US1879863A - Volume control - Google Patents

Description

Sept. 27, 1932. H. A WHEELER 1,879,863

VOLUME CONTROL Original Filed July 7. 1927 4 Sheet's-Sheet l Sept. 27,' 1932.

H. A. WHEELER VOLUME CONTROL Original Filed July 7, 1927 4 Sheets-Sheet 2 @eier i UO r bw P F H n N n f n H n m ww NW .SW uw@ 4 Sheets-Sheet '5 INVENTOR ATTORNEYS Harold waaier BY 'Hlm/2.4, .L4-ul NNY SePt- 27, l932- H. A. WHEELER VOLUME CONTROL Original Filed July 7, 1927 Sept. 27, 1932.

H. A. WHEELER VOLUME CONTROL 4 Sheets-Sheet 4 Original Filed` July 7, 1927 SSB@ ATTORNEYS Patented Sept. 27, 1932 UNITED STATES PATENT OFI-'ICE HAROLD A. WHEELER, OF GREAT NECK, NEW YORK, A.ASSIGNIOR TO HAZELTINE CORPO- RATION, A CORPORATION OF DELAWARE VOLUME CONTROL Original application led July 7, 1927, Serial No. 203,879, and in Great Britain July 3, 1928.

REISSUED Divided and this application led November 13, 1930. Serial No. 495,386.

This invention relates to amplifiers, and more particularly to amplifiers utilized in modulated carrier-current signaling systems wherein the limit of amplification is automatically maintained substantially at a predetermined level.

This application is a division of application Serial No. 203,879, filed July 7, 1927.

flien amplifiers are employed for amplifying a signal voltage it becomes desirable for various reasons to control automatically the amplitude of this amplified signal voltage. To this end the present invention provides means for effecting automatic amplification control. Such an arrangement, for example, is particularly advantageous in radio receivers such as are employed for receiving broadcast signals, because lit prevents the overloading of the last amplifier stage of the receiver, which overloading would result in distortion of the reproduced signal, as well as loud and harsh reproduction.

Another advantage resides in uniform reproduction of the amplified signal irrespective of whether the carrier-current signal is received from a nearby station or from a distant or a high-power station, or a lowpower station, since it has been found in former radio receivers that when the receiver was reproducing strong signals as from a nearby, or a high-power station, the audibly reproduced signal was very loud, whereas when the signal was received from a distant, or a low-power station, it was relatively weak, with the result that if signals were to be reproduced uniformly from both near and distant stations, and from high-power and low-power stations, it` became necessary to readjust some volume controlling mea-ns in the receiver to compensate for these unequal signals.

It has also been aI common experience in the use of former radio receivers that the reproduced signal was not uniform due to the phenomenon of fading, whereby the received signal occasionally, or periodically became much'weaker, or faded almost to the point of inaudibility. Since the present invention provides an amplifier which automatically compensates for inequalities in the lpower system, it has been found that received carrier-current signal strength, when fading takes place the degree of amplification is correspondingly increased and the reproduced signal maintained at its former volume, so that a listener is unaware that variation of the received carrier-current signal is occurring. This automatic compensation for .signal fading is especially advantageous in commercial radio telephony and like systems.

In existing radio receivers in which operating current is derived from the municipal when there is considerable variation in the line voltage supply, the volume of the reproduced signal is not uniform. An additional advantage of the present invention is that of automatically compensating for such line voltage variations with the result that the reproduced signal is uniform in volume. v

A still further advantage is the saving in plate current which is automatically effected during the reception of powerful signals, for the reason that this invention incidentally provides means for reducing the plate current of one or more amplifying tubes as the signal strength increases. y

Fig. lis a circuit diagram of a complete radio receiver which includes the present invention, and consists of a three-stage radiofrequencyamplifier followed by a rectifier, a two-stage audio-frequency amplifier, and a loud speaker, or other suitable indicating device.

Fig. 2 shows curves disclosing the relation between the radio-frequency antenna voltage and the radio-frequency amplified voltage, with and without ent invention.

Fig. 3 shows a circuit diagram of a second embodiment of the invention in which there is disclosed a three-stage tuned radio-frequency amplifier, a rectifier, and a three-stage audio-frequency amplifier.

Fig. 4 represents the present invention embodied in a radio receiver including a twostage radio-frequency amplifier, a detector, and a two-stage audio-frequency amplifier.

Fig. 5 shows a modified form of coupling arrangement and controlling'network which the application of the presmay be substituted for the correspondin elements inclosed in the dotted rectange of batteries now so widely used.

Fig. 7 is a circuit da am of a radio receiver in which several o the features of the by a variable condenser 22 present invention are combined in a single embodiment.

