Extract A

Any 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 depending on the class of operation (classes A, B, or C), which means that 40-70 % of input power 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 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 possesses a tungsten filament alloyed with 1-3 % thorium which reduces the work function of the metal, giving it the ability to emit sufficient number of electrons at about 2000 degrees Celsius. The "dull emitter" types also possess a tungsten filament but it is coated by 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 by 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 paralleled rather than some requiring special isolated power supplies such as specially insulated windings on power transformers or separate batteries.