US2023448A - Muted automatic volume control circuit - Google Patents

Description

Dec. '10,, 1935. J. 5. STARRETT I MUTED AUTOMATIC VOLUME CONTROL CIRCUIT Filed Nov 25, 1932 2 Sheets-Sheet 1 INVENTOR- JOHN '5 STARRETT M ATTORNEY- Patented Dec. 10, 1935 UNITED STATES PATENT OFFICE MUTED AUTOMATIC VOLUME CONTROL CIRCUIT Delaware Application November 25, 1932, Serial No. 644,160

18 Claims. (01. 250-20) My present invention relates to muted automatic volume control circuits for radio receivers, and more particularly to improved circuits for the accomplishment of detection, automatic volume control and automatic suppression, or muting of inter-channel noise.

There has been disclosed by D. G. Burnside in application Serial No. 644,149, filed November 25, 1932, various circuits for securing detection 1b and delayed automatic volume control functions from a single stage. It has also been shown by D. G. Burnside that receiver noises present when no carrier is tuned in could be prevented from appearing in the reproducer by a system of socalled delayed detection, in which system a detector diode anode is biased negatively by an amount just greater than the noise potential. It was pointed out that this biasing prevented the diode anode from becoming positive, and, therefore, prevented detection; consequently, the noise voltages are not able to pass the detector.

Furthermore, it has also been shown in the aforementioned Burnside docket that the three functions of detection, delayed automatic volume control and inter-channel noise suppression could be accomplished with a single tube of the duplex diode-triode type, a tube of the latter type having been disclosed by T. M. Shrader in application Serial No. 622,140, filed July 12, 1932.

While the inter-channel noise suppressor arrangement for an automatic volume control circuit shown by Burnside is entirely satisfactory for many purposes, at times, due to the different delay voltages which may be used for biasing the detector and volume control anodes, interaction of an undesirable kind between the two circuits may be experienced. If the aforedescribed detector bias is more than a small fraction of the signal voltage, it may cause distortion due to a deeply modulated carrier wave.

Accordingly, it may be stated that it is one of the main objects of the present invention to provide a method of eliminating, or suppressing, inter-channel noises in a receiver equipped with automatic volume control, and wherein the detection and automatic volume control functions are performed by a single tube, the method consisting in utilizing the said single tube, in the absence of a signal, to suppress the inter-channel noises by the application of a cut-off voltage to the grid of an audio frequency amplifier stage, such a method of noise suppression being adapted to function without distortion, and the presence or absence of a signal voltage greater than a noise voltage determining whether the bias on the grid of the first audio frequency amplifier stage is such as to repeat, or such as to reject the audio frequency voltage reaching the said stage; it being possible, in addition, to so design the time constant of the cut-off action so that no change 5 lo. the bias of the said audio stages takes place sufficiently quickly to produce distortion.

Another important object of the present in- 7 vention is to provide in a radio receiver a single stage capable of performing diode detection, de- 10 layed automatic volume control and inter-channel noise suppression functions, the noise suppression function being such that the grid of ,he first audio frequency amplifier tube is, in the absence of a signal biased to cut-off, but is 15 automatically removed from cut-off when a signal is received.

, Still another object of the invention is to provide a radio receiver equipped with a duplex diode-triode tube capable of accomplishing de- 20 tcction, automatic suppression of inter-channel noises, and delayed automatic volume control, the duplex-diode triode tube embodying at least three independent anodes with a common cathode, one of the anodes being used for detec- 25 tion, a second for automatic volume control, and

a third anode as a source of direct current potential for biasing a grid of a tube subsequent to the triple anode tube.

Another object of the present invention is to 30 provide a radio receiver wherein a single tube functions to perform detection, automatic volume control and muting, the tube including a triode section independent of at least two diode sections, the muting being accomplished by controlling 35 bias on a grid other than the signal grid of a multi-grid tube following the aforementioned multiple duty tube.

