1. MULTI-ELEMENT TUBES

2. The triode tube has some limitations which can be overcome by adding additional elements to the tubes. First, before studying multi-element tubes, let’s consider one of the big disadvantages of a triode in order to be able to understand the advantages of multi-element tubes better.

3. PLATE-TO-GRID CAPACITY You will remember from your lesson on capacitors that a capacitor consists of two metal plates placed close to each other. In capacitors, the metal plates are deliberately placed near each other in order to have capacity. The grid and plate of a triode tube, since they are placed near each other and are insulated from each other, actually form the plates of a capacitor.

4. PLATE-TO-GRID CAPACITY Because the grid wire is small and there is a reasonable amount of space between the plate and grid, the capacity is small, but in some circuits (particularly where fairly high frequencies are involved), there is a high enough capacity to introduce a number of undesired effects. As you know, at a high frequency, even a small capacitor has a fairly low reactance. Therefore if a triode tube is used to amplify a high-frequency signal, the amplified signal present in the plate circuit of the tube can be fed back into the grid circuit through the plate-to-grid capacity.

5. PLATE-TO-GRID CAPACITY Under certain circumstances, this signal fed from the plate back to the grid can be in phase with the signal applied to the grid of the tube so it will add to the input signal. We call this regenerative feedback where the feedback signal is adding to the input signal. This increase in the amplitude of the input signal in turn produces a still greater signal in the output. The increased signal in the output in turn increases the signal fed back into the grid circuit, which makes the grid signal still stronger.

6. PLATE-TO-GRID CAPACITY This increases the output signal still more. This action goes on and on until eventually a point is reached where the signal from the plate circuit takes control in the grid and the tube begins to generate its own signal voltage. This is called oscillation. The signal fed from the plate of the tube back to the grid is called a feedback signal.

7. PLATE-TO-GRID CAPACITY When the feedback signal (or more simply the feedback) is of the correct phase so that it adds to the grid signal, the tube will oscillate, meaning it generates its own signal and so will not work as an amplifier. When the feedback signal adds to the original signal, we call the feedback regenerative feedback. Not all triode amplifier stages will oscillate. It is possible to overcome this problem by means of suitable circuitry.

8. PLATE-TO-GRID CAPACITY However, in most applications, it is easier and more practical to overcome the problem in the tube itself. This is accomplished by the addition of another grid inside the tube. The second grid is placed between the plate and the first grid in order to shield or screen the grid from the plate. It is called the screen grid or simply the screen. The screen is also called the No. 2 grid.

9. THE SCREEN-GRID TUBE The screen-grid tube is a four-element tube and is called a tetrode. The four elements are the cathode, the grid, the screen grid, and the plate. A top View of a screen-grid tube is shown in Fig. l4(A), and the schematic symbol for the screen-grid tube is shown in Fig.l4(B).

10. Figure 14 A screen-grid tube. (A) A cross section showing the arrangement of the elements, and (B) the schematic symbol.

11. THE SCREEN-GRID TUBE The screen grid is usually made just like a spiral-wound coil. The screen-grid wires are placed directly behind the grid wires so they are completely hidden from the electrons by the grid wire. A cutaway view of the tube would look something like Fig.15. Notice that the screen-grid wires are immediately behind the control-grid wires.

12. Figure 15. A cutaway view of a screen-grid tube

13. THE SCREEN-GRID TUBE The effect of the screen grid is to break the capacity between the plate and the control grid into two separate capacitors. In other words, there is a small capacity between the plate of the tube and the screen, and another capacity between the screen of the tube and the grid. These two capacitors are connected in series so that the net plate-to-grid capacity is much smaller than it is in a triode tube. In addition, the screen grid of a tube is normally operated at signal ground potential.

14. THE SCREEN-GRID TUBE Therefore, any energy fed from the plate of the tube back toward the grid is fed to ground at the screen. This practically eliminates the feedback from the plate to the grid of the tube. The dc potential applied to the screen is always positive with respect to the cathode. Usually the voltage placed on the screen is about half the voltage applied to the plate of the tube, but in some tubes the screen-grid voltage may be as high as the plate voltage.

