US2142095A - Detecting and amplifying system - Google Patents

Description

Jan. 3, 1939. ELLIOTT I v 2,142,095

DETECTING AND AMPLIFYING SYSTEM Original Filed March 15, 1932 2 Sheets-Sheetl INVENTORw O0 wofllzm'o l 1 BY 95 ATTORNEY.

Jan. 3, 1939. H. F. ELLIOTT DETECTING AND AMPLIFYING'SYSTEM 2 Sheets-Sheet 2 INVENTOR.

, zwozdzz wbit BY W ATTdRNEY.

Original Filed March 15, 193.")

Patented Jan. 3, 1939 UNITED STATES PATENT OFFICE Application March 15, 1932, Serial No. 598,941 Renewed March 5, 1937 6 Claims.

This invention relates to an electrical wave communication system utilizing carrier waves, and more particularly to a system for receiving, amplifying and detecting signals superposed upon carrier waves.

Such carrier wave systems are in common use today, either for line telegraphy or telephony;

or radio telegraphy or telephony. The present invention can be utilized in connection with all such systems. However, to provide one concrete example of the invention, it is described as incorporated in a broadcast radio receiver. Specifically, the invention further relates to such a receiving system that utilizes electron emission devices for amplification and detection.

Such electron emission devices are now gener-- ally used for these purposes. Broadly considered, they include an evacuated envelope in which several electrodes are enclosed. One electrode acts as a cathode; that is, an electron emitter. It can comprise a filament heated by the passage of electricity through it; or alternatively, it can comprise a compound that serves as a source of a copious stream of electrons when heated, as by the aid of an electric heater unit, embedded in the compound,

The electrons emitted from the cathode are usually captured on an anode or plate, also usually enclosed in the envelope and surrounding the cathode. This anode is kept at a potential positive with respect to the cathode, as by the aid of an external circuit connecting the cathode and anode. The flow of electrons from cathode to anode in the envelope forms the space current.

The intensity or quantity of electron flow, and consequently the space current, can be controlled by the aid of a control electrode interposed between the cathode and anode. This electrode is usually in the form of a grid or a coil of a conductor, wound to provide spaces between the helical turns, and surrounding the cathode. By varying the potential difference between the oathode and the control electrode, as for instance in accordance with the impulses to be amplified, the space current can be varied within comparatively wide limits. This variation in potential difference is accomplished by the aid of an input circuit connecting the cathode and the control electrode externally of the evacuated envelope.

When the control electrode swings positive with respect to the cathode, the space current increases; and when the control electrode swings negative, the space current decreases.

Thus an amplification is effected by the device, since comparatively minute signaling impulses,

effective in the input circuit, cause correspondingly great variations in the space current. These variations can aifect an output circuit connecting the anode and cathode.

The degree of amplification of any device of this character can be adjusted as by biasing the potential of the control electrode. By making this control electrode negative when no signals are present, as by a battery included in the input circuit, the space current can be reduced very 10 materially; and if this negative potential is made high enough, the amplification factor can be brought down to a low value. These considerations are made use of in connection with this invention in a manner that Will now be discussed.

At the present time, there are a large number of broadcast transmitting stations, located in all parts of the country. An individual receiver at any particular locality is usually equipped with enough stages of amplification to bring in sta- 20,"

tions quite remote from the receiver. It is apparent that the degree of amplification must be reduced very materially for reception from a near-by or local station; that is, volume control must be resorted to if the quality of the reception is to be satisfactory. Otherwise, the badly overloaded tubes or electron emission devices, and particularly the detector tube, cause serious distortion. Various means for effecting this volume control have been proposed; as for example, 1

combined automatic and manual control; but in this instance the volume control is automatic over the whole range of operation and responds to the conditions of operation of the detector stage.

The arrangement is such that upon any increase in volume passing through the detector beyond a definite level, a sharp and immediate reduction in amplification by the prior stages is effected.

