US2039666A

US2039666A - Volume control for reflex circuits

Info

Publication number: US2039666A
Application number: US673389A
Authority: US
Inventors: Otto H Schade
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1933-05-29
Filing date: 1933-05-29
Publication date: 1936-05-05
Anticipated expiration: 1953-05-05

Description

Maj 5, 1936. c. H. SCHADE VOLUME CONTROL FOR REFLEX CIRCUITS Filed May 29, 1935 INVENTOR .msi

OTTO H. SCHADE BY/LQZ- ATTORNEY Patented May 5, 1936 UNITED STATES PATENT OFFICE Otto H. Schade, West Orange, N. J., assgnor to Radio Corporation of America, a corporation of Delaware Application May 29, 1933, Serial No. 673,389

14 Claims.

My present invention relates to radio receivers and volume controls therefor, and more particularly to improved volume control circuits adapted for use in connection with radio receivers of 5 the reflex type.

The use of a reliex circuit is of considerable advantage in radio receivers which, due to interests of economy and compactness, employ a limited number of tubes. The fundamental requirement for a reflex amplifier is zero, or negligible, rectification for high frequency and audio frequency signals in order to prevent mixing of the two signals. Correct values of operating voltages and high frequency and audio loads will minimize distortion and insure good operation of the reflex amplifier at normal signal voltages. Abnormal signal voltage conditions must be prevented to operate efliciently with a reflex ampli- 9 fier; and, this is done by correct control of signal voltages in the circuit.

There has been described by R. M. Smith in application Serial No. 678,410, filed June 30th, 1933, a radio receiver utilizing superheterodyne reception and employing an intermediate frequency amplifier tube which also functions as an audio frequency amplifier. This tube additionally includes within its envelope two separate diode rectiers, one of the diodes being used to 30 demodulate the high frequency (i. f.) signal, and the other diode providing a direct current voltage for automatic volume control purposes. In this last named application R. M. Smith has refie'xed the rectied output of the demodulation diode to the signal input grid of the amplier portion of the same tube, thereby causing the said amplifier portion to function as an amplifier of the demodulated signal.

No particular difficulty in positioning, or placing, the manual volume control instrumentality is encountered in receivers of the aforementioned Smith type which do not employ automatic high frequency amplification control. A more diiiicult problem arises, however, in a receiver of the aforementioned type which utilizes one of the diode rectifiers within the reflex amplifier tube for automatic volume control. In this case it is not enough to merely control the transmission between the audio signal diode and the grid circuit of the reex tube, thus varying the audio input signal of the reflex amplifier; since, it is possible only with a perfectly linear tube to obtain zero volume at zero audio input. As actual tubes have a certain amount of distortion, slight demodulation of the intermediate fre-l quency signal takes place in the reiiex tube, and causes a residual signal on the audio load in the plate circuit of the reflex tube.

Further, where automatic volume control is obtained by means of a diode rectifier within the same envelope as the reiiex amplifier electrodes, the intermediate frequency input to the refiex tube and the intermediate frequency gain for automatic volume control purposes should not be controlled, or reduced, by the manual volume control. Depending on the control characteristics of the preceding amplifier stages, the intermediate frequency input voltage on the reiiex tube departs more or less from a definite value for average received broadcast signals. A manual volume control in the plate circuit of the reflex tube, and thus in the grid circuit of the following tube, controlling the amount of audio voltage delivered to the grid of the following tube has several disadvantages. The reflex tube operates at maximum audio output at all times with consequent maximum distortion.

The combined grid voltage swing on the reflex tube is the sum of the audio and intermediate frequency peak grid voltages. It is necessary to limit the intermediate frequency input into the reflex tube to such a value that one hundred per cent modulation will not produce an audio peak voltage from the diode which will exceed the grid current point of the reflex tube if added to the peak intermediate frequency grid signal. A further limitation involves the fact that this particular value of audio grid swing should not cause excessivedistortion of the audio voltage in the plate circuit of the reflex tube. These requirements are very difficult to fulfill in receivers with low B- supply voltage and relatively poor automatic vol-- ume control characteristics. The disadvantages of both systems can be avoided with a manual volume control which not only regulates the audio signal on the grid of the reflex tube, but additionally, and simultaneously, regulates the audio voltage delivered to the grid of the following tube.

