RCA Power Tube
Electron Tube
POWER TUBE FUNDAMENTALS
Power tubes are devices for controlling the transfer of energy in electrical circuits. In this respect they are similar to rheostats, switches, and other circuit-type control devices. Tubes, however, permit much more rapid, precise and efficient control of electrical energy than mechanically operated devices.
GAS TUBES
In a vacuum tube, space charge inhibits the release of electrons from the cathode, and this limits the plate current at low and moderate plate voltages. Although the space-charge effect may be reduced by a reduction in the spacing between plate and cathode, it cannot be entirely eliminated by this method. The negative space charge can be neutralized, however, by other methods – for example, by the introduction of a controlled amount of mercury vapor or inert gas in the tube.
When a gas is present in a two-electrode tube, free electrons in the gas are attracted to the positive anode and add to the anode current. Positive ions created continuously by collisions between gas atoms and the free electrons neutralize the space charge so that large currents may be drawn at low anode voltages. In addition, the space-charge neutralization effectively increase the thermal efficiency of the cathode. These advantages make gas tubes particularly suitable for use as power rectifiers. The use of gas tubes, however, requires precautions in circuit design, physical installation and operation, which are not necessary with vacuum tubes.
MERCURY VAPOR TUBES
A mercury-vapor rectifier tube must be handled with special care to prevent dispersion of the liquid mercury from its normal position at the bottom of the bulb. Spattering of the mercury over other portions of the bulb or on the anode or filament must be avoided because it may lead to internal shorts or arcs when the tube is placed in operation. A mercury-vapor tube should always be transported, stored and operated in a vertical position with the filament end down, and should never be jarred, shaken or allowed to rest even momentarily in a horizontal position. The tube should never be rocked or allowed to snap into place in its socket or mounting, and should be protected against excessive equipment vibration.
If spattering occurs, the dispersed mercury must be completely reconcentrated before the tubes are placed in service by means of special preheating and conditioning treatments. In the preheating treatment, the mercury-vapor tube is operated at normal filament voltage, but without anode voltage, for 30 minutes to assure complete vaporization of the mercury content. When filament voltage is removed at the end of this preheating period, most of the vaporized mercury recondenses in a pellet or pool at the bottom of the bulb. The conditioning treatment is then applied to flash out any mercury which may have condensed on the bulb walls or in the vicinity of the anode and filament seals. In this treatment, the tube is operated at normal filament voltage and at about one-sixth normal anode voltage for 5 minutes. The anode voltage is then gradually increased over a period of about 30 minutes to the normal operating value. If an internal flashover occurs at any time during the conditioning treatment, the anode voltage should be reduced until the flashover ceases. It should then be held at this reduced value for a few minutes to assure complete vaporization of the mercury before the treatment is resumed.
From: RCA Transmitting Tubes, Technical Manual TT-4 June, 1956