Fundamentally, the present invention contempfates the use of a duplex diode-triode tube as a 4 detector, source of automatic volume control voltages, and additionally, as a special means for suppressing circuit noises. The tube is arranged in such a manner that when zero or very small signal is present at the detector, the grid of the 45 first tube following the detector shall be biased to cut-off; that is, shall pass no plate current. The circuits contemplate that when a signal is received, provided it is above a certain minimum and predetermined voltage, the tube following the 50 detector (that is, the first audio tube) shall be removed from cut-01f, and the signal passed in the usual manner. The removal of the grid from cut-oil is arranged to be accomplished by the supply of a reversed voltage from the rectified 55 current through an impedance in series with one the sets of diode elements in the duplex diodetriode tube.

The coupling for the audio or signal voltage frim the detector to the first audio tube is made by means of a coupling condenser arranged in any suitable manner. Ordinary automatic volume control, or delayed automatic volume control, may be used, and half wave or full wave diode detection and rectification may also be employed. Again, detection and rectification may be from diiferent sets of diode elements, or may be from the same ones. The audio tube control may be of any type ordinarily suitable for audio amplification. If the audio tube is a triode, the cut-off control should ordinarily be on the control grid. However, if the audio tube is of the multi-grid type, this control may be on the signal control grid, or on the suppressor grid, if the tube is of the pentode audio output type.

Finally, it may be stated that other objects of the present invention are not only to improve generally the simplicity and efiiciency of automatic'volume control circuits for radio receivers,

butto also provide receivers of this type which not only include the aforedescribed features, but are durable and reliable 'in operation, and economically manufactured and assembled.

The novel features 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 diagrammatically shows a superheterodyne type of receiver embodying the present invention,

Fig. 2 shows a modification of the invention of Fig. 1,

Fig. 3 shows still another modification,

Fig. 4 shows an arrangement similar to the modification in Fig. 3, a modified form of noise suppression function being shown.

Referring now to the accompanying drawings wherein like reference characters designate the same circuit elements in the different figures, there is shown in Fig. 1, in schematic fashion, a superheterodyne receiver embodying the present invention. As is well known to those skilled in the art, such a receiver usually embodies an antenna circuit, a tuned radio frequency amplifier, a local oscillator, a first detector stage, an intermediate frequency amplifier, a second detector stage, and one or more audio frequency amplifier stages followed by a reproducer. A uni-control means is shown for simultaneously tuning the radio frequency amplifier, first detector and local oscillator stages, it being understood that various expedients are known for tuning the receiver throughout the broadcast range while supplying a substantially constant intermediate frequency from the output of the intermediate frequency amplifier,

The output of the intermediate frequency amplifier includes a resonant network I which is fixedly tuned to the aforesaid intermediate frequency, the resonant circuit I being magnetically coupled to the resonant input circuit 2 of the second detector, this input circuit also being fixedly tuned to the operating intermediate frequency.

The second detector stage includes a tube 55, which is of the so-called duplex diode-triode type, and is adapted to cooperate with its circuits to 5 function as a combined second detector, automatic volume control, and inter-channel noise suppressor device. The first audio frequency amplifier stage includes a tube 46 having its anode arranged for connection to an output load circuit 1 which may include additional power amplifier stages and a reproducer (not shown).

Of course, it is to be clearly understood that the present invention is not limited in any manner to a superheterodyne receiver, but may be 15 employed with equal facility in connection with a radio frequency amplifier preceding the tube 55 and an audio frequency amplifier following the latter. Hence, in Figs. 2, 3 and 4 the symbol R. F. is to be understood as designating a 20 source of radio frequency energy, whether it be of intermediate frequency, or a frequency usually employed'prior to a detector in a tuned radio frequency amplifier receiver.

