Hsb

The grid-leak bias method has the advantage of simplicity and of automatically biasing the grid in proportion to the excitation voltage available. Because of this automatic action, the bias voltage developed across a grid leak is not critically dependent on the value of the grid-leak resistance. Therefore, considerable variation in the resistance of the leak can usually be tolerated. Special care must be observed when grid-leak bias is used because accidental removal of the r-f grid excitation will cause the grid bias to fall to zero and (in the case of a tube having a low or medium amplification factor) the plate current to rise to an excessive value. The use of a protective device designed to remove the plate voltage (and screen voltage, in the case of tetrodes and pentodes) on excessivt rises of plate current will minimize the danger of destructive overloads (see PROTECTIVE DEVICES).

Fig. 8 illustrates the use of cathode-resistor bias. In these circuits, the cathode current flowing through Rk builds up a voltage drop which makes the cathode positive with respect to ground. Since the grid is at ground potential with respect to all d-c voltages, the grid is biased negatively with respect to the cathode. The cathode current for triodes is the sum of the d-c plate current and the d-c grid current. For tetrodes and pentodes, the screen current must also be added.

Fig. 8 illustrates the use of cathode-resistor bias. In these circuits, the cathode current flowing through Rk builds up a voltage drop which makes the cathode positive with respect to ground. Since the grid is at ground potential with respect to all d-c voltages, the grid is biased negatively with respect to the cathode. The cathode current for triodes is the sum of the d-c plate current and the d-c grid current. For tetrodes and pentodes, the screen current must also be added.

Cathode-resistor bias, or self-bias, is advantageous in that it tends to protect the tube against heavy d-c plate-current overloads; that is, when the plate current increases, the bias voltage across the cathode resistor also increases so that the rise in plate current is automatically opposed. A disadvantage of self-bias is that the effective d-c plate voltage is reduced by the amount of the bias voltage. Thus, the voltage output of the plate supply must equal the desired plate voltage plus the required bias voltage.

The value of cathode resistor Rt can be determined by Ohm's law, R = E/I, where R is in ohms, E is the required bias in volts, and I is the total cathode current in amperes. For example, assume that the total d-c plate current (under normal load) is 100 milliamperes, that the total d-c grid current is 20 milliamperes, and that the required bias is -240 volts. Then, Rk = 2'40/0.120 = 2000 ohms. The power dissipated by Ri is equal to EI, or (240) (0.120) =28.8 watts. A 50-watt resistor is a logical choice, because it is desirable to operate a resistor at less-than-rated power in order to provide a suitable factor of safety.

Where two or more filament-type tubes are individually self-biased, the use of a separate cathode resistor and a separate filament-supply winding is necessary for each tube so biased (see Fig. 9). This arrangement provides a method of adjusting individually the bias of each tube in a push-pull amplifier stage.

Various combinations of biasing methods are sometimes desirable. Fig. 18 in the CIRCUIT SECTION shows a combination of grid-leak bias and self-bias in the second 807 stage. Part of the bias is supplied by R4 in the cathode circuit. The remainder is furnished by the grid leak R3. This method reduces the magnitude of plate-current overloads due to loss of grid-leak bias when the r-f grid excitation fails. In addition, the loss in d-c plate voltage (caused by the drop across the cathode resistor) is less than in the method employing self-bias alone. Fig. 17 (CIRCUIT SECTION) illustrates the use of grid-leak bias with fixed bias. Sufficient fixed bias is used to reduce the d-c plate current to a low value when no r-f grid excitation is applied (as when the oscillator is keyed, or accidentally goes out of oscillation). The grid leak furnishes enough additional-bias, under normal operating conditions, to provide the total bias voltage required. In a plate-modulated amplifier, the use of grid-leak bias combined with either cathode bias or fixed bias improves the linearity of the amplifier and thereby reduces distortion in the audio component of the modulated carrier.

The performance of a transmitting tube definitely depends on the characteristics of the circuit in which it is used. Because parallel-tuned circuits are almost universally employed for the plate, or output, circuit of vacuum-tube r-f amplifiers, except at ultra-high radio frequencies, considerations involving inductance (L) and capacitance (C) are very important in transmitter design.

The resonant frequency of the parallel-tuned circuits used in transmitters is given by the «relation,

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