Patent GB-440337-A (GB)
Title: Improvements in and relating to thermionic valve circuits
IPC: H03G7/04
Filed: 1934-06-15
Abstract: 440,337. Automatic volume control. WHITELEY, J. W., 60, Clifford Road, Hounslow West, Middlesex. June 15, 1934, No. 17802. [Class 40 (iv)] [See also Group XL] Variations in the impedance of a thermionic tube having a negative feed-back coupling between its anode and grid-circuits are used to control the output of an amplifier in accordance with the volume of signals received. It is shown that the impedance of such a tube depends on its mutual conductance and on the nature of the feed-back coupling and in carrying out the invention the tube has its mutual conductance varied by rectified signals and is used as an element in a potential divider from which signals are passed on to the next amplifying tube. The arrangement may be used in a volume-range expander or, by suitably designing the feed-back circuit, to cause an amplifier to amplify the middle. frequencies less relatively to the extreme frequencies at low than at high signal volumes. Volume-range expanders. As shown in Fig. 2, the output circuit of the triode portion of a double diode triode 1 in the signal path includes a potential divider consisting of a fixed resistance 12 and the anode-cathode space of a variable-Á pentode 13 having a negative back coupling between its anode and grid circuits provided by a potential divider 29, 30, 31 and coupling condenser 32. Signal voltages developed across the pentode are passed on by a transformer 27. Condenser 33 may be provided in the back-coupling circuit to compensate for the shunting effect as regards high frequencies of transformer 27. Signal voltages, rectified by the diode portion of the tube 1, control the grid voltage of the pentode so that as the signal level increases the impedance of 13 and therefore the proportion of signal passed on increases also. Values are given for the current limiting resistances 17, 18, load resistance 19 and condenser 23 in the diode circuit to ensure that the grid voltage of the pentode shall not be varied by very rapid changes of levels. In the example, the time constant for charging 23 is À1 second and for discharging, 1 second. In a modification, Fig. 3 (not shown), the signals for controlling the pentode are amplified before rectification by a tube which itself is subjected to automatic volume control. The expansion introduced by the whole arrangement is thus spread over a wider range of intensities. A similar result is produced by the arrangement shown in Fig. 4 in which the resistance 12 of the potential divider is included in the circuit of the controlling rectifier. In further arrangements the variable impedance tube is a double diode triode. As shown in Fig. 6, the control voltage is derived by full wave rectification of a current passed on by a transformer 44 from the potential divider resistance 12. In a similar arrangement, Fig. 5 (not shown), the transformer 44 is omitted and the diode portion acts as a single wave rectifier. In the above described arrangements the change in current flow through the variable impedance tube due to a sudden change in the control bias may cause a false signal in the amplifier output. This is avoided by the push-pull arrangement shown in Fig. 7. Tone controlling arrangements and means for masking background noise variations. The arrangements described above may be modified for automatic tone control by arranging that more feed-back occurs at the middle than at the extreme frequencies, e.g. by including a broadly tuned resonant circuit in the feed back path. In the arrangement shown in Fig. 9, the effect of background noise at low volumes is minimized by the provision of an additional variable impedance 161. The tube 16 acts in the manner previously described as a volume-range expander at the middle frequencies whereas the feed back circuit of the tube 16<1> is arranged to be most effective at the higher frequencies: Consequently any diminution in the main signal level causes a greater diminution in the high frequency and middle frequency levels but since the time constant for the discharge of 231 is smaller than that for the discharge of 23 the reduction in the high frequencies and therefore the noise takes place before the general level is much reduced. On the other hand the time constant for charging 23 is less than that for charging 23<1> so that when the general level rises there is a delay before the rise of the high frequencies and therefore of the noise. In a further modification designed to keep background noise constant in spite of the expansion in volume variations of the signals, the signal voltages are derived from a potential divider including an inductance L, Fig. 8, in series with the variable impedance tube. At high levels the impedance of L is negligible at all frequencies. At low levels however the impedance of L at high frequencies becomes considerable and accordingly the amplification of high frequencies and noise remains substantially constant after a given low level is reached whereas that of low and middle frequencies is progressively reduced as the level falls.