Accordingly, it may be stated that it is one of the prime objects of my present invention to provide in a radio receiver of the type including a reiiex amplifier for intermediatel frequency and audio frequency voltages, which receiver also includes an automatic volume control arrangement deriving its control voltage from the plate circuit of the reex amplier, a manual volume control adapted and constructed to simultaneously vary the intermediate frequency input to rIS) the receiver demodulator and the audio frequency signal input to the signal grid of the audio amplifier following the reflex amplifier tube.

Another importantobject of the present invention is to provide in a superheterodyne receiver a simple type of manual volume control device, the latter being positioned and connected to regulate the intermediate frequency signal input to a diode rectifier as well asthe audioV signal input to an audio amplifier, the intermediate frequency amplifier providing the signal input to the diode rectifier functioning to amplify the rectied output of the diode rectifier.

Still another object of the present invention is to provide an improved manual volume control for a radio receiver employing a limited number of tubes, and at least one of the tubes including within its envelope an amplifier section and a pair of independent diodes, one of s aid diodes functioning to rectify the high frequency energy in the anode circuit of the amplifier section for automatic volume control purposes, and the other diode being coupled through a resonant circuit to the said anode circuit for the demodulation of the high frequency energy, the amplifier section functioning to simultaneously amplify high frequency energy and the demodulated output of ,said demodulation diode, and at least another of the receiver tubes functioning as an additional amplifier for the demodulated energy, the aforementioned manual volume control consisting of a single resistor, which is variable, disposed in Vsuch relation with respect to said resonant circuit that variationl of said resistor results in a regulation of the transfer of high frequency energy from the anode circuit of saidamplier section to the demodulation diode, variation of said resistor additionally resulting in a control of the transfer of amplified demodulated energy from the anode circuit of said amplifier section to the signal grid circuit of the following amplier of demodulated energy.

Still other objects of the present invention are to improve generally the efiiciency of volume control circuits used in reflex receivers, and to particularly provide a manual volume control device for such a receiver which is not only efficient and reliable in operation, but simply constructed, and economically manufactured and assembled in the radio receiver. 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 toboth its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. Y

In the drawing,

Fig. 1 diagrammatically shows a receiver embodying the present lnvention,

Fig. 2 shows a modified form of the reflex portion of the receivercircuit.

Referring now to the accompanying drawing wherein like reference characters in the different figures represent similar circuit elements, there is shown in Fig. 1 a receiving circuit employing a reflex circuit in the intermediate frequency amplifier stage. The general, and even certain specific, features of the circuit shown in Fig. 1 are described, and claimed, by R. M. Smith in the application referred to above, and for this reason brief reference will be made to circuit fea tures which are not directly involved in the present invention. Thus, there has been shown'in the aforementioned application theI specific circuit details of the oscillator-converter network I. This network is the source of the intermediate frequency signal voltage, which intermediate frequency may be 175 k. c., and includes a 6A7 type tube, also designated as a pentagrid oscillator converter tube. 'Ihis type of tube is especially suited for receivers of the automobile type, and is designed primarily to perform simultaneously the functions of a mixer (first detector) tube and 'of an oscillator tube. in superheterodyne circuits.

