US2144920A - Automatic volume control circuits - Google Patents

Description

. Jan. 24,

NETWORK 1939. G, HQLST 2,144,920

AUTOMATIC" VOLUME CONTROL CIRCUITS Filed Sept.v 20, 1935 5 SheetsSheet 1 E Q E W i 7'0 701%? SUPPLY, NETWORK SIG/Ml SOUR INVENTOR POUL F. 6. HOLST SIG/I41 SOURCE ATTORNEY Jan. 24, 1939. P. F, G. HOLST AUTOMATIC VOLUME CONTROL CIRCUITS Filed Sept. 20, 1935 3 Sheets-Sheet 2 m R0 w mi. W A 6 R NE: W w W Q R gwk h I I; X? R w\ mw W & Ul IWIY M W 4 1 x w K 8 k$ mx QR M P W HIM BY ATTORNEY Jan. 24, 1939. P. F. G. HO LST 2,144,920 AUTOMATIC VOLUME CONTROL CIRCUITS Filed Sept. 20, 1935 3 SheetsSheet 3 70 .4. E NET/708K INVENTOR POUL F. 6. HOLST MWA/ ATTORNEY Patented Jan. 24, 1939 2,144,920 AUTOMATIC VOLUME CONTROL CIRCUITS Poul F. G. Holst, Oaklyn,

Corporation of America,

aware N. J., assignor to Radio a corporation of Del- Application September 20, 1935, Serial No. 41,359

11 Claims.

My present invention relates to gain control networks for radio signaling systems, and more particularly to automatic volume control arrangements for radio receivers.

5 One of the main objects of my present invention is to provide an automatic volume control arrangement for a radio receiver of the type utilizing a diode demodulator as a gain control tube, and wherein the anode and cathode of the 12 demodulator may be maintained at a normal positive potential with respect to the cathodes of the tubes to be controlled, and an auxiliary negative potential source being utilized to provide the normal minimum biasing voltage for the signal coni; trol grids of the controlled tubes.

Another important object of the invention is to provide an automatic volume control arrangement for a radio receiver of the type utilizing battery sources for energization of the receiver tube electrodes, and the volume control arrangement employing the diode demodulator of the receiver as the gain control tube, where the diode anode and cathode may be maintained at a normal positive potential with respect to the filaments or cathodes of the controlled transmission tubes and the diode filament, and where a batteryis utilized as a source of biasing potential to provide a normal negative minimum grid potential for the controlled transmission tubes.

Another object of the invention is to provide an automatic gain control arrangement for a battery operated radio receiver which employs a diode as a detector, wherein the anode of the diode may be prevented from assuming a normal potential with respect to the filaments of the controlled amplifiers which is negative, and a negative voltage source being connected in the automatic gain control connection to the signal control grids of the controlled amplifiers for providing the normal negative grid bias for the controlled ampli- .fiers.

Still other objects of the invention are to improve generally the simplicity and efliciency of automatic volume control networks for radio receivers, and more especially toprovide automatic volumecontrol arrangements which are not-only reliable in operation, but are economically manufactured and assembledin radio receivers whether of the battery or alternating current operated type.

The novelfeatures which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 is a circuit diagram of a battery operated receiver embodying the present invention,

Fig.2 is a modification plied to an A. C. operated Fig. 3 shows still anothe vention,

of the invention, as apreceiver,

r embodiment of the in- Fig. 4 shows an alternative form of the diode demedulator network of a receiver embodying the present invention,

Fig. 5 shows a modification of the arrangement of Fig. 4,

Fig. 6 shows another variation of the arrangement shown in Fig. 4,

Fig. '7 shows a receiver of the alternating current type embodying the present invention.

Referring now to the accompanying drawings,

wherein like reference figures correspond to si characters in the different milar circuit elements,

or more, stages of and is followed by work.

signal frequency amplification, the usual audio frequency net- Let is be assumed that the receiver shown in Fig. 1 is of the superheterodyne-type; in that case the demodulator is demodulator comprises a the second detector, and the diode of the indirectly heated cathode type. The diode I has its anode connected to its cathode ing the signal input circuit with the diode load resistor through a path includ- 2 arranged in series 3. The circuit 2 is resonated to the operating intermediate frequen- Y. condenser 4 is connected sistor 3. The heater, or

and the usual intermediate frequency by-pass in shunt with load refilament, 5 of the cathode I has its terminals connected across a heating battery A, the negative terminal of which may be grounded. The cathode of diode l is maintained at a predetermined positive potential with respect to ground by inserting a battery 6 between ground and the cathode lead. Thus, when signals are not impressed ode and cathode of tube 1 positivepotential with res on the circuit 2, the anare at a predetermined pect tothe heater 5.