15. THE SCREEN-GRID TUBE This may appear to be a contradiction because we said the screen is operated at the signal ground potential. Actually, it is possible to have a tube element at signal ground potential and still have a positive or negative dc voltage applied to it. You will remember that a capacitor offers a low reactance to the flow of ac through it. Therefore, we can ground the screen of a tube, as far as the signal is concerned, by putting a capacitor between the screen of the tube and ground.

16. THE SCREEN-GRID TUBE If the capacitor is large enough, its reactance to the signal is so low that the screen is practically at signal ground potential. At the same time, since a capacitor does not permit the flow of dc through it, you can connect it to a dc voltage source and apply a positive voltage to the screen. The schematic diagram of a typical screen-grid amplifier circuit is shown in Fig.16. Here the input signal is fed between the grid and cathode of the tube, as in the case of the triode.

17. FIGURE 16 Schematic diagram of a screen-grid amplifier

18. THE SCREEN-GRID TUBE Electrons flow from the cathode of the tube to the plate, as in the triode. However, in the tetrode tube, the number of electrons flowing will depend upon the plate voltage, the grid voltage, and the screen-grid voltage. In fact, the number of electrons flowing from the cathode toward the plate will depend much more on the screen and grid voltages than on the plate voltage. As in the triode tube, small changes in grid voltage produced by the ac signal applied to the input will produce comparatively large changes in plate current.

19. THE SCREEN-GRID TUBE These changes in plate current will cause the voltage between the plate of the tube and ground to vary, and this voltage will be the amplified output voltage. A few of the electrons flowing from the cathode of the tube toward the plate of the tube will strike the screen. Thus, there will be a small current flowing from the cathode of the tube to the screen grid, through R3 and through the power supply, and back to the cathode. Because the plate voltage has much less effect on the plate current in a screen- grid tube than in a triode tube, screen-grid tubes have a much higher amplification factor than triodes.

20. THE SCREEN-GRID This is due to the fact that it takes a much larger change in plate voltage than in a triode to produce the same change as a change in grid voltage.

21. DISADVANTAGES Electrons traveling from the cathode to the plate of a tube reach a very high speed, and when they strike the plate, they may knock other electrons off the plate. In a diode and a triode, the plate is the only positive element in the vicinity of these loose electrons, and therefore they are attracted back to the plate. However, in a tetrode, in addition to the plate, the screen grid has a positive voltage applied to it.

22. DISADVANTAGES If the plate voltage is substantially higher than the screen voltage, the electrons will be attracted back to the plate, but if the screen voltage is almost equal to or even higher than the plate voltage, the electrons knocked off the plate of the tube will be attracted to the screen instead of to the plate. Thus, for every electron reaching the plate from the cathode, there may be two or three electrons emitted by the plate. This is called “secondary emission.”

23. DISADVANTAGES If the control grid swings in a positive direction, more electrons will strike the plate, knocking still more electrons off the plate. The net result will be that the plate current decreases when the grid swings positive. To prevent this undesirable action, a third grid, called a suppressor grid, was added to develop the pentode tube.

24. PENTODE TUBES The pentode tube is a tube with five elements. The fifth element, called a suppressor grid, is placed between the screen grid and the plate. The third grid gets its name because it is put in the tube to suppress or eliminate the undesirable effects of secondary emission occurring at the plate of the tetrode tube. Because it is also the third grid, it is sometimes called the No. 3 grid.

25. PENTODE TUBES The suppressor is usually connected directly to the cathode of the tube, but sometimes it is connected directly to ground. It has very little effect on the electrons traveling from the cathode of the tube toward the plate. These electrons are attracted from the cathode by the positive potential on the screen of the tube. They are accelerated by the voltage, but as they approach the screen, instead of stopping there, they pass right through the screen wires.

26. PENTODE TUBES Once they get through these wires they are attracted to the plate by its positive voltage. In the pentode tube, after the electrons have passed through the screen, they are moving at a fairly high velocity and travel right on through the suppressor grid to the plate. When the electrons strike the plate, they knock other electrons off the plate, as in the case of the screen-grid tube. However, these electrons are traveling at a comparatively low speed when they are knocked off the plate.

27. PENTODE TUBES The suppressor, which is connected to the cathode or to ground, repels the electrons emitted from the plate by secondary emission, and these electrons move back to the plate of the tube. Thus, the addition of the suppressor grid, eliminates the undesirable current flow from the plate of the tube to the screen, which we found could occur in the tetrode tube. Because the pentode tube has a screen grid, it has the low plate-to-grid capacity that we found in the tetrode tube. In addition, the pentode has the advantage that it does not suffer from the adverse effects of secondary emission.