Such a pentagrid converter tube, and its specific circuits has been described and claimed by J. C. Smith in application Serial No. 654,421, filed January 31, 1933. As shown schematically in Fig. 1 the converter network I has thev tube thereof provided with ve grids. The tunable signal input circuit 2, which is coupled as at M to the usual signal pick-up circuit 3, is arranged for connection between the signal input grid 4' and the cathode circuit of the tube, the cathode circuit being omitted for simplicity of disclosure. It is sufficient for the purposes of the present application to point out that the cathode 5 and the first two grids may be regarded theoretically as a composite cathode which supplies a modulated electron stream, the tunable local oscillator circuit being disposed between the cathode 5 and the first grid 6, this tunable oscillator circuit having been omitted to preserve simplicity of dis closure.

The control grid 4, placed between the composite. cathode and the plate 1, provides for the introduction of the radio frequency signal, while the additional grids 8 are disposed on either side of the control grid 4 to shield the control grid electrostatically from the other electrodes and increase the output impedance of the tube, a desirable characteristic from a gain standpoint.

The intermediate frequency resonant circuit 9 is connected to the anode 1, details not being shown, and the resonant network 9 is coupled. as at Mi, to a second resonant network I0 also tuned to the operating intermediate frequency. The resonant networky II) is coupled to the input electrodes of the amplifier section of the electron discharge tube II, the amplifier section comprising the cathode I2, the signal grid I3, the screen grid I4, the suppressor grid I5, and the mananode I6. The envelope of tube II also includes a pair of independent diode anodes I'I, I8, these diode. anodes being disposed adjacent a portion I2 of the cathode I2 which only provides an electron stream to the anodes I1, I8. Y

The tube I I is of the 6B'7 type and is especially suited for receivers of the automobile type. While its specific construction is not a part of the present invention, it can be briefiy pointed out that the tube consists of two diodes and a pentode in a single bulb. The specific construction` of a tube of the type rep-resented by tube II has been disclosed by T. M. Shrader in application Serial No. 622,140, filed July 12, 1932. In this last named application the two diodes and the amplier electrodes are independent of each other except for a common cathodre sleeve, which has one emitting surface for the diodes and another for the amplifier electrodes. Further, in the said last application the common cathode extends beyond tne amplifier electrodes, and the diode anodes are disposed adjacent the cathode extension. Although the Shrader application does not show the suppressor grid I5 and screen grid I4, the aforementioned R. M. Smith application discloses the insertion of these two additional electrodes.

The signal grid I3 is connected to the high potential side of the network I0, while the cathode I2 is connected to the low potential side of the network through a path which includes a resistor I9 having a magnitude of about 2500 ohms in series with a second resistor 20 having a magnitude of about 1.5 megohms. A condenser 2l, having a magnitude of about 0.0001 mfd., is connected across the resistors i0 and 20, and a condenser 22, of .05 mfd., is connected across the resistor I9.

The main anode I0 of the pentode section of tube II is connected to the source of positive potential, conventionally represented by the numeral 23, through a path which includes the lead 24, the network 25 resonant to the operating intermediate frequency, the lead 26, resistor 21, having a magnitude of about 5000 ohms, the manual volume control resistor 23, having a magnitude of about 100 megohms, and the lead 20.

The screen grid electrode I4 of tube I I is connected to the positive potential source 23 through a path which includes the lead 30, the resistor 3l having a magnitude of about 200 megohms, and the lead 29. One side of the resistor 3I is connected to the negative side of the power supply source through the resistor 32 having a magnitude of about 100 megohms, and the negative side of the power supply source is conventionally designated by the numeral 23. The cathode I2 of tube II is connected to the negative side 23 through a path which includes the resistor I9, the resistor I 9 having a magnitude of about 5 megohms and the lead 33, the resistor I 9 being shunted by a condenser 33 of about .05 mfd.

The power supply source, which has been conventionally designated by the numerals 23, 23', is not specifically shown because it is not a part of the present invention; however, a power supply system of the type disclosed in my co-pending application Serial No. 670,069, filed May 9, 1933 may be utilized, particularly since such a power supply system is well adapted to tubes of the 6A7 and 6137 types, it being pointed out that these latter two tubes are especially suited to have their heaters operated in series from the power line.

