Figure 2-11.Concentric construction of a conventional tube

Figure 2-13.Internal structure of a typical planar tube

Neets Module 06-Introduction to Electronic Emission, Tubes, and Power Supplies
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2-12 placed over the grid. The result is a tube composed of concentric cylinders like the one shown in figure 2-11. Thus, the name concentric tubes. Figure 2-11.—Concentric construction of a conventional tube. At ultrahigh frequencies, the problems of producing small tube elements while reducing the spacing between elements become very difficult. Not only are the elements hard to keep parallel with each other during the manufacturing process, but they also have a tendency to warp and sag under normal operating conditions. Since these elements are already as close together as possible, any reduction in element spacing can cause arcing. Therefore, a new type of tube was developed to prevent arcing or element sagging in conventional tubes. This tube is known as the planar tube. Planar tubes are electron tubes in which the cathode, plate, and grids are mounted parallel to each other. Their physical construction greatly resembles a schematic diagram of a normal tube, as shown in figure 2-12. Figure 2-12.—Resemblance of a planar tube to a schematic diagram.

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