Plates

Edison's plate was just that-a plate, a flat piece of metal. Plates are no longer flat but are designed in many different shapes. Figure 1-8 shows two diodes, one with a directly heated cathode, the other with an indirectly heated cathode. Each plate is cut away to show the internal position of elements and the plate shapes.

Figure 1-8. - Cutaway view of plate construction.

Plates must be able to hold up under the stress of heat created by the flow of plate currents and the closeness of hot cathodes. They need to be strong enough to withstand mechanical shocks produced by vibration and handling.

Some typical materials used for electrontube plates are tungsten, molybdenum, graphite, nickel, tantalum, and copper.

Tube Bases

The base shown in figure 1-9 has two functions. First, it serves as the mounting for tube elements. Second, it serves as the terminal points for the electrical connections to the tube elements. This is accomplished by molding or otherwise bringing pins (or prongs) through the base. The internal ends of these pins are connected to tube elements. The pins themselves are male connections.

Figure 1-9. - Diode construction.

The base must be mechanically strong and made of an insulating material to prevent the tube elements from shorting.

Because they require relatively frequent replacement, most tubes are designed to plug into sockets permanently mounted in the equipment. Tube pins and sockets are so designed that tubes cannot be plugged in incorrectly.

Tube sockets must make secure mechanical and electrical contact with tube pins, must insulate pins from each other, and must provide terminals to which circuit components and conductors are connected.

Each element of a tube is connected to a pin in its base. To trace a circuit easily and efficiently, you must match elements with their pin numbers. This information is available in tube manuals and equipment schematics. Figure 1-10 shows these numbers on one example of a diode symbol. You will also note the designation V1 beside the tube. Electron tubes are often identified in schematic diagrams by the letter V and a number.

Figure 1-10. - Identification of tube elements.

Now, to use the information in the symbol, you need to know the system used to number tube pins and socket connections.

Figure 1-11 shows five common pin configurations as viewed from the bottom of each tube or socket. This is important. In every case, pins and pin connections on sockets are numbered in a clockwise direction - WHEN VIEWED FROM THE BOTTOM.

Figure 1-11. - Pin Identification; all tubes are viewed from the bottom.

In each of the five pictures in figure 1-11, there is an easily identified point from which to start numbering. In the 4-prong and 6-prong tubes, the point is between the two larger prongs. In the octal tube, the point is directly down from the keyway in the center of the tube. In the 7-pin and 9-pin miniatures, the point is identified by the larger distance between pins.

Q.9 Name two functions of the base of a vacuum tube.