The intermediate frequency signal energy which is impressed upon the resonant network I0 is produced in amplied form in the resonant network 25. The amplified intermediate frequency energy is transferred to a network resonant to the intermediate frequency energy through the coupling M2. This latter resonant network comprises the coil 40, which is inductively coupled to the coil 4I of the resonant network 25, the tuning condenser 42 and the resistor 21. It will be observed that the resistor 21 has one side thereof connected to the coil 40, while the other side thereof is connected to one side of condenser 42 through a lead 42.

To transfer the amplified intermediate frequency energy to the demodulation circuit, there is provided a coil 43, inductively coupled to the coil 40, and one side of the coil 43 being connected by a lead 44 to the diode anode I1, the other side of the coil 43 being connected to the cathode I2 of tube II to a path which includes the lead 45, and the resistor 46, the latter having a magnitude of about 1 megohm. The resistor 46 is shunted by a condenser 41 of about .0001

mfd., there being a condenser 48 connected between the cathode side of condenser 41 and the lead 30, the condenser 48 having a magnitude of about .05 mfd.

The low potential side of the network I0 is connected to one side of the resistor 46 through a path which includes the condenser 49 of .01 mfd. in series with the resistor 50 having a magnitude of about 200 megohms. Besides deriving amplified intermediate frequency energy for demodulation, there is derived from the main anode circuit of tube I I intermediate frequency energy for automatic volume control purposes. Briefly, this is accomplished by connectingthe diode anode I0 to the path which includes the condenser 5I of .00005 mfd., the lead 52, the resistor 53 having a magnitude of about 2 megohms, and the lead 54. The lead 54 is connected to the low potential side of the tunable input circuit 2, and the lead is designated by the expression A. V. C., thus designating this connection as the automatic volume control connection.

It is suflicient for the purposes of the present application to point out that in the aforementioned R. M. Smith application, it has been shown that the amplification gain in the oscillator converter network I may be controlled through this automatic volume control circuit, it being schematically shown in Fig. 1 that the signal grid 4 is the electrode through which gain control is accomplished. Of course, it is to be clearly understood that the network I could readily be a preceding stage of intermediate frequency ampliication, or a stage of radio frequency amplification ahead of the oscillator converter.

It is not believed necessary to explain in detail the functioning of the automatic volume control circuit since those skilled in the art well understand such functioning. It is pointed out that the cathode circuit lead 33 is connected to one side of resistor 53 through a resistor 55 having a magnitude of about 1.5 megohms. Of course, when a receiver of the type shown in Fig. 1 is employed as an automobile receiver the automatic volume control circuit serves an important function since it operates to maintain a substantially uniform input level to the demodulation diode, even though the signal pick-up by the collector 3 is reduced.

After demodulation of the amplified intermediate frequency energy by the diode demodulator, the audio signal is impressed, or reflexed back, to the signal grid I3 of tube I I, and there consequently is produced in the circuit of the main anode I6 amplified audio signal energy. This amplified audio signal is transferred to a second stage of audio frequency amplification which includes the pentode output tube 60. The connectionsbetween the loud speaker and the output electrodes of the tube 60 are not shown; furthermore, the energizing connections between the electrodes of tube 00 and the power supply source are not shown, these various connections being omitted since they do not form a part of the present invention, and may be accomplished readily by those skilled in the art.

The signal grid of tube 60 is connected to the lead 25 through a path which includes the resistor 0I having a magnitude of about 50 megohms in series with the condenser 62 of .01 mfd. The lead 26 is further connected to one side of the manual volume control resistor 23 through a path which includes the condenser 53 of .0001 mfd.

The manual 4volume control instrumentality consists of an adjustable .tap 64 having its free terminal in contact with the resistor 28 so that movement of the control 64 results in sliding of the free end of the tap over the resistor 28. The other side of the tap 24 is connected to the lead 26, which means that the variable tap 64 is connected to the path transferring the audio signal to the signal grid of tube 60. Variation of the tap 64 results in changing the intermediate frequency'input to the demodulator diode and the audio amplification simultaneously.