Considering, now, the specific nature of the network connected between the input circuit 2 and the tube 46, it is believed sufiicient for the purposes of this application to conventionally represent the tube 55. As shown, the tube comprises a triode section including a cathode 3 (of the indirectly heated'type), a grid 4, and a main anode 5; a diode section consisting of a diode anode D1 and a portion of the cathode 3 not emitting electrons for the main anode 5; and an additional diode section consisting of the diode anode D2 also disposed adjacent the cathode 3 out of the electron stream to the main anode 5. Such a tube has been specifically described and claimed in the aforesaid Shrader application, and the construction shown therein may be em- 40 ployed in designing tube 55.

Briefly, in actual construction the tube 55 is provided with a cathode sleeve, within which is mounted the heater element, a grid and main anode being concentrically positioned about the sleeve. The pair of diode anodes D1 and D2 are placed around an end of the sleeve projecting beyond the triode grid and anode. In operation, the two diodes and the triode are independent of each other except for the common cathode sleeve which has one emission surface for the diode anodes D1 and D2, and another for the triode grid and anode.

The circuits associated with the electrodes of tube may now be considered. Fig. 1 shows 55 half-wave diode detection, and separate halfwave diode rectification for delayed automatic volume control, together with coupling from the detector diode through a condenser to the grid of the first audio stage tube. The circuit is so arranged that when no signal is applied, the plate current of the triode element is used to bias the control grid of the first audio amplifier tube to cut-off, and prevent any relay action by the latter tube. The incoming signal is applied between the cathode and the diode anode D1, and the potential developed across resistor R1 by the audio component of diode current is coupled by means of the condenser C1 to the grid 6 of the first audio tube. This grid, however, in the absence of a signal above a predetermined threshold, is biased to cut-off by means of the plate current of the tube 55 flowing through the resistor R3. When a signal is received on diode anode D2 by means of the coupling condenser C3 the voltage drop through resistor R2 is such as will bias the grid of the tube 55 to cut-off. This eliminates the potential drop across resistor R3, and removes the audio frequency tube from cut-off.

In other words, when no signal is received, the triode section of tube 55 passes plate current which is used to develop a voltage for biasing the second tube 45 to cut-off. When a signal is received the first tube biases its own triode grid element 4 to cut-off, and thus removes the cut-off bias of the second tube. By the proper choice of values for the various resistances and condensers the circuit can be made to operate in a very satisfactory manner. It will be noticed that the diode D2, together with the grid 4 of the first tube, is given a negative bias controlled by the potentiometer C which enables a suitable adjustment of operation to be made. It is, of course, desirable that the audio tube be one which exhibits a sharp cut-off of plate current with change of grid voltage.

The automatic volume control function is accomplished through the path l (designated AVC) connected between the grid circuits of the radio frequency amplifier, first detector, intermediate amplifier stages and one side of the resistor R4. The opposite side of the latter is connected to the diode anode D2. The diode rectifier including anode D2, then, functions to control the bias of the grid 4 for the muting action, and also furnishes rectified current for securing a volume control voltage by the drop across resistor R4. The direct current connections to the controlled predetectorstages are too well known to require further description. The condensers C4, C5, C6 are radio frequency by-pass condensers. The bleeder resistor provides the electrodes D1, D2, 5, 4 of tube 55 with the required potentials; the grid 6 of tube 46 is given a normal bias, relative to the cathode, in virtue of the drop across resistors R6, R3, R5. The adjustable connection 8, between the resistor R2 and the potentiometer resistor C, furnishes a means for adjusting the sensitivity of the receiver at which muting occurs.

Recapitulating the operation of the tube 55, and its associated circuits, the rectified voltage across the resistor R2 occurs when a peak signal greater than the negative bias at the potentiometric resistor C is impressed upon the signal input circuit 2. Automatic volume control action then occurs, and the grid 4 is biased to cut-off by the additional voltage across the resistor R2. The voltage drop across resistor R3 is then zero, or very small, and the audio amplifier tube 46 is working with normal bias which passes the audio signal. With no signal delivered to the diode anode D2, the grid of the tube 55 is removed from cut-0.1T, and the increased potential drop across the resistor R3 biases the tube 46 to cut-off. As pointed out heretofore the audio signal voltage is fed to the amplifier tube 46 from the path including resistor R1.