The stages preceding circuit 2 are of the intermediate frequency amplifier type; the tube 1 has its signal grid and cathode connected to a signal input circuit 8, and its tuned plate circuit 9 is coupled to the tuned input circuit II) of the following amplifier I I. The tuned plate circuit I2 of amplifier II is coupled to the demodulator input circuit 2. It will be understood that each of circuits 8, 9, I0 and I2 is resonated to the operating intermediate frequency. The heater elements, or filaments, of tubes 1 and II are connected in shunt with the heater 5 of diode I across the heating battery A. The battery source for energizing the plates and screen grids of tubes 1 and II is not shown in order to preserve simplicity of disclosure, but those skilled in the art will fully appreciate that the usual battery sources are employed in the type of receiver shown in Fig. 1. The numeral I3 designates the intermediate frequency signal source feeding the input circuit 8. This signal source I3 may comprise the customarysignal collector followed by one, or more, stages of radio frequency amplification, and a frequency changer network. This frequency changer network may embody the usual first detector and local oscillator networks. For example, the networks preceding amplifier 1 may be of the type shown in Fig. 1 of my co-pending application, Serial No. 29,014, filed June 29, 1935, patented May 17, 1938, as U. S. Patent 2,117,664.

The audio component of demodulated intermediate frequency energy is impressed upon a subsequent audio frequency network through an adjustable connection including condenser I4, and the adjustable tap I5 may be adjusted to any point on resistor 3 to vary the audio input to the audio network. The latter may comprise one, or more, stages of audio frequency amplification followed by a reproducer. The direct current component of demodulated energy is em ployed for automatic gain control of one, or more, signal transmission tubes preceding the demodulator I.

The automatic volume control arrangement (denoted by the symbols AVC) comprises the lead I6 connected between the anode side of resistor 3 and the low alternating potential sides of the input circuits Ill and 8. The AVC connections to the input circuits 8 and I9 include filter resistors I1 and I1, and the filter resistor I8 is connected in the AVG connection to the anode side of load resistor 3. In order to provide the normal negative minimum grid bias for tubes 1 and II, a negative voltage source I9 is connected in the AVG lead. Thus, when no signals are impressed on the receiver, the effective operating negative grid bias for each of tubes 1 and I I is the sum of the negative and positive voltages of sources I9 and 6.

Of course, the AVG connection may be extended to any of the other tubes preceding the I. F. amplifiers, and, as shown in my aforesaid patent, the signal amplifiers preceding the frequency changer network may be regulated in gain, and the first detector may have its gain regulated. When signals are received, the eifective negative bias voltage on the controlled tubes is increased due to the connection of lead I6 to the anode side of resistor 3 In other words, as the amplitude of signals impressedon circuit 2 increases, the variable negative bias transmitted through lead IE to the signal grids of the controlled transmission tubes increases, and the gain of the controlled tubes decreases. This maintains the signal input to the demodulator I substantially-uniform in spite of wide variation of signal amplitude at the signal collector of the receiver.

In the case of the receiver being of the tuned radio frequency type, it will be understood that the stages preceding the demodulator will have their input circuits each tuned to the carrier frequency being received, and that the tunable input circuits will then be uni-controlled by the usual gang condenser construction. In any event the automatic volume control arrangement will be such that the diode demodulator of the receiving system is employed as the gain control tube, and the diode anode will be at a predetermined potential which may be positive with respect to the cathodes of the controlled transmission tubes. However, in so far as the signal grids of the controlled tubes are concerned, they are increased in negative bias as the signal amplitude increases. This is accomplished in a simple and effective manner by employing as the demodulator a diode having an indirectly heated cathode, and maintaining the cathode of the diode positive with respect to the interconnected tube filaments of the system; and utilizing an auxiliary negative grid biasing battery to provide the normal negative minimum grid bias for the controlled tubes.