This object is accomplished by utilizing the detected signaling impulses for building up a potential difference that can be made effective on one or more of the control electrodes in the amplifier stages preceding the detector stage. This automatic volume control covers the entire range, so that no manual operation is necessary 4.5

to reduce the signal volume of even very close locals to the desired value. Furthermore, the tuning for local stations is automatically made broader so that the quality is improved.

In connection with the detector stage, the invention also presents distinct advantages. In one common mode of detection, a three-electrode electronic emission device is used. The signaling impulses, which when passed to the detector stage, are in the form of modulations of a car- 55.

rier frequency wave, must be segregated from the carrier. The three electrode device is caused to act as a rectifier to accomplish this purpose, the rectification taking place between the cathode and the control electrode.

These rectified impulses are used to charge a condenser which is discharged through a resistance across the condenser. This discharge, by appropriate design of the condenser andits leak resistance, can be so timed as to be effective in accordance with the signaling impulses. The

electronic emission device then acts to amplify.

these discharged impulses. The output circuit thereof can be used to transmit'the signaling impulses to additional amplifiers, or to. a translating device such as a loud speaker or recorder.

This well-known system of detection has some serious disadvantages. The detector is effective only for half-wave rectification; and furthermore, the space current carries not only the component representing the signal, but also another component corresponding to the carrier wave. Thus the detector stage is apt to be overloaded by such extraneous or unnecessary currents;

With the aid of the present invention, this unnecessary loading is entirely obviated. This is accomplished by the aid of an electronic emission' tube of special construction, so arranged that carrier waves impressed on the input of the detector are automatically balancedout, and no current of the carrier frequency is present in the space discharge. Furthermore, the signaling impulses are obtained by a full-wave rectification of the carrier wave with its superposed signaling current.

This invention possesses many other advantages, and has other objects which may be made more easily apparent from a consideration of several embodiments of the invention. For this purpose there are shown a few forms in the draw ingsaccompanying and forming'part of the present specification. These forms shall now be described in detail, which illustrate the general principles of the invention; but it'is to be understood that this detailed description is not to be taken. in alimiting sense, since the scope of the invention is best defined by the appended claims.

Referringto the drawings:

-Figure. l is a diagram of an electron emission device, or audion, that is constructed in such manner 'as tov make it possible to practice the invention;

Fig. 2 is a wiring diagram showing the invention incorporated in a superheterodyne receiving system; and

Figs. 3, 4, 5 and 6 are fragmentary diagrams of slightly-modified forms of theinvention.

Although theparticular structure ofthe tube that can be used in the practice of the invention ,may be quite widely varied, itis preferred to use a tubeconstructed substantially as shown in the diagram of Fig. 1. In'this' diagram; the evacuated envelopa|- is shown as of the usual bulb form, provided as is also customary, with appropriate supports and leading in conductors for the electrodes. The cathode 2. is shown as a centrally disposed cylinder, having a leading-in conductor 3, and provided with a heater, the leads of which are shown at 4 and 5. The exterior surface of cathode 2 can be appropriately coated for electron emission. The anode 6 can be of hollow cylindrical form, surrounding the cathode 2 and having a leading-in conductor 1.

Disposed'b'etween the cathode 2 and anode 6 are a pair of control electrodes or grids, 8 and 9.

Both of these electrodes can be in the form of a helix or coil of wire, the turns of which are alternated, and uniformly spaced. To indicate this arrangement clearly, grid 9 is shown by the aid of dotted lines, while grid 8 is shown by the aid of full lines. Due to the fact that the helices of both grids 8 and 9 have substantially identical space relationships to the cathode 2 and anode 6, as by making them occupy substantially the same geometrical surface, the electrical functions of the grids are balanced; and both have substantially equal effects upon the tube characteristics. The leading-in conductor for electrode 8 can be provided by a cap II] at the top of the tube; and the leading-in conductor for electrode 9 can be provided through the base as by element H. Alternatively, both may-be brought out through the base or through the cap.

