SPECIAL-PURPOSE TUBES

LEARNING OBJECTIVES

Upon completion of this chapter, you will be able to:

• Determine the number and type of individual tubes contained within the signal envelope of a multi-unit tube.
• Explain the function and operating principle of the beam power tube and the pentode tube.
• State the difference between the capabilities of conventional tubes and variable-mu tubes.
• Describe the construction of uhf tubes, and explain the effects that ultra-high frequencies have on conventional-tube operation.
• Explain the operation of gas-filled diodes, thyratrons, and cold-cathode tubes.
• Explain the operating principles behind cathode-ray tubes, and the manner in which these tubes present visual display of electronic signals.

INTRODUCTION TO SPECIAL-PURPOSE TUBES

Because of their great versatility, the four basic tube types (diode, triode, tetrode, and pentode) covered in chapter 1 have been used in the majority of electronic circuits. However, these types of tubes do have limits, size, frequency, and power handling capabilities.

Special-purpose tubes are designed to operate or perform functions beyond the capabilities of the basic tube types discussed in chapter 1. The special-purpose tubes covered in this chapter will include multi-unit, multi-electrode, beam power, power pentode, variable-mu, uhf, cold cathode, thyratrons, and cathode-ray tubes.

MULTI-UNIT AND MULTI-ELECTRODE TUBES

One of the problems associated with electron tubes is that they are bulky. The size of an electron tube circuit can be decreased by enclosing more than one tube within a single envelope, as mentioned in chapter 1. There is a large variety of tubes that can be combined into this grouping of "specialty tubes" called MULTI-UNIT tubes. Figure 2-1 illustrates the schematic symbols of a few of the possible combinations found in multi-unit tubes.

Figure 2-1. - Typical multi-unit tube symbols.

An important point to remember when dealing with multi-unit tubes is that each unit is capable of operating as a separate tube. But, how it operates, either as a single tube or as a multi-unit tube, is determined by the external circuit wiring. When you analyze the schematic of a circuit, simply treat each portion of a multi-unit tube as a single tube, as shown in figure 2-2.

Figure 2-2. - Multi-unit tube Identification.

Another type of special-purpose tube is the MULTI-ELECTRODE tube. In some applications, tubes require more than the three grids found in conventional tubes. In some cases, up to seven grids may be used. These types of tubes are called multi-electrode tubes and are normally classified according to the number of grids they contain. An example of this is illustrated in figure 2-3. Here, you see a tube with five grids; hence, its name is "pentagrid." The application of these tube types is beyond the scope of this module, but because multi-electrode tubes have been commonly used you should be aware of their existence.

Figure 2-3. - Pentagrid multi-electrode tube.