In Fig. 2 there is shown a modification of the arrangement of Fig. 1 wherein the auxiliary normal negative grid biasing voltages for the controlled tubes are different, but supplied by common battery I9. It will be seen that the circuit elements of the receiving system are not shown substantially the same as those shown in Fig. 1. That is to say, the diode I has its cathode ele vated to a predetermined potential, which may be positive, above ground by the battery 6, and the AVG lead I6 includes the auxiliary negative biasing battery I9. An additional amplifier 29 is shown preceding the amplifiers I and I I.

The signal grid of amplifier 29 is connected to the negative terminal of the source I9; the signal grid of amplifier 1 is connected to a point of less negative potential on the source I9; and the grid of tube II is connected to a point on source I9 which is still less negative in magnitude.

The tubes 20, 1, II, are shown, by way of example, as of the indirectly heated type, and can be used in the A. C. operated set. The heater of diode I and the cathode of tubes 20, 1 and II are shown at ground potential. It will be seen that the normal negative grid bias for each of the controlled tubes difiers, and in each case is equal to the sum of the positive voltage from source 6 and the particular value of negative voltage supplied from the source I9. When the AVG action commences, due to the reception of signals, there will be an increase in the negative bias supplied to the signal grids of each of the controlled amplifiers, but the magnitude of these biases will decrease in the order of the arrangement of the tubes looking towards the demodulator I.

In Fig. 3 is shown a still more generalized embodiment of the invention wherein each of the signal grids of amplifiers 29, I and II not only has a different normal minimum negative bias, but the AVG bias differs. Thus, the anode side of the diode load resistor 3 is connected by lead 2| I to the signal grid of amplifier II through a path which includes the filter resistor 22, the auxiliary negative grid biasing battery 23, and the filter resistor 24. The signal grid of the next preceding amplifier 1 is connected to a point on load resistor 3 which is closer to the cathode of tube I. The lead 25 includes the filter resistor 26, the auxiliary grid biasing battery 21, and the filter resistor 28. The signal grid of the next preceding amplifier 20 is connected to a point on load resistor 3 through a connection 30, and this latter point on resistor 3 is still closer to the cathode side of resistor 3. The connection 30 includes the filter resistor 3|, the auxiliary grid biasing battery 32, and the filter resistor 33.

In order to preserve simplicity of disclosure, the couplings between the amplifiers 20, I and I I are not shown, and the common heater circuit is also omitted. It is. believed that those skilled in the art will readily understand the manner in which the normal negative signal grid bias on each of the controlled amplifiers is calculated. Thus, the negative grid bias for tube 20 will be equal to the sum of the voltage of source 6 and the negative voltage of source 32; in the case of tube I the normal grid bias will be equal to the sum of the negative voltage from battery 21 and the voltage from source 6; and in the case of tube II it will be equal to the sum of the voltages from source 6 and battery 23. When signals are received and the AVG bias is generated, the effective negative grid bias in the case of each of the controlled tubes will depend upon the point of its connection on resistor 3. For example, the signal grid of tube I will have impressed on it the voltage drop across resistor 3; in the case of tube 1 there will be impressed on the signal grid the voltage drop between the cathode side of resistor 3 and the point 46; and in the case of tube 20 the gain control voltage will be equal to the voltage drop developed across that portion of resistor 3 which is between point 4| and the cathode side of the resistor.

In each of Figs. 1, 2 and 3, it will be observed that the battery 6 may be used to raise the diode anode at a normal positive potential with respect to the cathodes of the controlled tubes. But in cases where it is not desired to have this voltage relation the battery may be omitted. In Fig. 4 is shown a modification of the demodulator network wherein a positive voltage for the diode anode is obtained by connecting the diode cathode to a suitable point on the supply voltage bleeder resistor. Thus, the numeral 42 denotes a bleeder resistor one side of which is. grounded, and the other side of which is connected to a point of positive potential of the power supply network. The anode of the diode is connected to the cathode through the load resistor 3, and the oathode of the diode is connected to an intermediate point on the bleeder resistor. The symbol e2 denotes the positive potential which is employed for raising the diode anode and cathode above ground with respect to the heater elements of the controlled tubes. A by-pass condenser 43 is connected between ground and the cathode lead of tube L It will be understood that the remaining portions of the circuit elements are the same as those shown in Fig. l. The bleeder resistor may be connected across any desired source of D. C. power. This may be the filter network of the rectifier output of an alternating current sup-ply. As is the case of Fig. 1, the auxiliary biasing battery is disposed in the AVG lead.