Although the particular dimensions of the tube can obviously be varied to suit various conditions, for ordinary broadcast reception the general structure of the tube can be the same as the conventional radio receiver type amplifier tubes, such for example as the 227 type. Each of the grids 8 and 9 can be wound about twelve turns, more or less, to the inch. The wire forming the grids can be the usual type used for grid purposes, of the order of .008 inch in diameter.

Fig. 2 shows the tube used in a superheterodyne radio receiving circuit, for the second detector. The heater I2 is indicated as a filament, adapted to be supplied with heating current from a secondary winding I3 of a power pack transformer M. This transformer is adapted to be supplied from commercial mains, such as house lighting mains. The primary winding l5 ofthe transformer also energizes the usual secondaries l6 and l which are needed for providing the necessary direct current potentials for the anodes and control electrodes.

Thus secondary Hi can be connected to a fullwave rectifier tube I8, having a filament l9 supplied with heating current from the secondary Since the operation of such rectifier tubes is well understood, it is merely necessary to mention that the rectified current is supplied to the leads 20, 2|, and that various filter elements are connected in series and across the leads to reduce pulsations. Such elements can be, for example, the bridging condensers 22, inductances 23, 24, and-resistance 25. The negative main 2| can be grounded, as at 26.

The potential difference across mains 20, 2| can be applied to a series of potentiometer resistances, such as 21, 28, 29, to which connections can be made, as hereinafter described, to provide the desired positive potentials.

The operation of tube I will now be described. In this description, the origin of the impulses to be detected and amplified will not be carefully discussed, as such a discussion is, at the present place, unnecessary to an understanding of the operation of the detector. Further on in this specification, the remaining stages of the system will be fully disclosed.

It is presumed, then, for the present, that modulated carrier frequency current impulses are passed to a winding 30, as by a primary coil 3| coupled thereto. The opposite ends of this winding 30 are respectively connected to the control grids 8, 9. A tuning condenser 32 can be bridged across coil 30, for tuning circuit 30-42 to the carrier wave frequency, Cathode 2 is connected to the coil 30, as for example, its center point, to form a rectifying circuit, electrodes 8 and! acting as rectifying anodes for full wave rectification of the modulated carrier waves passed to winding 30. In this input circuit may also be included a means for impressing a potential on grids 8 and 9 that is positive with respect to the cathode, to the extent of two or three volts. This positive bias is useful for reducing distortion of Weak signals. It prevents the grids assuming an initial negative bias due to grid current or to extraneous potentials induced on the grids or their associated circuits.

This positive bias is obtained in this instance by the aid of a pair of potentiometer resistances 33, 34, bridging theleads 20, 2|. The potential difference across these leads is quite high, of the order of 250 to 275 volts, to provide the maximum anode potentials. Consequently, to obtain a few volts positive bias, the resistance 33 can be of the order of one megohm, and resistance 34 of the order of 10,000 ohms, so that the potential drop across this latter resistance will correspond to the desired positive bias. The rectifying circuit involving the electrodes 8, 9 and cathode 2 can thus include cathode 2, connection 35, resistance 34, another resistance 36, connection 31 to coil 30 (in this instance to the mid-point of coil 30), and the grids 8, 9.

It is apparent that carrier wave impulses affect electrodes 8, 9 with exactly opposite polarities. During one half-cycle, grid 9 is more positive with respect to cathode 2 than grid 8, so that the carrier wave current flows through this grid, connection 31, resistances 36, 34, to connection 35 and cathode 2. During the other half-cycle, grid 8 is active and the current flows in the same direction through the common path 3'I36 34-352. Thus the carrier wave is rectified in this common path, which is of very high resistance due to the inclusion of resistance 36, of about one megohm.