It will now be observed that the-manual volurne control consists of a single potentiometer Z8, 64. The resistance value of the potentiometer, which is the audio plate load resistor on the reflex tube, is selected according to best operation of the reex tube. The operation of the present invention will no-w be described.

The intermediate frequency grid signal voltage, which is 175 k. c., of the reflex tube I I is supplied from the secondary coil 40 of the transformer 40, 4I. The voltage is impressed, for demodulation, upon the coil 43. However, prior to this demodulation the input network I has impressed upon it the intermediate frequency signal voltage, and the latter is supplied from the secondary of the coupling M1 direct to the signal grid I3, and on the low potential side through the condenser 2 to the cathode I2. The amplified intermediate frequency signal is built up in the tuned circuit 25, the audio load resistor 28 being by-passed by the condenser 63, and optimum coupling is adjusted between resonant network and. the resonant network including condenser 42 and coil 46 for the condition of full audio output. This adjustment is obtained with the sliding contact 64 in the position shown in Fig. 1; that is, at the topy of resistor 28. Y

The network including condenser 42 and coil 4Q, hereinafter briefly designated by the numeral 25', tunes over the sliding contact 64. Maximum energy transfer from circuit 25 to circuit 25 is obtained for the position o-f the slider 64 shown in Fig. 1. The intermediate frequency current in circuit 25 induces an intermediate frequency voltage in the audio diode winding 43, the latter bei-ng tightly coupled to the coil 40 of circuit 25. A separate winding is used in order to prevent feed-back of audio signal voltage from the plate circuit into the grid circuit, and also to make possible the use of a series diode circuit, the latter having lower losses than the parallel diode circuit.

The audio signal is developed on the diode load resistor 46, and coupled back to the signal grid circuit of tube I I over an intermediate frequency filter resistor and a condenser, the latter consisting of the resistor 50 and condenser 49. The heavy full line in Fig. 1 denotes the diode demcdulator circuit, the diode load resistor 46, and the r-eex path for the audio signal.

The signal grid I3 of tube II is returned over resistor 26, functioning as a grid leak, to the proper biasing voltage. The amplified audio voltage, flowing in the circuit of the main anode i6, builds up on the volume control resistor 28 in the circuit of the main anode, and is applied to the signal grid of the following tube 60 over the path including resistor 6I and condenser 62.

Movement of the sliding contact 64 to a lower position on resistor 28 introduces series resistance into the resonant circuit 25', thereby lowering the energy transfer from circuit 25. This occurs because the coupling is below optimum for increased resistance of circuit 25. Consequently,

the intermediate frequency voltage on the coil 43 decreases, and thus also the audio voltage across the diode resistor load 46, and the audio voltage impressed on signal grid I3, decreases.

The amount of series resistance which is introduced into circuit 25 is limited by the resistor 2'I to 5000 ohms in the particular case, as the audio grid voltage on the signal grid I3 should not be reduced to zero. Further movement of the slider 64 decreases the audio plate load, thereby decreasing the audio gain of the reflex tube until zero. It will be observed that the intermediate frequency voltage on circuit 25 is not reduced, and in fact it rises slightly. Consequently, the action of the, automatic volume controlY diode I8 is not disturbed. It will therefore be seen that manipulation of the manual volume control in the present case in no way controls, or reduces, the intermediate frequency input to the reflex tube or the intermediate frequency gain for automatic volume control purposes.

Further, it will be seen that although the manual volume control is not in the plate circuit of the reflex tube I I, yet it regulates both the audio signal on the grid of the rei-lex tube and also the audio voltage delivered to the grid of the following tube 66, without involving any of the disadvantages discussed heretofore inherent in directly disposing the manual volume control in the plate circuit o-f the reflex tube, or between the demodulation diode and the Signal grid circuit of the reflex tube.