Fig. 8 shows graphically a comparison between the performance of the two-electrode valve or rectifier, and of the three-electrode detector. l Referring in detail to Fig. 1, there is shown an antenna 5 connected to ground '1 through the primary winding 6 of a radio-frequency transformer, the secondary winding 7 of which, tuned by a variable condenser 8, is connected at one point to the filament of the vacuum tube 9 in the first radio-frequency amplifying stage and at another point to the grid 11 o this vacuum tube. The output circuit of this vacuum tube extends from the filament system, through a high-voltage battery B, a milliammeter 10, primary winding 13 of a second radio-frequency transformer to the anode or plate 14 of this-vacuum tube. In order to neutralize the inherent capacity between the grid 11 and the plate 14, and thereby to prevent oscillations, and otherwise to increase the effectiveness of the present invention as hereinafter described, a neutralizing winding 19, electromagnetically coupled to winding 13, and a neutralizing condenser 3 are employed in the manner described in the U. S. patents to Hazeltine Nos. 1,489,228 and 1,533,858.

A second stage of radio-frequency amplification including the vacuum tube 15 neutralized by cooperation of coil 26 and condenser 4, like the first stage, comprises the secondary winding 16 of the last-mentioned radio-freuency transformer tuned by a variable condenser 17 connected between the filament system of the vacuum tube 15 and the grid 18 thereof. The output circuit of this vacuum tube also includes the high-voltage battery B and a primary winding20 of a second radio-frequency transformer, while the secondary winding 21 of this transformer tuned is included in the input circuit of a third stage of radio-frequency amplification which includes vacuum tube 23. The inherent capacity effective between the electrodes 24 and 25 is neutralized by a network including the neutralizing condenser 28 and the neutralizing winding 29 as described in the mentioned atents. The output circuit of the vacuum tu e 23 includes the primary winding 30 of a third radio-frequency transformer and the high-voltage battery B. The secondary winding 31 of this lastmentioned transformer, tuned by a variable condenser 32, is connected in the input circuit. of a rectifier 33 which input circuit includes the -fixed condenser 2, the rectifier employed may be of the type commonly known in the art as a two-electrode Fleming valve, or may consist of an e uivalent such as a threeelectrode vacuum tu e, as shown, having its grid 12 and its plate or anode 35 directly connected together to comprise in effect a single anode.

It may here be noted that throufrhout the present specification and claims the terms rectifier and detector aref, in general, used interchangeably, the terms rectifyng and converting being employed in the general sense to include the process of changing alternatin g current into a form of direct current or modulated unidirectional current. Likewise, the terms carrier-current and modulation current may be substituted, respectively, for radio-frequency current and audiofrequency current, since the description herein of radio-frequency amplifiers and audio-frequency amplifiers is merely by way of example of a typical embodiment of the present invention.

In the absence of the present invention including the control circuit 36, to be described, the three-stage amplifier functions in a manner well-known in the art to amplify the incoming signal intercepted on the antenna 5. The output circuit of the rectifier 33 includes what may be termed a rejector circuit for stopping radio-frequency currents which have passed through the rectifier, and consists of a network including a resistance 34 and a by-pass condenser 37 connected between the anode 35 and the filament 38 of the rectifier. The output circuit of the rectifier is coupled to the input circuit of an audio-frequency amplifying vacuum tube 39 through an audio-frequency-pass filter including a fixed condenser 40 and an impedance 41 connected between the filament 42 and the grid 43 of this vacuum tube. The output circuit of this amplifier is connected between the filament 42 and plate 44 through the highvoltage battery B and the primary winding 45 ofan audio-frequency transformer, the secondary winding 46 of which is connected in the input circuit of a second audio-frequency tube 47, while a resistance 48 connected across the winding 46 serves to give the audio amplifier substantially uniform amplification over the desired. frequency range. Instead of employing resistance 48, a closed copper band of suitable size may be placed around the transformer winding so as to be electromagnetically coupled thereto. A loud speaker or other reproducing device 50, or if required, a coupling device for a telephone system, is connected in the output circuit of the last audio-frequency amplifying tube 47. It is presumed that adequate precautions against undesired electromagnetic coupling between the various radio-frequency coupling transformers are included in all of the arrangements herein disclosed.

In accordance with the main feature of the present invention, means are provided to control automatically the degree of amplification effected in the radio-frequency amplifying stages. These means include a resistance 51, connected between the filament 38 and the anode 35 of the rectifier, through which the I pulsating rectified or converted current fiows,

thereby developing a negative voltage at terminal 52. This negative voltage is applied over conductor 3G through the impedance 53 and the secondary winding 7 of the first radio-frequency transformer to grid 11 of the rst radio-frequency stage. Impedance 53,

together with blocking condenser 54, is effective to filter out and reject any audio-frequency currents which otherwise might be present in the conductor 36.

To complete the description of the system illustrated in Fig. l certain design data or constants are given herewith. It should be understood, however, that these, as well as all other constants appearing in the present specification, are mentioned merely by way of example in describing certain specific embodiments which in practice have proved eminently satisfactory, and are not intended to suggest any specific limitations as to the scope of this invention. Accordingly, fixed condenser 2 may be of 0.0005 microfarads; 37 of 0.0001 microfarads; 54 of 0.01 microfarads; 40 of 0.005 microfarads; resistance 51 of 1 megohm; 34 of 1 megohm; and 41 and 53 of 2 megohms each.

