SUMMARY OF THE CRT

This summary will not only review the CRT, but will also point out the similarities between the CRT and other tubes.

Look at figure 2 - 28. Here you see both a schematic diagram and a pictorial representation of a CRT. Each element is identified by a circuit number. We will review briefly the function of each element in a CRT and its similarity to elements in conventional tubes. This summary will help you tie together everything you have learned about the CRT and electron tubes in general.

Figure 2-28. - Summary of the CRT.

The Heater - serves as the source of heat for the cathode in both the CRT and indirectly heated tubes.

The Cathode - serves as the source of thermionically emitted electrons in both the CRT and conventional tubes. The major difference is that in the CRT, the cathode is circular in shape and the outer surface is coated to ensure that electron emission is roughly unidirectional.

The Control Grid - in both the CRT and conventional vacuum tubes, the control grid controls the number of electrons that will be fired across "the gap." The major difference is in the physical construction. Conventional tubes use a wire-mesh ladder-type grid, while the CRT uses a cap-like grid.

The Focusing Anode - in the CRT, this anode serves a dual purpose of attracting electrons from the area of the control grid and focusing the electrons into a beam. Its function of attracting electrons from the area of the grid is similar to the action of the plate in a conventional tube. The focusing action of the anode is similar to that performed by beam-forming plates in the beam-forming tetrode. Bear in mind, though, that beam-forming plates are negatively charged and repel electrons into electron sheets, while the focusing anode is positively charged and attracts electrons into beam.

The Accelerating Anode - in the CRT, this anode is used to accelerate the electrons toward the front of the tube. Its action is similar to the screen grid of tetrodes and pentodes. But remember, while the screen grid in conventional tubes accelerates electrons toward the plate, its primary purpose is to reduce interelectrode capacitance, NOT accelerate electrons.

The Vertical-Deflection Plates - in the CRT, these plates move the electron beam up and down the screen. The input signal is usually applied to these plates. While no equivalent element is found in conventional tubes, the principle employed (electrostatic attraction and repulsion) forms the heart of all vacuum tube opertion.

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 televison sets, electronic signals are supplied to these plates to trace or paint a horizontal line.

The Aquadag Coating - in the CRT, this coating performs the same function as the suppressor grid in conventional tubes; namely, eliminting 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.

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.

Q.14 What is the unique ability of the CRT?
Q.15 What are the three main parts of CRT?
Q.16 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?
Q.17 The electron beam in a CRT is made to sweep from left to right across the screen. What tube element causes this sweeping motion?
Q.18 In applications where electronic waveforms are displayed on a CRT screen, the input signal is normally applied to what CRT element?