The results, so far as described, are thatthe signaling impulses are rectified for full waves, instead of only for half-waves, as in the conventional detector. Furthermore, the rectified potentials charge a capacity 38 such as indicated by dotted lines, which capacity may be supplied by the inherent stray capacity of the system; and the charge is allowed to leak off through the high grid leak resistance 36. But this potential difference across the grid leak 36 is effective to impress the same potentials on grids 8 and 9 in parallel, so that they act as a single grid for affecting the space current in tube I, for the rectified impulses. However, since grids 8 and 9 are always at substantially opposite potentials so far as the carrier waves are concerned, no carrier wave frequency currents can exist in the space current to anode 6.

It is therefore seen that the space current, corresponding solely to the detected impulses, is not uselessly made greater by any current at carrier wave frequency. Accordingly, the detector tube I can carry a large detected current without overloading the tube.

The electrodes 8, 9 act together as a control electrode for amplifying the detected impulses, and anode 6 accordingly receives electrons at a density corresponding to the detected signals. The direct current positive potential is impressed on plate 6 with respect to cathode 2 by the aid of a circuit including a carrier wave choke coil 49, a resistance 4 I, to positive main 20, then negative main 2|, connection 35, and cathode 2.

The rectified potentials across grid leak 36 are used to provide automatic and full range volumecontrol by appropriate connections to one or more of the input circuits of the preceding stages of amplifiers in a manner to be shortly described. For full range volume control, this rectified potential should be capable of reaching a value of the order of fifty to seventy-five volts. In order to obtain these high values when needed, it is essential to provide a comparatively high potential between the cathode 2 and the anode 6, and which automatically increases as the signal strength increases. It is in this way that distortion is prevented when using high negative biases for the automatic volume control. These results are attained by providing a comparatively large potential drop across mains 20, 2I, by appropriate design of the power circuits including rectifier tube I8; and by making resistance 4| at least equal to the internal resistance of the tube I at zero signals and preferably considerably greater. For ordinary tubes, this potential drop can be of the order of 250 volts, and resistance 4I may be of the order of 100,000 ohms or even as high as 300,000 ohms. This resistance, in conjunction with the normal anode-potential anode current characteristic of the tube I, causes the potential applied to the plate or anode 6 torise as the signal strength rises. Thus for zero signals, a 270 volt drop across mains 29, 2I may be divided so that there is a drop of 235 volts across resistance 4|, and a drop of 35 volts from the anode 6 to cathode 2. For a strong local station, the drop across the resistance 4I may be only 50 volts, and the drop; from anode to cathode, about 220 volts.

The use of resistance 4| in this way also obviously affects the gain of the amplifiers succeeding the detector I. For strong input signals the amplification is reduced because of the lowered potential drop across this resistance. For weak signals the amplification is correspondingly increased. The resistance 4I gives an extended cut-off of the detector I; in other words, the detector I continues to operate withthese controlling functions, from very weak signals to very strong signals.

Various by-pass condensers have been indicated in connection with the detector'stage, such as condenser 42 across resistance 34; condenser 43 between the anode 6 and the negative main 2I; and condenser 44 across the mains. The condensers 42 and 44 can be of the order of .5 microfarad, and condenser 43, which is used for carrier wave filter purposes, may be as low as .001 microfarad.

There will now be described one form of cascade amplification preceding the detector stage just described, so connected as to make use of the potential difierences across grid leak 36 for automatic volume control, whereby strong signals will produce a large negative bias capable of being used on thegrids of these preceding stages.

Thus there is shown a conventional radio pick-up circuit including the elevated conductors 45, inductance coil 46, and ground connection 41. The inductance coil 46 is coupled to a coil 48, forming part of a tunable radio frequency circuit including the variable condenser 49. The electromotive force across condenser 49 is impressed upon the input circuit of the radio frequency amplifier tube 50. This tube has a heater5I for a cathode 52, the heater 5| being supplied with heating current, as for example from the secondary coil I3 of the power transformer. The control electrode 59 and anode 53 are of conventional form, the anode 53 being surrounded by a screen grid 54, as is now common.

