ELECTRON TUBES

2-29 found in conventional tubes, the principle employed (electrostatic attraction and repulsion) forms the heart of all vacuum tube operation. 7. The Horizontal-Deflection Plates—in the CRT, these plates move the electron beam by electrostatic attraction and repulsion, horizontally across the CRT screen. In most equipment using the CRT, including television sets, electronic signals are supplied to these plates to trace or paint a horizontal line. 8. The Aquadag Coating—in the CRT, this coating performs the same function as the suppressor grid in conventional tubes; namely, eliminating the effects of secondary emission. In conventional tubes, the suppressor grid is negatively charged and repels secondary emission electrons back to the plate. In the CRT, the aquadag is positively charged and attracts secondary emission away from the screen. 9. The Screen—also called the face, is a unique element of the CRT. When struck by electrons, the phosphor coating becomes luminous, or glows, thus enabling the tube to visually present electronic signals. From this comparison of the CRT and other types of electron tubes, one fact should be clear. Almost all tubes, no matter what their function, operate on two principles: electrostatic attraction and repulsion, and thermionic emission. By keeping these two principles in mind, you should be able to analyze any type of tube operation. Q14. What is the unique ability of the CRT? Q15. What are the three main parts of CRT? Q16. What term is used for the ability of a spot on a CRT screen to continue to glow after the electron bean has struck it and moved away? Q17. The electron beam in a CRT is made to sweep from left to right across the screen. What tube element causes this sweeping motion? Q18. In applications where electronic waveforms are displayed on a CRT screen, the input signal is normally applied to what CRT element? SAFETY There are certain safety precautions you should follow when you work with or handle the special tubes covered in this chapter. We will examine these tubes and their associated precautions in the following sections. ELECTRON TUBES The average electron tube is a rugged device capable of withstanding the shocks and knocks of everyday usage and handling. However, they are not indestructible. You should remember that most electron tubes contain a near vacuum enclosed by a glass envelope. Because of this, the glass is under constant stress from atmospheric pressure. Any undue stress, such as striking the envelope against a hard surface, may cause the envelope to shatter, resulting in an IMPLOSION. An implosion is just the opposite of an explosion. When the glass envelope of an electron tube shatters, the outside atmosphere rushes into the tube to fill the vacuum. As the air rushes into the tube, it