In the operation of the receiver shown in Fig. 1 a signal intercepted on the antenna 5 is `successively amplified through the neutralized radio-frequency stages indicated by the vacuum tubes 9, 15 and 23. This amplified signal voltage is then rectified by the rectifier 33, and the Vrectified pulsating current is successively amplified by the audio amplifying stages including vacuum tubes 39 and 47, after which it may be reproduced as sound by the loud speaker 50. When the rectified or converted signal current fiowing through the resistance 51 is greater than-a predetermined value. there is developed at the termin al 52 sufficient negative biasing voltage which in turn is impressed, through the conductor 36, upon the grid 11 ofthe vacuum tube 9. to reduce the amplification of this tube. It will be apparent that as the magnitude of the rectified current flowing through resistance 51 decreases with weaker signals, the voltage at terminal 52 becomes less negative, and the negative biasing voltage impressed upon the lgrid 11 also diminishes so that the vacuum tube 9 effects an increased degree of amplification. In this manner, the radio-frequency voltage applied to the input of the rectifier is maintained at a nearly constant predetermined value, and the .volume of the reproduced 'signal is substantially uniform under all conditions. The degree of volume of the reproduced signal is then determined by adjustment of rheostat 49 which controls the heating current in the filament 42 of the first audio-frequency amplifying tube 39. The neutralization of the gridplate capacity of the radio-frequency amplifying tubes is, in combination with the present invention, particularly valuable in that it allows an increase in the effectiveness of the amplification control, because such neutralization prevents radio-frequency energy from passing through the grid-plate capacity of the tubes. Thus the relay action of the tubes is almost entirely subject to the control by grid bias voltage provided in accordance with this invention.

The time required for operation of the control system would ordinarily be determined by the lowest audio-frequency modulation which must be reproduced. Fading, for example, might be considered a form of modulation; the frequency of the rise and fall of signals due to fading being the frequency of modulation. If this frequency of modulation be increased sufficiently, the effect will be audio-frequency modulation. It will thus be seen that if the automatic control attained by the present invention be allowed to respond too quickly, it will tend to smooth out the desired modulation ofthe signals at the lower audio frequencies. Hence, a time constant of operation is chosen which will be greater than the period of the audio frequencies which the system is intended to amplify. This time constant of the control circuit is equal to the product of the series resistance and the shunt capacitance of the grid bias circuit, represented in Fig. 1 by resistance 53 and capacitance 54. However, since the time constant can always be reduced to a value equal to the period of the lowest modulation frequency, it may readily be set to meet the requirements of nearly any special case which may arise. For example, a value of two million ohms resistance and of 0.1 microfarad capacitance gives a time constant of one-fifth of a second, which does not appreciably affect the modulation of frequencies above five cycles. While this constant is greater than required from the point of view of satisfactory audio-frequency quality in the reproduction of music, there appears to be no need for more rapid control under the conditions usually encountered. The use in this connection of condensers of large capacitance, such as one-tenth microfarad, likewise introduces another convenience in that the condensers may also serve to by-pass radio frequencies in order to prevent undesired coupling between the detector circuit and the first radio-frequency amplifying tube because of some impedances common to those two portions of the appadio-frequency voltage is indicated by curve 103 from which it will be seen that when at least a certain predetermined radio-frequency antenna voltage is present, (herein referred to as the threshold antenna voltage) the amplified radio-frequency voltage approaches-but is always less than-another certain predetermined voltage value (herein referred to as the cut-off voltage).

The modification illustrated in Fig. 3 is an especially desirable form of the present invention, and includes antenna 56, connected by means of a transformer 57 to a neutralized three stage tuned radio-frequency cascade amplifier including the vacuum tubes 58, 60 and 62coupled by transformers 59 and 61. The last stage of the amplifier is connected by a transformer 63 to a two-electrode rectifier 64 of the type already described, the output circuit of which, including the resistance 65, is connected between the anode 66 and filament 67 of the rectifier, as previously explained. Resistance 72 and condenser 68 associated with this output circuit, constitute a rej ector network which filters out the radio-frequency current component in the output circuit of the rectifier 64, While the network including condenser 69 and resistance 70 constitutes an audio-frequency-pass filter for coupling the output circuit of the rectifier to the input circuit of the audio-frequency amplifier which includes vacuum tube 71. Rheostat 7 3 controls the heating current supplied to the filament 74 of this vacuum tube, and thereby permits a manual adjustment of the volume of the reproduced signal desired by thelistener. Audio-frequency transformer 76, which is preferably of a low ratio of transformation, couples the output circuit of vacuum tube 71 to a second audio-frequency amplifying tube 77. This last vacuum tube in turn is coupled by a second audio-frequency transformer 78 to a third audio-frequency amplifying tube 79 in the output circuit of which there is included a loud speaker 80.