The input circuit of amplifier thus includes the tunable circuit 48-49, resistance 29, providing'a.,slight negative bias, and cathode 52. A by-pass condenser of about .5 microfarad can be used across resistance 29. The requisite positive potential of screen grid 54 can be obtained by connecting the screen grid through resistance 56 to the. topterminal of resistance 28 which is in the series of resistances 2128-29 forming a potentiometer. Resistance 56 is used as a filter resistance and can be of the order of 200 ohms. The positive potential of plate or anode 53 is provided through a circuit including the radio frequency choke coil 51, and resistance 58 to positive main 20. This resistance 58 is a filter resistance and can be of the order of 200 ohms.

Due to the varying potential differences between cathode 52 and control electrode 59, the space current between cathode 52 and anode 53 is'caused to varyin accordance therewith. This variation is impressed upon the first detector tube 60, through the medium of choke 51, coupling condenser 6| and tuned circuit 61. The cathode 63 of the tube can be heated by the heater 64, also supplied from secondary I3. The anode 65 and the screen grid 66 are of conventional form.

The input circuit between the cathode 63 and control electrode 62 includes the tunable radio frequency circuit 61, resistances 29 and 68, coil 69 and cathode 63. The resistance 68 which can be of the order'of 2000 ohms is used to provide the proper initial negative bias for the control electrode 62. The coil 69 acts as the recipient of the. heterodyning frequency generated by the oscillator tube 10.

In the present instance, oscillator tube 10 is shown as having a heater type cathode 1| heated by element 12. It also has the anode 13 and control electrode 14. A tunable circuit 15 is connected between the control electrode and cathode through a grid capacity 16. This circuit is tuned for the heterodyning frequency. The grid resistance 11 is shunted directly across the electrodes 1|, 14 and may be of the order of 100,000 ohms.

The plate or anode circuit of the oscillator 10 is coupled to. the coil of the tunable circuit 15 as well as to the coil 69 in the input circuit of intermediate detector 60. This is accomplished by the aid of a coil 18. The anode circuit for the oscillator thus includes coil 18, resistance 56 to potentiometer 212829, to the negative lead 20; through any of the various grounds, to the grounded cathode 1| of the tube 10. Since the anode circuit and the grid-cathode circuit are :thus interlinked, oscillations are generated in the tunable circuit 15, which are passed to coil 69.

The output circuit of the tube 60 is coupled to the input of the intermediate frequency amplifier tube 19. The screen grid 66 of tube 60 is shown as maintained'at the same positive potential as screen grid 54.

The coupling of the two stages 60 and 19 can be accomplishedby the aid of a tunable circuit in the anode circuit of tube 60. This circuit can be tuned to the intermediate frequency. The anode circuit for tube 60 also includes a connection through resistance 58 to the positive main 20.

The intermediate frequency amplifier 19 has a heater type cathode 8|, its heater 82, control electrode 83, anode 84 and screen grid 85. Its input circuit includes cathode 8|, to a tap on potentiometer 21-28-29 for including an initial negative bias in the input circuit, thence from negative main 2|, any ground connection, through connection 86, condenser 810, the tunable circuit 89, back to thegrid 63. The capacity 81C is used for automatic volume control in a manner to be hereinafter described.

The output circuit of the intermediate frequency amplifier 89 includes the tunable circuit 3|-90 in which coil 3| is coupled, as heretofore stated, to the input ofdetector I. The screen grid is connected as by a lead 9| to an appropriate point on potentiometer 212829 for obtaining the proper screen grid positive potential.

A condenser 81 is connected through resistor 92, which may be of the order of one megohm, across the grid lead resistance 36 so that it is charged in accordance with the potentials across this grid leak. Automatic control of volume is obtained by applying the potentials across condenser 81 to the grids of the preceding amplifier and detector tubes, through the medium of bus 88, and resistors 92A, 92B and 92C, which may be of the order of one-half megohm. Bypass capacities 81A, 81B and 816 may be of the order of 0.05 microfarad.

For very strong signals the arrangement can be such that relatively high potentials are de veloped across grid leak 36. This is accomplished without appreciable distortion in tube Ibecause of the high anode potential used therefor.