Fig. 2 shows full-wave diode detection with diode bias, and automatic volume control so arranged that when no signal is received the plate current of the triode section flowing through a resistance biases the control grid of the first audio tube to cut-off, and thus prevents noise from being passed. When a signal is received the grid of the triode section of the tube 55 is biased at, or near, cut-off, and the signal applied through the coupling condenser from the grid of the tube 55 to the control grid of the first audio tube is relayed by that tube.

More specifically Fig. 2 is similar to Fig. 1, except that there is provided full-wave diode detection and automatic volume control rectification from the same elements, and a slightly different method of connections to the first audio tube is given. As before, the rectified current is led off in the usual manner for automatic volume control, and the audio component is coupled to the succeeding tube by means of the condenser C1. When no signal is received, the grid 6 of the first audio tube 57 is biased to cut-off by means of the normal bias and the voltage due to the plate current of the tube 55 flowing through the resistance R2.

When a signal is received the direct current drop across resistor R1 biases the grid 4 to cut-off, reduces, or eliminates, the current flowing through resistor R2, and removes the grid of the audio tube from cut-off, thus passing the signal. This arrangement provides for suppression of noise in that a signal of a predetermined value is required to bring the tube 55 to cut-off, and to remove the audio tube from cut-off. Adjustment of volume may be had by means of a potentiometer C in that the signal voltage applied to the grid of the audio tube may be varied at will. The circuit shows ordinary automatic volume control; that is, without delayed action.

Briefly, then, in Fig. 2 the rectified voltage across the resistor R1 brings the grid 4 to cut-off, and removes the audio stage from cut-off, functioning in the same manner as in Fig. 1. Full wave rectification occurs between the two diode plates D1 and D2, these two diode plates being connected to opposite sides of the input circuit coil 2. The resistor R1 is connected between the grounded leg of the cathode of tube 55 and the midpoint of 'coil 2, the condenser C7 functioning as a radio frequency by-pass condenser connected between one side of resistor R1 and the lead 9 between the grid 4 and the automatic volume control path. The resistor R7 and the condenser C2 provide a filter network for the automatic volume control path.

The condenser C9 provides a radio frequency by-pass path around the resistors R2 and R8. The potentiometer at C provides a manually operable audio volume control, and it will be observed that the tube 57 is of the pentode type including a suppressor grid, disposed between the anode and screen grid, at cathode potential. In the modification shown in Fig. 2 the automatic volume control is obtained from the voltage drop across the resistor R1, and, as stated above, this resistor also provides the audio signal for the tube 51, as well as control of the muting bias.

Passing on to Fig. 3, the latter shows halfwave diode detection; separate diode rectification for delayed automatic volume control voltage; and the use of a third diode for the development of direct current for the cancellation of cutofi bias on the grid of the first audio tube when a signal is received. In the case of Fig. 3 a triple diode arrangement is shown; the grid and plate of the triode of the tube 55 are connected together to serve as a diode anode D3. Detection is secured by means of diode anode D2 which is coupled to the radio frequency input circuit 2 by means of condensers Cz and C's. The detected audio component is fed to the first audio tube by means of condenser C1 in the usual manner.

When no signal is being received the voltage supplied from across the voltage bleeder points EF biases the grid 6 of the audio tube 46 to cutoff; hence, no circuit noises are passed. An incoming signal rectified by diode anode D1, and flowing through the resistance R1, produces a counter-bias which removes the audio tube from cut-off. Automatic volume control voltage is supplied by the rectified current flowing through the diode anode D3 and the resistance R's. This may be delayed by the desired amount by means of the automatic volume control bias from across the bleeder points It will be noted that the potential of the circuit D2, R2 and R'i varies as a unit with the voltage developed across R1 so that this voltage supplies the bias for the audio tube.

