Planar Tubes

PLANAR TUBES are so named because of their construction. The ordinary (conventional) tube you studied earlier uses concentric construction. This means that each element (cathode, grid, and plate) is cylindrical in shape. The grid is placed over the cathode, and the plate, which is the largest cylinder, is 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.

A typical planar tube is depicted in figure 2-13. Notice that the tube elements are mounted close to each other and are parallel to one another. The oxide coating of the cathode is applied to the top surface only. Therefore, the emitting surface of the cathode is parallel to the plate and the grid.

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

The plate of the tube consists of a cylindrical stud. This stud-plate construction has two purposes. Its flat lower surface serves as a parallel plate, and its external upper end serves as the external-plate connection from the tube to the circuit.

Because of its construction, the planar tube cannot use the ladder-type grid, with which you are familiar. Instead, the grid, formed into a circle, is composed of a wire mesh similar to that of a common screen door.

The cathode structure is manufactured in two parts. Point A of figure 2-13 is the metallic shell of the tube and is used to couple (or connect) unwanted radio frequency signals from the cathode to ground. This connection is not, however, a direct coupling. The wafer at point C of figure 2-13 is composed of mica, which serves as a dielectric. The lower extension of the cathode serves as one plate of the capacitor, while the other plate is formed from the flattened upper portion of the cathode connector ring. The cathode has a direct connection to the tube pin through the connector labeled point B. You might think that this is a rather complicated method to connect the cathode to a circuit, but it serves a purpose. At high frequencies, the wiring of a circuit can pick up radio frequency signals and retransmit them. If the wiring involved happens to be the wiring used to supply dc voltages to the circuit, all the tubes in the circuit will receive the signal. The result will be massive distortion throughout the circuit.

The problem can be eliminated by isolating the dc and radio frequency circuits from each other. In planar tubes, this is fairly simple. The point A ring is grounded. Any rf signals that the cathode may pick up through tube conduction are grounded or shorted to ground through the capacitive coupling with the point A shell. In other words, the point A shell (capacitive ground) serves the same function as the bypass capacitor in a cathode-biased circuit. Because the capacitor will not pass dc, bias voltages can be applied to the cathode through the tube pins.

Notice the external shape of the planar tube in figure 2-13. The tube is composed of five sections, or cylinders. As you go from the top to the bottom, each cylinder increases in diameter. Because of this piled cylinder construction, the tube resembles a lighthouse, and is therefore known as a LIGHTHOUSE TUBE.

Another type of planar tube is shown in figure 2-14.

This type of tube, because of its external appearance, is called an OILCAN TUBE. The major difference between it and the lighthouse tube is the addition of cooling fins to allow it to handle more power than the lighthouse tube. Because of their planar construction, both types of tubes are capable of handling large amounts of power at uhf frequencies.

Figure 2-14. - Oilcan planar tube.

Q.6 What effect does transit time have on a conventional triode operated at uhf frequencies?
Q.7 How do uhf tubes counter the effects of transit time?
Q.8 Why can acorn and doorknob tubes NOT handle large amounts of power?
Q.9 What type of uhf tube was developed to handle large amounts of power?