28. PENTODE TUBES One of the important characteristics of both the tetrode and pentode tube is that the plate voltage has very little effect on the plate current. The plate voltage on these tubes can be varied over wide limits without appreciably changing the plate current. For a given grid voltage, the plate current will depend primarily upon the screen voltage. The screen is the electrode that starts the electrons moving from the cathode to the plate, and its voltage (rather than the plate voltage) determines how many electrons will flow from the cathode to the plate of the tube.

29. PENTODE TUBES This is because the screen is closer to the cathode than the plate. You will remember that the amplification factor of a tube is the ratio of plate voltage required to produce a given change in plate current, to the grid voltage change required to produce the same change in plate current. The formula is:

30. PENTODE TUBES The symbol e p stands for the change in plate voltage and the symbol e g stands for the change in grid voltage. Since the plate voltage has very little effect on the plate current in a tetrode or pentode tube, it takes a very large change in plate voltage to produce the same change in plate current that can be produced by a small change in grid voltage. Thus, the amplification factor of a tetrode or a pentode tube is very high. A high-mu triode may have an amplification factor of 100, whereas the pentodes typically have amplification factors in excess of 1000.

31. BEAM-POWER TUBES Another tube that solves the problem of secondary emission in the screen-grid tube is the beam-power tube. The beam-power tube is a tetrode or four-element tube. A sketch of a beam-power tube is shown in Fig. 17.

32. FIGURE 17 A beam-power tube

33. BEAM-POWER TUBES Notice the shape of the cathode of the tube. It has two flat surfaces. The construction of the cathode is such that most of the electrons will be emitted by the flat surfaces and therefore there is a tendency for the electrons to form into two beams, one on each side of the cathode. Only one of these beams is shown in the drawing.

34. BEAM-POWER TUBES Notice the two small additional plates between the screen grid and the plate of the tube. These plates are called the beam-confining or beam-forming plates. The beam-forming plates are connected internally to the cathode of the tube and repel electrons. These plates act to keep the electrons concentrated into the two beams that are formed at the cathode.

35. BEAM-POWER TUBES The electrons flowing from the cathode of the tube toward the plate are bunched together in these beams. You will remember that an electron has a negative charge and will repel other electrons. Therefore, if an electron traveling at a high rate of speed from the cathode of the tube to the plate knocks additional electrons off the plate, these loose electrons, which will be traveling at a low speed, will be repelled back to the plate by the negative charge on the electron beam.

36. BEAM-POWER TUBES The beam-power tubes have been used in radio, TV, and industrial electronic equipment. Also, large beam-power tubes are used in transmitters. The beam-power tube has proven superior to the pentode in applications where large amounts of power must be handled.

37. OTHER TUBE TYPES The diode, triode, tetrode, and pentode are the basic tube types. However, there are many special tubes that have been manufactured for special applications. In addition, there are tubes that are simply combinations of several tubes in the same envelope. An example of a tube where several types have been combined in one envelope is the tube with a triode and two diodes in it. A schematic symbol of this type of tube is shown in Fig.18.

38. FIGURE 18 A combination tube containing two diodes and a triode. This is called a duo-diode triode.

39. OTHER TUBE TYPES This tube is called a duo-diode triode. As the name indicates, it has two diodes and a triode in one envelope. Another tube type consists of two triodes in one envelope, as shown in Fig. l9. In the schematic shown at (A), a common cathode is used for the two triode sections; in the one shown at (B), each triode section has its own separate cathode. Another combination type is the triode-pentode shown in Fig.20. This type has been used in many applications.

40. FIGURE 19 DUAL TRIODE TUBES. The tube at (A) has a common cathode. The tube at (B) has two separate cathodes. Notice this tube has a single heater with a center tap. Not all tubes of this type have tapped heaters.

41. FIGURE 20 A pentode-triode

42. OTHER TUBE TYPES The pentagrid tube, shown in Fig. 21, is an interesting tube. It is called a pentagrid tube because it has five grids. In this tube, the first grid is used as a grid and the second grid is used as the plate of a triode section. Grids 3 and 5 act as screen grids, and grid 4 acts as the grid of a pentode tube.