Summarizing, the operation of the modification shown in Fig. 3, it will be observed that efiectively the tube 55 includes within its envelope three independent diodes, two of which diodes include the anodes D1 and D2 while the grid and plate of the tube 55 are tied together to serve as the additional one. The circuit is arranged so that with no signal to the diodes, the bias on the amplifier tube 46 is at, or beyond, cut-elf. With signal, the voltage across R1 bucks out part of the applied bias, and removes the amplifier tube from cut-off; thus, passing the signal. Detection takes place in the diode having resistance R2, while automatic volume control voltage is developed across R3 when the peak signal equals the delay bias on this diode.

The modification shown in Fig. 4 is substantially similar to that shown in Fig. 3, but with the cut-01f bias of the first audio tube applied to the suppressor grid H) of the audio tube 51. This tube 5'! is of the pentode type including a signal grid 6, a positive screen grid, and the grid i5 disposed between the screen grid and the anode. The suppressor grid 26, a grid other than the signal grid, is connected by a lead I i to the positive side of the resistor R1, and the cut-off bias for the audio tube 57 is applied to the grid id, rather than to the signal grid 6. This is arranged by means of a positive bias of approximately 45 volts, applied to the cathode from the resistance strip of the voltage supply.

In other words, the cathode of tube 5? is connected by a lead !2 to the positive side of the bleeder. The signal grid 5 of tube 5? is connected through the adjustable tap of the volume control C and lead is to a point on the bleeder which is less positive than the oint to which the lead I2 is connected. The negative side of the resistor R1 is connected by a lead id to a point on the bleeder which is less positive than the point to which the lead I3 is connected.

When a signal is impressed on the resonant input circuit 2, the audio frequency tube 57 is removed from cut-off by means of the direct current drop across the resistor R1. The audio voltage from the diode anode D2 is supplied to the grid 6 of the audio tube by means of the condenser C1. With no signal impressed on the input circuit 2, the suppressor grid it} is 45 volts negative with respect to the cathode of tube The rectified voltage developed across the resistor R1, when a signal is present, bucks the applied voltage, and brings the suppressor grid 1 E! to zero potential, and passes the signal. Otherwise, the operation of the arrangement shown in Fig. 4 is substantially the same as that described in connection with Fig. 3.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent 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 combination with a high frequency amplifier whose gain is to be controlled, and an audio frequency amplifier, a network coupling the two amplifiers, said network including a pair of independent diodes and a triode, means for coupling one of the diodes to the output of said high frequency amplifier, means for coupling the other diode to the output of said high frequency amplifier, a direct current connection between said other diode and a gain control electrode of said high frequency amplifier, a path of low impedance to audio frequency signals connected between said one diode and an input electrode of said audio frequency amplifier, a direct current connection between said other diode and the grid of said triode, and means common to the output electrode of said tr ode and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inoperative when signal energy below a predetermined intensity level is impressed upon said network.

2. In combine. with a high frequency amplifier whose gain .15 to be controlled, and an audio frequency amplifi r, a network coupling the two amplifiers, said net including a pair of independent diodes and a triode, means for coupling one of the diodes to the output of said high frequency amplifier, means for coupling the other diode to the output of said high frequency amplifier, a direct current connection between said other diode and a gain control electrode of said high frequency amplifier, a path of low impedance to audio frequency signals connected between said one diode and an input electrode of said audio frequency amplifier, a direct current connection between said other diode and the grid of said triode, and resistive means common to the output electrode of said triode and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inoperative when signal energy below a predetermined intensity level is impressed upon said network.