In this arrangement automatic amplification control is effected in a manner slightly different from that shown in the diagram of Fig.' 1, since in this instance the radiofrequency voltage of the signals intercepted by the antenna 56 is successively amplified by three neutralized tuned radio-frequency amplifying stages including the vacuum tubes 58, 60 and 62, of which two are controlled in accordance with the present invention. The amplified radio-frequency current is rectified by the rectifying valve 64, and successively amplified at audio-frequency by the vacuum tubes 71, 77 and 79. The rectified current in the output circuit of the rectifier flows through the resistance 65, and thereby develops a negative-voltage at the terminal 81, which voltage is applied through the impedances 72, 82, 83 and 85 to the rids 84 and 86 of the radio-frequency ampli ying tubes 58 and 60. By thus simultaneously controlling the degree of amplification of two successive radio-frequency amplifying stages agreatly increased uniformity of regulation is attained. Impedance 82 and the condenser 87 constitute an audio-frequency-stop filter, so that substantially only direct-voltage is impressed upon the grids 84 and 86. It will be understood that the voltage developed at terminal 81 is a function of the amplified radio-frequency voltage delivered to the input circuit of the rectifier by the radio- frequency amplifying tubes 58, 60 and 62, and therefore, as the negative voltage at terminal 81 tends to increase with the increased signal, the resulting increase of biasing voltage impressed upon the grids of the tubes 58 and 60 limits the degree of amplification effected in the radio-frequency stages including those tubes.

In this arrangement the constants for the varlous resistances and condensers may, for example, be the same as those for the corresponding elements in Fig. 1. In addition the grid resistances 83 and 85 may have a value of 2 megohms each; and the grid condensers connected at the junction of these resistances and the grid electrodes 84 and 86 may each be of 0.001 microfarad capacity.

The modification shown in Fig. 4 differs from the arrangement of Figs. 1 and 3 mainly in that it employs a. three-electrode vacuum tube which functions in the manner of the Well-known three-electrode detector, and also effects rectification, or conversion, of the radio-frequency carrier current to control amplification in the first radio-frequency stage of the receiver. As in the preceding arrangements, there is here employed an antenna or other suitable signal interceptor 88 coupled by means of a radio-frequency-transformer 89 to a two-stage neutralized tuned radio-frequency amplifier including the vacuum tubes 90 and 91 coupled by means of a radio-frequency transformer 92. The output circuit of the last stage of the amplifier is connected by means of radio-frequency transformer 93 to the tuned input circuit of a three-electrode vacuum tube detector 94, which inputk circuit is tuned by the inductance of the secondary winding of transion former 93 in shunt to variable condenser 95. A suitable negative voltage is maintained on the grid 96 of the detector tube, through the secondary Winding of transformer 93, by C battery 97 and by potentiometer 98 connected across the filament of the detector tube. By means of this potentiometer connection, a negative voltage may be applied to the grid 96 varying from one volt to a maximum of five volts; the minimum value being the difference between the six Volts of C battery 97 and the voltage drop across the filament of the detector. The output circuit of the detector includes the primary winding of transformer 99, a 45-volt battery 100 and a 500,000 ohm resistor 101, connected in series between the anode 0f the detector and the common B battery. A fixed condenser 104 by-passes the radio-frequency current that has passed through the detector, while the audio-frequency component of the rectified, or converted, current is transferred through the audio-frequency transformer 99 to the input circuit of an audio-frequency amplifying vacuum tube 105, the filament circuit of which includes rheostat 106 for controlling the arbitrary volume level of the amplified signals. The output circuit of the audio-frequency amplifier 105 is coupled by means of an audio-frequency transformer 107 to the input circuit of a second audiofrequency amplifying tube 108. The output circuit of this vacuum tube includes the usual loud speaker or indicating device 109. A resistance 110 is connected across the secondary winding of transformer 107 to secure substantially constant amplification within the frequency range of the audio-frequency amplifier, especially when the plate resistance of the preceding tube 105 is high as a result of the adjustment of rheostat 106.

For controlling the amplitude of the radiofrequency voltage applied to the input circuit of detector 94, a conductor 111 is con' nected at point 112 common to a terminal of resistor 101 and battery 100, and thence through the secondary winding of transformer 89 to the grid of the first radio-frequency amplifying tube 90. A by-pass condenser 113 connecting the conductor 111 to the filament system serves to filter out and reject any audio-frequency currents present in the circuit including the conductor 111, thereby insuring that these currents have no effecton the grid of the vacuum tube 90. Battery 1 00 is the source of negative biasing voltage applied to the control grid, or controlelectrode, of the radio-frequency amplifying tube, this battery being so connected to the output circuit of the detector, or rectifier, that fluctuations of voltage in the detector output circuit cause equal fluctuations in the negative biasing voltage impressed upon the control-grid.

In this embodiment, condensers 104 and 113 may be of 0.0005 microfarad and 1 micrfarad capacityrespectively; while resistance 101 may have a value of 0.5 megohm.