A modification of the manual volume control arrangement of Fig. 1 is shown in Fig. 2. This; modication is preferable for use with reflex tubes which do not permit reduction of the audio plate load to Zero on account of distortion. The high frequency action is the same as in Fig. 1, but the audio action is different. tiometer takes the place of the grid leak of the following tube. That is to say, the

potentiometer

28, 64 takes the place of the grid leak resistor I0 of Fig. 1.

It will therefore be seen that in the modification shown in Fig. 2 movement cf the slider 64 downwardly not only results in a reduction of the transfer of intermediate frequency .energy to the demodulation diode circuit, but also results in a reduction of the magnitude of the grid leak resistance of tube 60 to zero. The reduction of the audio plate load 12, having a magnitude of about 100,000 ohms, is limited by the lter resistor 13, having a magnitude of about 50 megohms, to a definite value, and the latter value is determined by distortion requirements. The grid coupling condenser I4 o-f 0.1 mfd. is made large enough to prevent loss of low frequencies at low volume levels. Since, with the exception of the aforementioned differences, the operation of this modification is substantially the same as that given in connection with Fig. l, it is not believed necessary toexplain Fig. 2 any further.

The aforegoing invention is particularly adapted for use in receivers with low B-supply voltage and relatively poor automatic volume control characteristics. For example, the invention may be employed to advantage in a radio receiver of the automobile type.

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 organization shown and described, but that many modifications may be made without The potenf departing yfrom the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination with an amplifier provided with a resonant signal input circuit vand a resonant signal output circuit, a demodulator circuit provided with an input network, a resonant signal circuit coupling said demodulator circuit input network and said resonant output circuit, a connection of low impedance to the demodulated Signal energy between said resonant input circuit and said demodulator circuit, and means electrically associated solely with said resonant coupling circuit for varying the damping of said coupling circuit to thereby control the transfer of signal energy from said resonant output circuit to said demodulator circuit'input network.

2. In combination with an amplifier provided with a resonant signal input circuit and a resonant signal output circuit, a diode demodulator circuit provided with an input network, a resonant signal circuit coupling said demodulator circuit input network and said resonant output circuit, a connection between said resonant input circuit and said demodulator circuit, and resistive means electrically associated solely with said resonant coupling circuit for varying the damping of said coupling circuit to thereby control the transfer of signal energy from said resonant output circuit to said demodulator circuit input network.

3. In combination with an amplifier provided with a resonant signal input circuit and a resonant signal output circuit, a diode rectifier circuit provided with an input network, a resonant signal circuit coupling said rectifier circuit, input network and said resonant output circuit, a connection between said resonant input circuit and said rectifier circuit, and a variable resistor connected solely to said resonant coupling circuit for controlling the transfer of signal energy from said resonant output circuit to said diode circuit input network.

4. In a receiver of the type including a radio frequency amplifier provided with a resonant signal input circuit and a resonant signal output circuit, a diode detector circuit, an audio frequency amplifier circuit, an audio signal connection between said resonant signal input circuit and said diode circuit and another audio signal connection between said resonant output circuit and said audio amplifier, the combination of a resonant signal circuit magnetically coupled to said diode circuit and said resonant signal output circuit and a manual volume control means comprising a variable impedance connected to said resonant coupling circuit for varying the damping thereof.

5. In a receiver of the type including a radio frequency amplifier provided with a resonant signal input circuit and a resonant signal output circuit, a diode detector circuit, an audio frequency amplifier circuit, an audio signal connection between said resonant signal input circuit and said diode circuit and another audio signal connection between said resonant output circuit and said audio amplifier, the combination of a coupling circuit, tuned to the frequency of said input and output circuits, magnetically coupled to said .diode circuit and said output circuit and a manually operable volume control device comprising a variable resistor connected to said coupling circuit whereby variation of said resistor simultaneously controls the transfer of signal energy to said detector circuit and the magnitude of Vaudio rsignal impressed ony said audio amplifier. f