3. In combination with a high frequency amplifier whose gain is to be controlled, and an audio frequency amplifier, a network coupling the two amplifiers, said network including a tube provided with a pair of independent diode sections and a triode section, means for coupling one of the diode sections to the output of said high frequency amplifier, means for coupling the other diode section to the output of said high frequency amplifier, a direct current connection between said other diode section and a gain control electrode of said high frequency amplifier, a capacitative path of low impedance to audio frequency signals connected between said one diode section and an input electrode of said audio freq ncy amplifier, a direct current connection be men said other diode section and the grid of said triode section, and means common to the output electrode of said triode section and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inoperative when signal energy below a predetermined intensity level is impressed upon said network.

4. In combination with a high frequency amplifier whose gain is to be controlled, and an audio frequency amplifier, a network coupling the two amplifiers, said network including a pair of independent diodes and a triode, means for coupling one of the diodes to the output of said high frequency amplifier, means for coupling the other diode to the output of said high frequency amplifier, a direct current connection between said other diode and a gain control electrode of said high frequency amplifier, a path of low impedance to audio frequency signals connected between said one diode and an input electrode of said audio frequency amplifier, a direct current connection between said other diode and the grid of said triode, and means common to the output electrode of said triode and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inop erative when signal energy below a predetermined intensity level is impressed upon said network, and an adjustable bias means connected to said direct current connection to the high frequency amplifier.

5. In combination with a high frequency amplifier whose gain is to be controlled, and an audio frequency amplifier, a network coupling the two amplifiers, said network including a tube provided with a pair of independent diode sections and a triode section, means for coupling one of the diode sections to the output of said high frequency amplifier, means for coupling the other diode section to the output of said high frequency amplifier, a direct current connection between said other diode section and a gain control electrode of said high frequency amplifier, a path of low impedance to audio frequency signals connected between said one diode section and an input electrode of said audio frequency amplifier, a direct current connection between said other diode section and the grid of said triode section, and means common to the output electrode of said triode section and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inoperative when signal energy below a predetermined intensity level is impressed upon said network, said couplings between said diode sections and said amplifier output comprising a signal input circuit including a coil, and the cold electrodes of said diode sections being connected to opposite ends of said coil.

6. In combination with a high frequency amplifier whose gain is to be controlled, and an audio frequency amplifier, a network coupling the two amplifiers, said network including a pair of independent diodes and a triode, means for coupling one of the diodes to the output of said high frequency amplifier, means for coupling the other diode to the output of said high frequency amplifier, a direct current connection between said other diode and a gain control electrode of said high frequency amplifier, a path of low impedance to audio frequency signals connected between said one diode and an input electrode of said audio frequency amplifier, a direct current connection between said other diode and the grid of said triode, and means commonto the output electrode of said triode and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inoperative when signal energy below a predetermined intensity level is impressed upon said network, and additional means for adjustably connecting the input electrode of said audio frequency amplifier to said path to provide a manual volume control device.

'7. In combination with a high frequency amplifier whose gain is to be controlled, and an audio frequency amplifier, a network coupling the two amplifiers, said network including a tube provided with a pair of independent diode sections and a triode section, means for coupling one of the diode sections to the output of said high frequency amplifier, means for coupling the other diode section to the output of said high frequency amplifier, a direct current connection between said other diode section and a gain control electrode of said high frequency amplifier, a path of low impedance to audio frequency signals connected between said one diode section and an input electrode of said audio frequency amplifier, a direct current connection between said other diode section and the grid of said triode section, and means common to the output electrode of said triode section and said input electrode of said audio frequency amplifier for rendering the audio frequency amplifier inoperative when signal energy below a predetermined intensity level is impressed upon said network, and a resistor, common to both said diode sections, connected to the grid of said triode section, said direct current connection to the high frequency amplifier, and said path to the audio frequency amplifier input electrode.

