GAS-FILLED TUBES

GAS TRIODE

The point at which the gas ionizes can be controlled more accurately by inserting a grid into the gas diode. A negative voltage on the grid can prevent electrons from going to the plate, even when the plate voltage is above the normal firing point. If the negative-grid voltage is reduced to a point where a few electrons are allowed through the grid, ionization takes place. The grid immediately loses control, because the positive ions gather about the grid wires and neutralize the grid's negative charge. The gas triode then acts as a diode. If the grid is made much more negative in an effort to control the plate current, the only effect is that more ions collect about the grid wires - tube continues to conduct as a diode.

Only by removing the plate potential or reducing it to the point where the electrons do not have enough energy to produce ionization will tube conduction and the production of positive ions stop. Only after the production of positive ions is stopped will the grid be able to regain control.

Such gas-filled triodes are known as THYRATRONS. Thyratrons are used in circuits where current flow in the thyratron's output circuit is possible only when a certain amount of voltage is present on the thyratron's grid. The flow of plate current persists even after the initiating grid voltage is no longer present at the grid, and it can be stopped only by removing or lowering the plate potential. The symbols for the gas-filled diode, the voltage regulator, and the thyratron are the same as those for high-vacuum tubes except that a dot is placed within the envelope circle to signify the presence of gas. Some examples of gas-filled tube schematic symbols are shown in figure 2-15.

Figure 2-15. - Schematic diagram of gas-filled tubes.

Before leaving this section, you should be aware of one precaution associated with mercury-vapor tubes. The mercury vapor is not placed in the tube as a vapor; instead a small amount of liquid mercury is placed in the tube before it is sealed. When the liquid mercury comes in contact with the hot filament, the mercury vaporizes.

To ensure that the mercury has vaporized sufficiently, the filament voltage must be applied to mercury-vapor tubes for at least 30 seconds before the plate voltage is applied. If vaporization is incomplete, only partial ionization is possible. Under these conditions, the application of plate voltage results in a relatively high voltage drop across the tube (remember E = I X R), and the positive ions present are accelerated to a high velocity in the direction of the cathode. As the ions strike the cathode, they tear away particles of the emitting surface, usually causing permanent damage to the cathode and the tube. When the mercury is completely vaporized, the action of the gas is such that the voltage drop across the tube can never rise above the ionization potential (about 15 volts). At this low potential, positive-ion bombardment of the cathode does not result in damage to the emitting surface.

Generally, when gas-filled tubes are in the state of ionization, they are illuminated internally by a soft, blue glow. This glow is brightest in the space between the electrodes and of lesser intensity throughout the remainder of the tube envelope. This glow is normal and must not be confused with the glow present in high-vacuum tubes when gases are present. A high-vacuum tube with a bluish glow is gassy and should be replaced. The ionization of these gases will distort the output of the tube and may cause the tube to operate with much higher plate current than it can carry safely.