In Fig. is shown a modification of the arrangement in Fig. 4 wherein the positive voltage e2 is obtained by the voltage drop across an impedance in the space current path of an electron discharge device. For example, tube 44 is of the multi-function type wherein there is included Within the tube envelope an indirectly heated cathode, a diode anode 45, a grid 46 and a plate 41. The diode anode 45 is disposed adjacent a portion of the cathode, and is outside the electron stream flowing through the grid 46 to the plate 41. The diode load resistor 3 develops the direct current and audio frequency components of the signal currents. An impedance 48, in particular a resistor, is disposed in the grounded cathode circuit of tube 44, the resistor being suitably by-passed by a condenser 49. The voltage drop 62 across resistor 48 is utilized as the positive voltage for raising the diode anode 45 above ground by the predetermined positive voltage. Of course, in this modification the controlled amplifiers have their cathodes at ground potential, and as in the case of Fig. l, the auxiliary negative biasing battery is disposed in the AVG lead. In this modification, the audio component is derived from grounded grid leak 54. The tap 53 from grid 46 acts as a manual volume control. The ungrounded side of leak 54 is connected to the anode side of resistor 3 through an audio by-pass condenser. Thus, the voltage e2 is used here to bias the grid 46 to a proper negative value.

In Fig. 6 is shown still another modification wherein the demodulator diode 50 is disposed outside the electron discharge tube envelope 5|. The load resistor 3 again supplies the gain control voltage and the audio component. The control grid 52 of tube 5| is adjustably connected, by adjustable tap 53', to the grounded resistor 54', the ungrounded side of which is connected to the anode side of resistor 3 through condenser 55. The resistor 56, disposed in the cathode circuit of tube 5|, has one side thereof grounded,

and its other side is connected to the cathode side of resistor 3. A suitable by-pass condenser 51 is connected in shunt with resistor 56. The voltage drop across resistor 56 is employed for two functions. In the first place it acts to raise the anode of demodulator 56 above ground in the absence of signals; and in the second place it provides the normal negative grid bias for the control grid 52 of audio amplifier 5|.

In Fig. '7 is shown a modification of this invention as applied to the receiving system shown in Fig. 1 of my aforesaid patent. This receiving system is of the alternating current operated type, and it is suflicient for the purposes of the present disclosure only to show a portion of the bleeder section of the power supply network. Thus, there is shown a series connection of two resistors 60 and 6|, one end of resistor 6| being grounded. One side of resistor 66 is connected to the positive side of the power supply network, while the grounded side of resistor 6| is connected to the negative side of the power supply network. The receiving system is of the superheterodyne type, and there is only shown one of the intermediate frequency amplifier tubes 63, and the diode tube 64. This tube 64 includes a diode which acts as a control device for the signal grid biasing network of amplifier 63.

The signal input circuit 65 of the demodulator diode is connected between the anode 66 and the cathode 61, a resistor 63 being connected between the low alternating potential side of input circuit 65 and the cathode 61. The signal input circuit of tube 63 has its low alternating potential side connected to an intermediate point on load resistor 68 through a path which includes the filter resistor the auxiliary negative biasing battery 12 and the filter resistor 13. A lead 83 connects the cathode 61 to the junction of resistors 6| and 60. The audio frequency component of demodulated signal current is transmitted through condenser 90 to the subsequent of the demodulator, a load resistor connected audio frequency network. The numeral l 00 designates the intervening transmission network between amplifier 63 and the diode device. It will now be seen that in Fig. 7 the -4.5 volts auxiliary source 12 performs the function of the normal biasing diode of Fig. l of my patent.

While I have indicated and described several systems for carrying my invention into efiect, it will be clear to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a receiving circuit, an electron discharge tube disposed in a signal transmission network, said tube including at least a cathode, signal grid and output electrode, a demodulator device including at least an electron emission element and a cold electrode, a load impedance connected between said cold electrode and emission element, a transmission network coupling the output electrode of the first tube and said cold electrode and emission element, voltage means for maintaining said cold electrode and emission element at a potential which is positive with respect to the cathode of said signal transmission tube in the absence of signals, said voltage means being connected between said cathode and the junction of said load impedance and said emission element, a gain control connection between the signal control grid of said ransmission tube and a point on said load impedance which is negative, when signals are received, with respect to the emission element side of the impedance, and a voltage source disposed in said gain control connection in polarity opposition to said maintaining means for normally maintaining the signal grid of the transmission tube negative with respect to the cathode thereof during the absence of signals thereby to establish the maximum gain of said tube.