43. FIGURE 21 A pentagrid tube

44. OTHER TUBE TYPES You will also run into a tube called the compactron. The compactron is simply a tube where several types are combined in one envelope. You may find a compactron that has three triodes in it, you may find one with two pentodes, or you may find one with two triodes and two diodes. Tubes such as compactrons offer an advantage over tubes in separate envelopes because they are less expensive to manufacture than separate tubes.

45. OTHER TUBE TYPES Another advantage is that they take up less space because the space occupied by the multi-function tube is considerably smaller than the space that would be required for separate tubes. It is easy to identify a compactron since compactron tubes have a base with twelve pins. Another tube you might encounter is the nuvistor. Most nuvistors are triode tubes but they are very small.

46. OTHER TUBE TYPES They look almost like a flat thimble. The nuvistor has the advantage of being not only small, but also having a very low capacity between the elements and having a comparatively high amplification factor. The nuvistor was a late development in tube design and probably would have been a much more important tube if it had not been for the fact that it was developed at about the time when transistors began to push tubes into the background.

47. GAS-FILLED TUBES At one time, gas-filled tubes were used for special purposes. A mercury-vapor tube is an example of a gas-filled diode. The schematic symbol shown in Fig.22 was used to indicate a gas-filled diode.

48. FIGURE 22 Dot inside the envelope is used to indicate a gas-filled tube. This is the schematic symbol for a gas-filled diode.

49. GAS-FILLED TUBES Another type of gas-filled diode is the voltage-regulator tube, often called a VR tube. The schematic symbol for this type of tube is shown in Fig. 23. VR tubes were designed to maintain a constant voltage across them. For example, a VR75 would maintain a voltage of 75 volts across it, a VR105 a voltage of 105 volts, and a VR 150 a voltage of 150 volts. Voltage regulator tubes were used in circuits like the one shown in Fig.24.

50. FIGURE 23 Schematic symbol of a VR tube

51. FIGURE 24 A circuit using a VR tube.

52. GAS-FILLED TUBES The current flow through the VR tube would cause a voltage drop across the series resistor. If the regulated voltage tended to increase, the tube would draw more current so that the voltage drop across the series resistor would increase. If the voltage tended to drop, the tube would draw less current so that the voltage drop across the series resistor would decrease to maintain the regulated output voltage constant.

53. GAS-FILLED TUBES Another gas-filled tube is the gas filled triode called the thyratron. The construction of a thyratron is shown in Fig.25(A) and its schematic symbol in Fig.25(B). In use, a negative voltage is applied to the grid of the thyratron to prevent current flow through the tube. Once the negative voltage is reduced to a point where current flow can begin, the gas in the tube ionizes so that ions flow over to the grid to neutralize the effect of the negative voltage applied to the grid.

54. FIGURE 25 (A) The construction of a thyratron, and (B) the schematic symbol.

55. GAS-FILLED TUBES Current flow begins from the cathode to the plate and it cannot be stopped except by removing the plate voltage from the tube. At one time, thyratrons were quite widely used in electronic control devices, but they have been almost entirely replaced by solid-state devices.

56. QUESTION What is the purpose of the screen grid in a tetrode tube?

57. ANSWER A screen grid reduces the plate-to-grid capacitance and prevents oscillation due to feedback from the plate to the grid of the tube.

58. QUESTION What dc operating potential is applied to the screen of a tube?

59. ANSWER The screen of a tetrode tube is operated with a positive potential applied to it.

60. QUESTION How was the screen of a tetrode tube operated in order to provide maximum shielding between plate and grid?

61. ANSWER The screen is operated at signal ground potential. We accomplish this by connecting a suitable capacitor between the screen of the tube and ground. The capacitor offers a low reactance to ac signals on the screen so, insofar as the signal is concerned, the screen is essentially at ground potential.

62. QUESTION Name the five elements in a pentode tube.

63. ANSWER The five elements in a pentode tube are the cathode, the grid, the screen grid, the suppressor grid, and the plate.

64. QUESTION What elements control the flow of plate current in the pentode tube?

65. ANSWER The grid and screen grid.

66. QUESTION How are the undesirable effects of secondary emission overcome in the beam- power tube?

67. ANSWER In a beam-power tube the electrons are focused in two beams. Electrons knocked off the plate are moving at a slow speed and they are repelled back to the plate by the high-speed electrons in the beam.