In adjusting the receiver of Fig. 4, it is necessary to determine the correct setting of the detector grid potentiometer 98. This adjustment should be made while there is no signal being received, as follows: First, the switch 115 is closed, and the normal plate current of the tube 90 is noted on milliammeter 116. Then the switch is opened, thus placing the control circuit in operation. In general, the plate current of vacuum tube 90 will change when the switch is opened, since the grid voltage of this tube is dependent upon the cont-rol circuit. By varying the grid voltage of the detector by potentiometer 98. the plate current of tube 90 is then adjusted to the normal value; and the apparatus is ready for operation. Upon receiptV of an amplified signal at the detector, the effect of the control circuit is to decrease the plate current through milliammeter 116, thereby reducing the amplification of the tube 90. When the receiver is tuned to e signal frequency, a minimum amplificatio is required, so that when the condition of sonance is attained, the plate current of t be 90 is at a minimum value.

It is believed unnecessary to explain the operation of the two radio-frequency amplification stages and of the detector, or of the two audio-frequency amplification stages, for

inoperation they are substantially similar to those of the now well-known type of radio receiver employing neutralized two-stage radio-frequency and two-stage audio-frequency amplifiers. The control'circuit operates, in the arrangement of Fig. 4, substantially in the same manner as in Figs. 1 and 3, to apply a negative biasing voltage to the grid of the radio-frequency amplifying vacuum tube 90, this voltage being a function of the radio-frequency voltage which has been amplified by the vacuum tubes and 91 and then applied to the input circuit of detector 94.. Since the voltage applied over conductor 111 is a function of the amplified radio-frequency voltage, there is a maximum, or cut-oli", detector voltage determined b the constants of the circuit, as shown in Fig'. 2, beyond which the radio-frequency amplifier is prevented from effecting further amplification. This arrangement maintains the finally-amplified radio-frequency voltage at substantially constant value.

Fig. 5 shows an alternative system for coupling the detector to the first audio-frequency amplifying tube of Fig. 4. The coupling arrangement of Fig. 5 included within the dot-ted rectangle, when substitutedfor the corresponding portion enclosed within the rectangle of Fig. 4, provides a modified form of the invention. Corresponding elements of Athese two figures are identified by the same reference characters, from which it will be seen that Fig. 5 differs from Fig. 4 in that the transformer coupling between the detector 94 and the first audio-frequency amplifying tube 105 has been replaced by an impedance coupling arrangement including the condenser 117 and the impedance 118.

While this modification does not utilize a transformer having a step-up ratio such as is included ink the arrangement of the former figure, it, nevertheless, introduces the advantage of effecting a more nearly constant degree of amplification at audio frequencies. When the modicationl of Fig. 5 is substituted in the system of Fig. 4, as described, the values of elements 104 and 101 may be the same as mentioned above; the resistance connected in lead 111 may be of 2 megohms; 1'18 of 2 megohms; 117 of 0.005 microfarad; and 113 of 0.5 microfarad.

Referring to Fig. 6, there is shown a radio receiver of the unneutralized type in which the so-called A, B and C batteries have been replaced by a source of rectified and filtered alternating current. In this embodiment of the invention there are provided three stages of tuned radio-frequency amplification in which the vacuum tubes of the successive stages are designated 119, 120 and 121, respectively. These several stages are transformer-coupled; and the last stage of the amplifier is coupled to a three-electrode vacuum tube detector 122, the grid bias voltage of which is controlled by potentiometer 128. In the output circuit of detector 122, there is provided a rejector circuit, similar to that previously described, for filtering out radio-frequency currents that have passed through the detector; and also an audiofrequency network, or impedance coupling, including condenser 123 and impedance 124, for passing the audio-frequency component of the rectified signal to the first audio-frequency amplifying vacuum tube 125. The filament of this tube is shunted by rheostat 129 which functions as a manual volume control. This last tube is transformer-coupled to a second stage of audiofrequency amplification, including the vacuum tube 126, in the output circuit of which there is provided a loud speaker, or other suitable indicating device, 127, which on occasion may be replaced by a coupling device in a telephone system. The filaments of these siX vacuum tubes are connected in seriesv across a suitable resistance in the rectified, filtered source of power supply, giving a potential difference of 30`volts, thus taking the place of an A battery. The necessary C, or bias, voltage is derived from a potential difference across a. resistance in that portion of the power supply indicated by the reference character C, while theplate-current supply is similarly derived from a resistance in that portion of the power supply indicat- Lenses ed by the reference-letter, B. It should be noted that the filament of the first radiofrequency amplifying tube 119 is connected to the positive terminal of the 30-volt A section of the power source, and that' the filament of the detector tube 122 is connected to the negative terminal of this section. The constants 'of the elements of this embodiment may in general be similar to those suggested with reference to Fig. 5.