2. In combination with a signal amplifier, said amplifier including an electron discharge tube provided with a cathode, control grid and plate, a diode demodulator including a signal input circuit connected between the anode and cathode thereof, a heater element for the cathode of the demodulator, a load resistor connected between the demodulator anode and cathode electrodes, means for establishing the cathode of the amplifier and the demodulator heater element at a common fixed potential, voltage means for establishing the demodulator cathode and anode at a potential which is positive with respect to said fixed potential in the absence of signals, said voltage means being connected between said amplifier cathode and the junction of said resistor and diode cathode, and a gain control connection between the anode side of said load resistor and the signal grid of the amplifier, said gain control connection including a negative voltage source for maintaining the amplifier grid at a normal negative potential with respect to the amplifier cathode in the absence of signals thereby to establish the maximum gain of said amplifier tube.

3. In combination with a signal amplifier, said amplifier including an electron discharge tube provided with a cathode, control grid and plate,

. a diode demodulator including a signal input circuit connected between the anode and cathode thereof, a heater element for the cathode between the demodulator anode and cathode electrodes, means for establishing the cathode of the amplifier and the demodulator heater element at a common fixed potential, means for preventing the demodulator cathode from assuming a potential which is negative with respect to said fixed potential in the absence of signals, and a gain control connection between the anode side of said load resistor and the signal grid of the amplifier, said gain control connection including a negative voltage source for maintaining the amplifier grid at a normal negative potential with respect to the amplifier cathode in the absence of signals thereby to establish the maximum gain of said amplifier tube, and said preventing means comprising a voltage source connected in polarity opposition to said negative source to maintain the demodulator cathode and anode positive with respect to the said heater element and said positive voltage source being connected between said amplifier cathode and the junction of said resistor and diode cathode.

4. In combination with a signal amplifier, said amplifier including an electron discharge tube provided with a cathode, control grid and plate, a diode demodulator including a signal input circuit connected between the anode and cathode thereof, a heater element for the cathode of the demodulator, a load resistor connected between the demodulator electrodes, means for establishing the cathode of the amplifier and the demodulator heater element at a common fixed potential, means for maintaining the demodulator cathode and anode at a desired positive potential with respect to said fixed potential, and a gain control connection between the anode side of said load resistor and the signal grid of the amplifier, said gain control connection including a negative voltage source connected in polarity opposition to the main- -taining means for maintaining the amplifier grid at a normal negative potential with respect to the amplifier cathode, at least one additional amplifier disposed between said demodulator and the first amplifier, and a direct current connection between the signal grid of the additional amplifier and a point on said voltage source which is positive with respect to the point thereon to which the signal grid of the first amplifier is connected.

5. In combination with a signal amplifier, said amplifier including an electron discharge tube provided with a cathode, control grid and plate, a diode demodulator including a signal input circuit connected between the anode and cathode thereof, a heater element for the cathode of the demodulator, a load resistor connected between the demodulator electrodes, means for establishing the cathode of the amplifier and the demodulator heater element at a common fixed potential, means for maintaining the demodulator cathode and anode at a positive potential with respect to said fixed potential, and a gain control connection between the anode side of said load resistor and the signal grid of the amplifier, said gain control connection including a negative voltage source in polarity opposition to said maintaining means for maintaining the amplifier grid at a normal negative potential with respect to the amplifier cathode, an additional amplifier disposed between the first amplifier and the demodulator, a direct current connection between the signal grid of the additional amplifier and a point on said load resistor which is at a different potential when signals are received than the point to which the first amplifier is connected, and said second connection including a source of negative biasing voltage for the signal grid of the additional amplifier.

6. In an automatic volume control arrangement for a radio receiver, which receiver is of the type including a diode demodulator, a signal transmission tube feeding the demodulator, and an audio amplifier, means for maintaining the diode anode and cathode at a positive potential with respect to a predetermined fixed direct current potential point, means for maintaining the cathode of the signal transmission tube at the potential of said fixed point, additional means connected in series with said maintaining means between said fixed potential point and the signal grid of said transmission tube and in polarity opposition to the maintaining means for normally maintaining the transmission tube signal grid negative with respect to the cathode thereof in the absence of signals, and said first means additionally providing the audio amplifier grid bias.

