Tetrodes

Author: J.B. Hoag

There are tubes which contain four electrodes: a filament, a plate, and two grids located between the filament and the plate. As usually operated, the grid nearest the filament serves in the same manner as the single grid of a triode. It is called the control grid. The additional grid is generally constructed so as to surround the plate as completely as possible. This type of tetrode is known as a screen-grid tube, inasmuch as the new grid structure shields the control grid from changes on the plate without influencing the fixed or static voltages on the plate. A tetrode with its associated circuits is shown in Fig. 15 A.

Fig. 15 A. A screen-grid tube. (From E. & N. P.)

In this circuit, the current I_p flows (in the conventional sense) through r₀ to P, to the filaments, and back to the battery E_b. As it passes through the resistance r₀, a voltage drop is set up, equal numerically to I_p · r₀. Thus the voltage on the plate of the tube is less than that of the battery E_b by an amount equal to the potential drop in the load resistance r₀. Now, when a signal is applied at the input terminals, a change occurs in the plate current, and likewise a change occurs in the potential drop across the load, of an amount i_p · r₀. In other words, the voltage between the filament and the plate changes by the same amount, ip · r₀ This change of plate voltage would induce a new voltage on the control grid were it not for the shielding action of the screen grid. In the absence of the screen, voltages induced upon the control grid can be sufficiently great to start and maintain oscillations throughout the tube circuits. These are undesirable if the tube is to be used in a receiver. In short, the screen grid is intended to prevent oscillations in the circuits.

Tubes of the screen-grid type have high voltage-amplification constants ; as great as 800. This is an advantage. But they are also characterized by high plate resistance, of the order of 1,000,000 ohms, which is, generally speaking, a disadvantage.

Fig. 15 B. Plate current Ip vs. plate voltage Ep of a screen-grid tube

Figure 15 B shows the plate current at different plate voltages. The arrow points to the fixed voltage of the screen grid. In order to understand the peculiar dip in this curve let us first start with zero voltage on the plate and gradually increase its value. At first, the plate current increases as with a triode. However, the electrons which reach the plate eject secondary electrons from it. As the original or primary electrons are sped up more and more by the increase of the plate voltage, starting at zero volts, the number of secondary electrons ejected from the plate gradually increases. Since the screen grid is more positive than the plate, these secondary electrons flow to the screen grid. The net current in the plate circuit is thus a composite of the original electrons coming to the plate minus the secondary electrons leaving the plate. According to the relative numbers of the primary and secondary electrons, the plate current will increase or decrease as the plate voltage is increased. Throughout the region AB of Fig. 15 B, this ratio of secondaries to primaries is on the increase. When the plate voltage approaches that of the screen grid, the secondary electrons have increasing difficulty in escaping from the plate.

When the plate voltage is greater than the screen-grid voltage, they fail to do so altogether. Then the secondary electrons-, although copiously emitted, execute small curved paths and plunge back again into the plate from which they arose, and the net current flowing to the plate is that of the primary electrons alone. When all electrons emitted from the filament are drawn to the plate, further increase in its potential does not increase the current and we have the horizontal section at the upper right of the curve.

There is a different kind of tetrode known as a space-charge-grid tube which will be discussed in a later chapter.