Automatic Gain Control

Vacuum-tube amplification factor is constant under certain conditions of operation. With high current operation the amplification factor in the region of high anode current and low anode voltage is no longer constant.

Some tubes are designed to have large variations in amplification factor. These are known as variable-mu, remote cut-off, or super-control tubes. The mutual conductance of such tubes is highly variable with grid bias.

Fig. 202. Variable-mu tube (6SK7) mutual conductance curve.

Figure 202 is the curve of mutual conductance for a tube of this kind. Such a characteristic can be used to reduce gain at high amplitudes and thus prevent overmodulation in audio systems. In Fig. 201 the circuit shown automatically reduces gain for excessive values of applied grid voltage e_g on the grid of the 6SK7 tube. This tube drives a 6L6 output tube through transformer T₁. On this transformer there is an auxiliary winding S₂ which is connected to rectifier tube 6H6-1 and produces the rectified output across resistance R₂ having a negative potential at the point shown. With large signals, the voltage rectified across R₂ is large and reduces the mutual conductance and plate voltage swing of the 6SK7 tube. Nearly constant output voltage is maintained in the 6L6 output. If the power output of the 6L6 tube is delivered mainly into a linear a-c impedance, the slight additional load imposed by the gain control makes little difference. But if all the output is delivered to rectifier loads, as it is in Fig. 201, the non-linearity of both tube and load causes output distortion. This is true particularly of beam or pentode output tubes. The normal class A output of a 6L6 beam tube is 6 watts but, if the output power is all rectified, only 50 mw can be drawn without excessive distortion. Half-wave rectifiers and capacitor-input filter outputs are worst in this respect, because of the current discontinuities. If the automatic gain control rectifier input is taken from a tuned amplifier, these difficulties decrease. The tuned circuit capacitor readily supplies irregular current wave forms, provided the amplifier has sufficient power output available.

Automatic volume control (AVC) is applied in receivers to either the r-f or audio stages, to maintain approximately constant volume in spite of fading or other causes of input voltage variations. It is applied in audio amplifiers to maintain better output volume with differing voice levels.

If the input grid resistor R₁ in Fig. 201 is connected to a fixed negative bias the AVC is inoperative below the value of bias voltage. This is called delayed AVC; with it, no AVC is applied until a certain output level is reached. In some receivers, more than one stage may be controlled, and the AVC action is amplified.

Circuits similar to this are used in power-line carrier receivers. The carrier frequency is 40 to 200 kc, and audio frequencies are employed for modulation. In transformer T₁ some special problems are encountered because the transformer operates over a range of 40 to 200 kc, delivers the correct amount of voltage to the automatic gain control tube 6H6-1 for proper AVC action, delivers the proper output to the audio load without distortion, and obtains these voltages from a nearly constant current source. The transformer ratio is obtained by estimating the r-f voltage swing obtained with a square primary input current wave, and dividing this by the voltage required to produce the necessary audio output after choke L₁ smooths the rectified lobes to the average value shown by the heavy dotted lines in Fig. 200. Transformer voltages and currents are calculated as in Table VII for a single-phase full-wave rectifier, but with peak audio current and voltage taking the place of d-c output.