'7. In an automatic volume control arrangement for a radio receiver, which receiver is of the type including a diode demodulator, a signal transmission tube feeding the demodulator, and an audio amplifier, means for maintaining the diode anode and cathode at a positive potential with respect to a predetermined fixed direct current potential point, means for maintaining the cathode of the signal transmission tube at the potential of said fixed point, additional means connected between said fixed potential point and the signal grid of said transmission tube for normally maintaining the transmission tube signal grid negative with respect to the cathode thereof in the absence of signals, and said first means additionally providing the audio amplifier grid bias, said demodulator and audio amplifier electrodes being disposed in a common tube envelope.

8. In an automatic volume control arrangement for a radio receiver, which receiver is of the type including a diode demodulator, a signal transmission tube feeding the demodulator, and an audio amplifier, means for maintaining the diode anode and cathode at a positive potential with respect to a predetermined fixed direct current potential point, means for maintaining the cathode of the signal transmission tube at the potential of said fixed point, additional means connected in series with said maintaining means between said fixed potential point and the signal grid of said transmission tube for normally maintaining the transmission tube signal grid nega-' tive with respect to the cathode thereof in the absence of signals, and said first means additionally providing the audio amplifier grid bias, said fixed potential point being ground, and said positive potential means being dependent upon the space current of the audio amplifier.

9. In combination with a source of electrical waves, a wave transmission tube coupled to the source and being provided with at least a cathode, output electrode and a gain control electrode, a wave rectifier tube including at least a cathode and a cold electrode, means for impressing waves upon the rectifier, an impedance connected between the rectifier cathode and cold electrode for developing thereacross a uni-directional voltage dependent in amplitude upon the wave amplitude, a voltage source establishing said rectifier cathode and cold electrode at a positive voltage with respect to a fixed potential, means for establishing the transmission tube cathode at said fixed potential, said voltage source being connected between said transmission tube cathode and the junction of said impedance and rectifier cathode, a direct current voltage connection, acting as a gain control circuit, between the said gain control electrode and a point on the impedance such that the uni-directional voltage is applied to the gain control electrode in a negative sense, and a voltage source being included in said gain control connection and providing a negative voltage in polarity opposition to the positive voltage thereby to provide a minimum negative potential for the said gain control electrode in the absence of waves and establishing the maximum gain of said transmission tube.

10. In an automatic volume control arrangement for a radio receiver, which receiver is of the type including a diode demodulator, a signal transmission tube feeding the demodulator, and an audio amplifier, means for maintaining the diode anode and cathode at a positive potential with respect to a predetermined fixed direct current potential point, means for maintaining the cathode of the signal transmission tube at the potential of said fixed point, additional means connected in series with said maintaining means between said fixed potential point and the signal grid of said transmission tube and in polarity opposition to the maintaining means for normal- 1y maintaining the transmission tube signal grid negative with respect to the cathode thereof in the absence of signals, and said first means additionally providing the audio amplifier grid bias, and a gain control connection between a gain control electrode of the transmission tube and said diode.

11. In combination with a source of electrical waves, a wave transmission tube coupled to the source and being provided with at least a cathode, output electrode and a gain control electrode, a wave rectifier tube including at least a cathode and a cold electrode, means for impressing waves upon the rectifier, an impedance connected between the rectifier cathode and cold electrode for developing thereacross a uni-directional voltage dependent in amplitude upon the wave amplitude, a voltage source establishing said rectifier cathode and cold electrode at a positive voltage with respect to a fixed potential, means for establishing the transmission tube cathode at said fixed potential, said voltage source being connected between said transmission tube cathode and the junction of said impedance and rectifier cathode, a direct current voltage connection, acting as a gain control circult, between the said gain control electrode and a point on the impedance such that the uni-directional voltage is applied to the gain control electrode in a negative sense, a voltage source being included in said gain control connection and providing a negative voltage in polarity opposition to the positive voltage thereby to provide a minimum negative potential for the said gain control electrode in the absence of waves, an electron discharge device having input electrodes coupled to points of different alternating potential on said impedance, and an impedance in the space current path of said device to provide said first voltage source. 7

' POUL F. G. I-IOLS'I.

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