The output side of tube I can be coupled to an audio frequency amplifier 93 as by the aid of a coupling condenser 94 and a potentiometer resistance 95. The input of audio frequency amplifier 93, which may be of the pentode type, thus includes the control electrode 96, a tap 91 on resistance 95, a filter resistance 98 and C-bias resistance 25, grounded connection to the center of secondary I3, and cathode 99. The potentiometer resistance can be of the order of 500,000 ohms. Various filter or by-pass capacities such as I00, IOI, I02 can be provided between the various screen grids and the grids to the cathodes.

The anode I03 of the power amplifier 93 can be connected through a coupling coil I04 to an intermediate grid I05. Another grid I08 at the same potential as the cathode 99 can be used to prevent secondary emission from anode I03, as is common in pentode tubes. Connection I01 to positive main 20 supplies a positive potential to the electrodes I03 and I05. The coil I04 forms theprimary of an audio frequency transformer I08. The secondary I09 of this transformer is connected to the movable coil of an electrodynamic speaker H0. The stationary field for the speaker can be provided by the filter inductance 24 in the main 2|.

The volume level of the output can be set by hand bythe aid of tap 91 on resistance 95. It is noted particularly that this volume level is obtained by elements occurring subsequently to the detection stage.

The audio frequency amplifier can be arranged in other ways than that shown in Fig. 2. For example, in Fig. 3 the-output circuit of detector I is shown as including the high resistance III which serves the same purpose as resistance 4| of Fig. 2, as' well as resistance 95 of Fig. 2. Potentiometer tap I I2 serves to adjust the volume level by adjusting the potential drop across the primary coil 3 of an audio frequency transformer II4. This primary circuit also includes the coilpling condenser H5 and the lower portion of resistance I I I.

The secondary coil I I6 of transformer II 4 is connected at its opposite ends to the control electrodes H1 and H8 of a pair of tubes II9, I20,

arranged in push-pull relation. The center tap of coil H6 is connected by lead I20 to the source of negative bias for the grids H1, H8. The output circuit of the push-pull system includes the anodes I2I and I22 in parallel together with the upper and lower halves I23 and I24 of the output transformer I25. The center tap of the primary I23, I24 is connected to the positive main 20 to provide the positive potentials for plates I2I and I22. The moving coil of the dynamic speaker I20 is connected to the secondary coil I21.

The push-pull arrangement can also be varied, as for example, as shown in Fig. 4. In this case the output of detector I includes the primary I28 of an audio frequency transformer I29, as well as the high resistance I30 which corresponds to the resistance 4| of Fig. 2. A filter capacity I3I can be used across source 20--2I and resistance I30.

The secondary coil sections I32 and I33 are connected respectively to potentiometer resistances I34 and I35. These can be used for volume control in place of the potentiometer resistance I II of Fig. 3, as by providing the taps I 36 and I3! for the control electrodes I38, I39 of the tubes I40, I4I arranged in push-pull relation. The output circuits of these tubes I40 and MI are arranged similarly to output circuits H9 and I20 of Fig. 3.

I have also found that it is immaterial to some extent how resistances 36 and 92 are connected in input circuit 30-32. Thus Fig. 5, instead of connecting the resistance 36 to the midpoint of coil 30, it can be connected to the lower terminal thereof; and resistance 92 can be connected to the upper terminal thereof.

Another variation is shown in Fig. 6 in which both resistances 92 and 36 are shown as connected to the lower terminal of coil 30 instead of to the center point of this coil.