6. In a superheterodyne receiver, an intermediate frequency amplier stage including a tube of the type provided with an amplifier section and an independent diode section, an audio frequency amplifier succeeding said tube, means for impressing upon the input electrodes of said amplifier section intermediate frequency energy, a coupling network, resonant to said intermediate frequency, between the output electrodes of said amplifier section and said diode section, means for impressing the audio component of the energy. rectified in said diode section upon the input elec-I trodes of said amplifier section, and means for controlling the efficiency of said coupling network to vary the reproducer volume of the receiver.

'7. In a superheterodyne receiver, an intermediate frequency amplier stage including a tube, an audio frequency amplifier succeeding said tube, means for impressing upon the input electrodes of said amplifier section intermediate frequency energy, a'coupling network, resonant to said intermediate frequency, between the output electrodes of said amplifier section and said diode section, means for impressing the audio component of the energy rectified in said diode section upon the input electrodes of said amplifier section, and means for controlling the efficiency of said coupling network, and means for impressing the intermediate frequency output of said amplifier section upon said second diode section to produce a direct current voltage adapted for automatic Volume control purposes.

8. In a superheterodyne receiver of the type including a combined detector-oscillator, an intermediate frequency amplifier, an audio frequency amplifier, a pair of independent diodes, connections between the output circuit of said amplifier of the intermediate frequency energy and one of said diodes whereby the latter functions as the receiver demodulator, additional connections including elements having a low impedance to audio frequency energy and high impedance to intermediate frequency energy between the demodulator diode and the input circuit of said intermediate amplifier whereby the functions as a stage of audio frequency amplification for the audio component of the demodulated energy, connections between the output circuit of the intermediate amplifier, the other of said diodes and the input circuit of said detectoroscillator whereby the gain of the latter may be automatically regulated, an audio frequency connection between the input circuit of the audio amplifier and the output circuit of said intermediate amplifier, and a manual volume control device for controlling the transmission efficiency of said first named connections.

latter fof 9. In a system of the type defined in claim 8,

volume control device being arranged to control the degree of magnetic coupling. i c

12. In combination with a reflex amplifier provided With a signal input circuit and a resonant output circuit tuned to the signal frequency, a demodulator provided with a signal input circuit and a demodulated signal output circuit, a demodulated signal path between the reflex amplifier input circuit and said demodulator output circuit, a resonant coupling circuit, tuned to said signal frequency, coupled to said demodulator input circuit and said amplifier output circuit, and means including an adjustable impedance connected in the coupling circuit for varying the damping thereof whereby the coupling between said demodulator input circuit and said amplifier output circuit is varied.`

13. In combination with a reflex amplifier provided with a signal input circuit and a resonant output circuit tuned to the signal frequency, a demodulator provided with a signal input circuit and a demodulated signal output circuit, a demodulated signal path between the refiex amplifier input circuit and said demodulator output circuit, a resonant coupling circuit, tuned to said signal frequency, tightly and magnetically coupled coupled toY said demodulator input circuit and said amplier output circuit, and means including an adjustable resistive impedance connected in the coupling circuit for varying the damping thereof whereby the coupling between said demodulator input circuit and said amplifier output circuit is varied.

14. In combination with a reflex amplifier provided with a signal input circuit and a resonant output circuit tuned to the signal frequency, a demodulator provided with a signal input circuit and a demodulated signal output circuit, said demodulator being of the diode type and being included in the same tube envelope which houses the amplller electrodes, a demodulated signal path between the reflex amplifier input circuit and said demodulator output circuit, a resonant coupling circuit, tuned to said signal frequency, coupled to said demodulator input circuit and said amplifier output circuit, and means including an adjustable impedance connected in the coupling circuit for varying the damping thereof whereby the coupling between said demodulator input circuit and said amplifier output circuit is varied.

OTTO H. SCHADE.