In adjusting the receiver of Fig. 6, the potentiometer 128 is adjusted with switch 115 open, as described in connection with the adjustment of the receiver of Fig. 4. This arrangement with thevacuum tube filaments connected in series obviates the necessity of a separate battery corresponding to 100 of Figs. 4 and 5, since the plate of the detector tube 122 can be positive relative to the filament of .that tube, and at the same .time maintains the grid of the first radio-frequency amplifying tube 119 negative relative to the filament of the same tube, due to the difference of potential between the two filaments. Thus the biasing voltage applied to the control-grid is derived from the voltage across the filaments instead of from a battery as in Figs. 4 and 5.

n Figs. 4 and 5, batteries B and 100 are connected in series in the plate circuit of detector 94 and both contribute to the detector plate current. The B battery supplies the voltage drop in resistor 101, while battery l 100 supplies the plate voltage of detector 94. The presence in the detector plate circuit of the B battery, which is directly connected to the grounded filament circuit, in addition to auxiliary battery 100, allows the use of a high impedance 101 in the detector plate circuit With a resulting high sensitivity of the detector circuit. The same result is achieved in the arrangement of Fig. 6 by the cooperation of the A and B voltages in the detector plate circuit, as described before. In the event that tubes having an indirectly heated cathode are used instead of those having an incandescent filament cathode as represented in the figure, the same advantages may be obtained as pointed out in connection with Fig. 6, if the detector cathode is maintained at a potential much more negative than the cathode of the controlled tube or tubes, which, in the ure, is the first radiofrequency amplifier tu e.

The circuit arrangement shown in Fig. 7 incorporates several advantages introduced by the present invention, some of which have been individually described above. Briefly,

this arrangement includes a combination of the features illustrated in and described in connection with Fig. 3 and Fig. 6. The reference characters of-Fig. 7 correspond to l those employed in Fig. 3 and have the same significance. It will be noted in addition to the apparatus represented in Flg. 3, that lli there is here illustrated a power source of rectified and filtered alternating current which replaces the `so-called A and B batteries represented in Fig. 3. and in addition, includes a source of 0, 0r grid bias voltage for tube 79. The grid of tube 7 7 is biased by connecting the grid return lead to an appropriate point in the series filament circuit, as shown. The power source is similar to that shown in and described in connection with Fig. 6. The present arrangement thus includes the advantages of neutralized radiofrequency amplifying stages. automatic volume control applied to the first two stages of the radio-frequency amplifier, a twoelectrode valve, or rectifier, and the complete elimination of all batteries for supplying operating potentials to the system. As is also true ofthe arrangement of Fig. 6, the automatic volume control not only compensates for fluctuations in the strength of the incoming signals. but also compensates for reasonable variations in line voltage of the alternating current powertsupply. p

As in Fig. 6 the variable timing condensers (i1-C, are grounded in order to eliminate undesirable capacity effects as well as to make it practicable to connect the condensers on a single shaft for uni-control, if desired. As in Fig. 6, it will be seen that the power supply of Fig. '7 is not grounded. thus eliminating the danger of short-circuiting the direct current supply when a separate ground is necessary for the alternating-current rectifying and filtering system. In certain. instances, Figs. 6 and 7 differ in the connection of certain by-pass condensers. The purposes and reasons for the positions of these by-pass condensers should be apparent to those skilled in the art.

Assuming that the vacuum tubes employed are of the type having five-volt filaments, volts of filament, or A, supply is needed. As above mentioned, the automatic volume control is here applied to two tubes, namely 58 and 60; the cut-ofiI being effected with the use of two different plate, or B, voltages. The plate electrode 66 of rectifier 64 is more negative relative to the filament of tube 58 than relative to the filament of tube 60, by the 5-volt drop across one filament. To compensate for this difference, volts higher B voltage is applied to the plate of tube 58 than to the plate of tube 60, which makes both tubes eut off vpractically at the same time. The reason for applying a B voltage of 85 volts (which. of course, is to be added to the 35 volts A voltage) tothe plate of tube 60, whereas a B voltage of 90 volts is applied to the plates of the radio-frequency amplifying tubes of Fig. 6, is that the arrangement of Fig. 7 employs one more tube than Fig. 6; the difference of 5 volts being included in the A supply, as will be seen y comparing the two figures. Thus, actually, the plates of the radio- frequency amplifying tubes 119, 120 and 121 of Fig. 6 are provided with 90, 95 and 100 volts, respectively. Similarly the plates of the amplifying tubes of Fig. 7 are supplied with 85 to 155 volts.