8. In a radio receiver, a radio frequency amplifier, an audio frequency amplifier, a network including a pair of diode rectifiers, a signal input circuit coupling one of the diode rectifiers to said radio amplifier, an audio frequency connection between the signal grid of the audio amplifier and said one diode rectifier, means for impressing signals from said input circuit on the other diode rectifier, an electron discharge device, a direct current connection between the signal grid circuit of said audio amplifier and the space current path of said device, and a direct current connection between said other diode rectifier and said device for impairing the transmission efficiency of said audio amplifier when signal energy below a predetermined intensity level is impressed on said radio amplifier, and a common tube envelope housing the electrodes of said rectifiers and said discharge device.

9. In a radio receiver, a radio frequency amplifier, an audio frequency amplifier, a network including a pair of diode rectifiers, a signal input circuit coupling one of the diode rectifiers to said radio amplifier, an audio frequency connection between the signal grid of the audio amplifier and said one diode rectifier, means for impressing signals from said input circuit on the other diode rectifier, an electron discharge device, a direct current connection between the signal grid circuit of said audio amplifier and the space current path of the device, and a direct current connection between said other diode rectifier and a space current control electrode of the device for decreasing the transmission efliciency of said audio amplifier when signal energy below a predetermined intensity level is impressed on said radio amplifier, and a common tube envelope housing the electrodes of said rectifiers and said discharge device.

10. In a radio receiver, a radio frequency amplifier, an audio frequency amplifier, a network including a tube housing at least a pair of diode rectifiers, a signal input circuit coupling one of the diode rectifiers to said radio amplifier, an

audio frequency connection between the signal grid of the audio amplifier and said one diode rectifier, means for impressing signals from said input circuit on the other diode rectifier, an electron discharge device, a direct current connection between the signal grid circuit of said audio amplifier and the space current path of the device and a direct current connection between said other diode rectifier and a control element disposed in said space current path for controlling the transmission efficiency of said audio amplifier when signal energy below a predetermined intensity level is impressed on said radio amplifier, and the electrodes of said discharge device being disposed in said tube.

11. In a radio receiver, a radio frequency amplifier, an audio frequency amplifier, a network including a pair of diode rectifiers, a signal input circuit coupling one of the diode rectifiers to said radio amplifier, an audio frequency connection between the signal grid of the audio amplifier and said one diode rectifier, means for impressing signals from said input circuit on the other diode rectifier, and an electron discharge device having an output electrode connected to the signal grid circuit of said audio amplifier and an input electrode connected to the anode of said other diode rectifier for controlling the transmission efiiciency of said audio amplifier when signal energy below a predetermined intensity level is impressed on said radio amplifier, and a com mon tube envelope housing the electrodes of said discharge device and said rectifiers.

12. In a radio receiver, a radio frequency amplifier, an audio frequency amplifier, a network including a pair of diode rectifiers, a signal input circuit coupling one of the diode rectifiers to said radio amplifier, an audio frequency connection between the signal grid of the audio amplifier and said one diode rectifier, means for impressing signals from said input circuit on the other diode rectifier, a resistor in the circuit of said last diode rectifier, an automatic gain control connection to a negative point on the resistor, and an electron discharge device having an output electrode connected to the signal grid circuit of said audio amplifier and an input electrode connected to said other diode rectifier for controlling the transmission efiiciency of said audio amplifier when signal energy below a predetermined intensity level is impressed on said radio amplifier.

13. In a receiver, a multiple function tube comprising a cathode and at least three cold electrodes, a signal transmission tube preceding the first tube, means for transmitting signals from the said preceding tube to a diode circuit connected between the said cathode and a first of said cold electrodes to develop a direct current voltage from said signals, means for impressing the said voltage upon the said transmission tube to regulate its signal transmission efficiency, means for transmitting signals from said preceding tube to a second diode circuit connected between the said cathode and a second one of the cold electrodes to develop a demodulated signal voltage from said signals, a third tube following said first tube, means for impressing said demodulated voltage on the third tube, means including a circuit connected between the said cathode and the third cold electrode for normally impairing the transmission efficiency of the third tube, and means, responsive to signal variations, for controlling the space current flow between the cathode and third cold electrode.

JOHN S. STARRETT.

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