate). 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 so that it can itself reradiate heat generated from within. Sometimes it is fabricated from expanded metal mesh, acting as a Faraday cage but allowing sufficient infrared radiation from the anode to escape. Types 6BX6/EF80 and 6BK8/EF86 are typical examples of this shielded type using expanded mesh. Types 6AU6/EF94 and 6BE6/EK90 are examples which use a gray sheet metal cylindrical shield.