In addition to the combined advantages just outlined, the arrangement of Fig. 7 also includes an additional feature which has not previously been described, namely, the means 130 for determining the filament current supplied to one of the amplifying tubes. As has been explained in connection with Fig. 6,-when operating the filaments of the several vacuum tubes on rectified and filtered current from an alternating current power source, it is desirable that the filaments be connected in series since it is at present more practicable to provide a current supply at a comparatively high voltage and lowv current. Fig. 6 shows a shunt rheostat 129 connected in parallel with the filament of tube 125 so that the current divides between the rheostat and the filament. iVhile this means for controlling the filament emission of a single tube, as shown in Fig. 6, is fairly satisfactory, the arrangement,shown in Fig. is a substantial improvement. With the former method, an increase in current throughthe controlled filament is accompanied by a smaller increase in current through the other filaments in series. The improved arranvement shown in Fig. 7, on the other hand, providing three resistances, two of which are simultaneously variable, allows a variation of the voltage on one or more filaments without affecting the current through the other filaments; or, more generally, without changing the load on the filament-current supply. It is apparent that the benefits of this device will be especially manifest in an arrangement such as the present, wherein the current supply for the filaments is obtained from a rectified, filtered alternating current source, particularly when the rectifying device is of the common type without automatic voltage regulation. The compound rheostat 130 comprisingresistances R1 and R2 is so arranged that a movement of the control knob Will increase the one resistance, while diminishing the other in proportion. ne of these resistances, namely R1 is connected in series with resistance R0 in shunt with filament 74 of tube 7l, the resistance of filament 74 being represented by Rf It may here be pointed out that While Fig. 7 illustrates the manual control of the filament of only one tube, namely 74, the filaments of other tubes could be connected either in series or parallel with filament 74 if it were desired that independent simultaneous control be had of more than one filament. Rf may, therefore, be taken to represent the effective resistance of the filaments to be controlled, When the two resistances R1 and R2 and the fixed resistance 4resent the Ro are properly proportioned to the normal gr operating reslstance Rf of the filament or filaments of the tubes to be controlled,the resistance of the system as a whole will remain substantially constant during adjustment of the control device 130. By way of illustration, the following data are given for Fig. 7, assuming the tubes to be all of the well-known 201A type, of 20 ohms resistance: It, will equal 20 ohms; RQ may equal Rf; R1 may equal 8Rf; and R2 may equal l/zRf. Accordingly, to control one tube when Rf equal 20 ohms, R1 R2, 10 ohms.

It is believed unnecessary to describe the method of controlling the signal amplification in the arrangementsof Figs. 5, 6 and 7 since they are substantially similar in operation to that of the systems described in reference to Figs. 1, 3 and 4. It should be mentioned, however, that the advantages of the present invention are especially apparent in systems such as shown in Figs. 6 and 7, because of the fact that any reasonable fluctuations in the voltage of the power supply line are thus automatically compensated for, and uniform volume of signals is assured.

There are advantages attending the use, in connection with the present invention, of the two-electrode rectifier circuit typified by Figs. 1, 3 and 7, which may not be apparent from the foregoing discussion. It is impossible to overload this type of rectifier, and the rectified output voltage is directly proportional to the applied alternating signal voltage when this voltage is large, say over two volts. The control system in the circuits of the figures referred to requires a large operating voltage, say ten Volts, so that the latter condition of large signal voltage is realized. No such simple relationship is possible in a three-electrode detector, whose rectified output never exceeds a limiting upper value, and is never proportional to the applied voltage, except over a very small range of voltages. This distinction will be seen from Fig. 8 where the abscissae A. C. repalternating sional voltages, whereas the ordinates D. C. represent the rectified output voltages. It is well known that the linear curve is much more desirable when minimum distortion of a modulated signal is desired, and it will be observed from will equal 160 ohms, and

' Fig. 8 that the preferred type of curve is obbe construed as a llmitatlon, but merely as tained from the two-electrode rectifier.

A further advantage of the linear type detector with the automatic volume control connection and a visual resonance indicator in the anode circuit of the amplifier whose grid bias is being automatically controlled, lies in the fact that the visual resonance indicator will give an indication which is proportionate to the received signal intensity. This follows from the fact that the negative each filament being equals 2O ohms, R0 willl id bias on the amplifier is directly proportional to the strength of the signal; and hence the anode current bears a similar relation to the signal.

The three-electrode detector is useful for relatively small applied voltages, and the rectified output voltage is then approximately proportional to the square of the applied voltage, i. e., to the power associated with thel applied Voltage. For this reason the rectified voltage increases with the carrier wave modulation. When such a detector is used in the control system, as in Figs. 4.-, 5 and 6, the total power from the radio-frequency amplifier is maintained at a substantially constant level, the amplitude of the carrier wave being decreased in the presence of modulation. It is desirable to maintain vthe carrier wave at al constant amplitude at the output of the amplifier, and this is accomplished by the two-electrode rectifier as shown in Figs. 1, 3 and 7. The control system maintains constant the average signal amplitude which is equal to the carrier wave amplitude and independent of the degree of modulation.

It will be observed that in a system employing a two-electrode rectifier such as represented by valve 33 of Fig'. 1, and 64. of Figs. 3 and 7, the control bias voltage is independent ofthe B or anode vbattery voltage. Since the rectifier is not an amplifier, is not critical, and requires neither anode nor bias: ing battery, no adjusting devices are required. This is not the case in the three-electrode detector circuits, so that a potentiometer, 98 or 128 in Figs. 4 or 6, respectively, must be adjusted as described to accommodate the control bias to any particular combination of tubes and B voltage. On the other hand, the latter type of detector is more sensitive because it is also an amplifier, so that the control system operates on aA smaller applied alternating voltage.