I claim:

1. In a carrier wave communication system, one or more amplifier stages each employing an electronic emission device having a cathode, an anode, and a control electrode; a detector stage employing an electronic emission detector having a cathode, a pair of rectifying anodes and a main anode, the rectifying anodes being located between the cathode and the main anode and capable of passing the electrons from the cathode to the main anode, whereby one or both rectifying anodes can be used as a control electrode; means comprising a center tapped coil for applying the carrier wave from the last amplifier stage across the rectifying anodes; a capacitive connection between the center tap and the detector cathode, the capacity in the connection being thus charged with both half-waves of each carrier wave cycle; means for increasing the average potential difference between the cathode and the main anode in response to an increase in the amplitude of the impulses passed to the detector, whereby large potential diiferences can be supplied to the capacity in the connection between the center tap and the cathode without distortion of the impulses in the space current of the detector; and means for applying the potential difference across the capacity to one or more of the preceding stages to control the amplification thereof.

2. A carrier wave receiving system, including a detector tube having a cathode, an anode, and twin control electrodes; an input circuit including a center tapped coil between the twin control electrodes and the cathode; an output circuit for the tube; said output circuit including a potentiometer for controlling the output potential effective in the output circuit; one or more amplifier stages preceding the detector tube; and means in the control electrode circuit whereby rectified potentials from the detector stage are fed back from the input circuit to one or more of the preceding amplifier stages for automatially controlling the volume of the output of the system.

3. A carrier wave receiving system, including one or more amplifier stages each employing an electronic emission device; a detector tube having a cathode, an anode, and twin control electrodes symmetrically arranged with respect to the cathode and the anode; an input circuit including a center tapped reactor for said tube; an output circuit for said tube, said output circuit including a source of potential for impressing a positive potential on the anode, and also including means operating in response to increases in the amplitude of the carrier wave potential diiferences applied to the detector for proportionately increasing the potential difference between the anode and the cathode; and a circuit associated with the input circuit of the detector whereby rectified potentials from the detector stage are fed back from the input circuit to one or more of the preceeding amplifier stages for automatically controlling the volume of the output of the system, said circuit including an impedance and a capacity, and connections between said impedance and capacity and the inputs of said preceding stages.

4. A carrier wave receiving system, including one or more amplifier stages each employing an electronic emission device; a detector tube having a cathode, an anode, and twin control electrodes symmetrically arranged with respect to the cathode and the anode; an input circuit for said tube, said input circuit including a source of positive bias for the twin control electrodes and a center tapped transformer winding interconnecting the twin electrodes and the cathode; an output circuit for said tube, said output circuit including a source of potential for impressing a positive potential on the anode, and also including means operating in response to increases in the amplitude of the carrier wave potential differences applied to the detector for proportionately increasing the potential difference between the anode and the cathode; and a circuit associated with the input circuit of the detector whereby rectified potentials from the detector stage are fed back from the input circuit to one or more of the preceding amplifier stages for automatically controlling the volume of the output of the system, said circuit including an impedance and a capacity, and connections between said impedance and capacity and the inputs of said preceding stages.

5. A carrier wave receiving system having a series of electronic emission amplifier tubes with their associated circuits; a detector tube succeeding the amplifier tubes and provided with twin control electrodes symmetrically arranged with respect to a cathode and an anode in said detector; said twin electrodes being intermeshed so as to occupy alternate spaces in substantially the same geometric surface; means including a center tapped transformer winding whereby said twin electrodes serve to rectify both half-waves of each carrier wave; an input and an output circuit for said detector so arranged that the twin electrodes have a neutralizing effect upon the carrier wave impulses so that the detector amplifies only the signaling impulses to the substantial exclusion of the carrier wave impulses; means for accumulating in the input circuit a charge corresponding to the rectified impulses and of suflicient potential difference for impressing a potential difierence on at least one of the amplifier stages to provide full range automatic volume control of that stage; and means including a resistor in the detector output circuit for increasing the potential diiference across the detector output terminals at high input levels, thereby minimizing distortion.

6. The method of increasing the output and reducing distortion in a grid leak detector having one or more grids, which comprises reducing anode rectification by rectifying substantially all of the input impulses before reaching the anode, reducing weak signal distortion by biasing the grids initially positive relative to the cathode and reducing overload distortion by increasing the anode-cathode potential as the applied signal increases.

HAROLD F. ELLIO'I'I.

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