In the foregoing description, tuned radiofrequency receivers of the neutralized and unneutralized types have been referred to. It should be pointed out however, that the present invention may be employed with equaleffectiveness -to any radio receivers in wired radio and space radio systems, and that it has been found especially applicable. to receivers of the super-heterodyne type. For this reason the present disclosure of typical embodiments of the invention should not illustrative of the principles' of the invention, the scope of which is defined in the appended claims.

What is claimed is:

1. In a signaling system a vacu-um tube amplifier having a cathode and a control electrode, a vacuum tube detector coupled to said amplifier, said detector having an output electrode, means-for maintaining said outnevases put electrode normally negative relative to at least part of said amplifier cathode, means for causing said output electrode to become more negative in the presence of an amplified signal, and a direct-current connection between said control electrode and said output electrode, whereby the amplification or said amplifier is regulated automatically.

2. In a carrier-current signaling system, in combination, a vacuum tube amplifier having a cathode and a control electrode, a vacuum tube detector coupled directly to the output of said amplifier, said detector having'y a cathode and an output electrode, means for maintaining said detector cathode at substantially the same potential as said amplifier cathode, means for maintaining said output electrode at a negative potential with respect to said cathodes, means causing said output electrode to become more negative in the presence of an amplified signal, and a directcurrent connection between said output electrode and said control electrode.

3. A combination according to claim 2 in which the means for maintaining the detector output electrode at a negative potential with respect to said cathodes is a resistance connected between said output electrode and the detector cathode.

4. In a modulated carrier-current signalling system employing a carrier-current amplifier and rectifier, which rectifier produces a modulated uni-directional voltage, a direct-current connection from said rectifier to an element of said am lifier whereby the amplification is regulated) automatically, and a connection from said rectifier to a modulation current amplifier whereby the signal is further amplified, said connection from said rectifier to said modulation current amplifier including a condenser in series for preventing the uni-directional component from being impressed upon the input of said modulation current amplifier.

In a signaling system, a vacuum tube amplifier having a cathode and a control electrode, a detector coupled to said amplifier, said detector having an output electrode, means for maintaining said output electrode normally negative relative to at least part of said amplifier cathode, means for causing said output electrode to become more negative in the presence of an amplified signal, and a direct-current connection between said control electrode and said output electrode, whereby the amplification of said amplifier is regulated automatically.

6. In a signaling system a vacuum tube amplifier having a cathode and a control electrode, a second vacuum tube having an out` put electrode, means for coupling the output of said amplifier with said second tube, means for maintaining said output electrode normally slightly negative relative to at least part of said cathode, means for causing said for causing the said output electrode to become more negative in the presence of an amplified signal, a tuning arrangement for tuning said amplifier to a desired signal, a direct current connection between said control electrode and said output electrode, whereby the amplification of said amplifier is regulated automatically, and means for visually indicating the condition of resonance 1n saidtuning arrangement, whereby tuning is facilitated.

8. An arrangement according to claim 7 in which said tuning arrangement and said means for visually indicating the condition of resonance are connected in the anode circuit of said amplifier.

9. In a signaling system a vacuum tube amplifier having a cathode and a control electrode, a vacuum tube detector coupled to the output ofsaid amplifier, said detector having a cathode and an output electrode, means for maintaining said detector cathode at a potential greatly negative relative to said amplifier cathode, means for maintaining said output electrode at a potential normally slightly negative relative to said amplifier cathode, means for causing said output electrode to become more negative in the presence of an amplified signal, and a-direct-current connection between said output electrode and said control electrode whereby the amplification of said amplifier is regulated automatically. l0. In a signaling system, a vacuum tube amplifier having a cathode and a control electrode, a diode detector coupled to said amplifier, said detector having an anode, means for maintaining said anode normally negative relative to at least part of said amplier cathode, means for causing said anode to become more negative in the presence of an amplified signal, and a direct-current connection between said control electrode and said anode, whereby the amplification of said amplifier is regulated automatically.

1l. In a signaling system, a vacuum tube amplifier having a cathode and a control electrode, a diode detector coupled to said amplifier, said detector having a cathode and an anode, means for maintaining said cathodes at substantially the same potential, means including a high resistance connected between the detector anode and cathode for maintaining said anode normally slightly nega-tive relative to said cathodes, means for causing said anode to become more negative in the presence of an-amplied signal, and

a direct-current connection between said control electrode and said anode, whereby the amplification of said amplier is regulated automatically. 12. In a signaling system a vacuum tube amplifier having a cathode and a control electrode, a second vacuum tube fhaving an output electrode, means for coupling said amplifier with said second tube, means for maintaining said output electrode normally slightly negative relative to at least part of said cathode, means for causing said output electrode to become more negative in the presence of an amplified signal, a tuning arrangement for tuning said 'amplifier to a desired signal, and a direct-current connection between said control electrode and said output electrode, whereby the amplification of said amplifier is regulated automatically, and means for visually indicating the condition of resonance insaid tuning arrangement whereby tuning is facilitated.

In testimony whereof I afx my signature.

HAROLD A. WHEELER.

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