US2432560A - Carrier current telephone apparatus - Google Patents

Description

Dec, 36, 1947. R. K. CROOKS Q 2,432,560

CARRIER CURRENT TELEPHONE APPARATUS Filed March 18, 1946 INVENTOR. Haiphii poahw.

' ari iwg Hi5 Ammwx Patented Dec. 16.1947

UNITED STATES PATENT OFFICE CARRIER CURRENT TELEPHONE APPARATUS Ralph K. Crooks, OHara Township, Allegheny Cunty,.Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application March 18, 1946, Serial No. 655,071 13 Claims. (01. 1792.5)

1 2 My invention relates to communication appathe transmitting tube to provide a substantially ratus, and more particularly, to portable carrier uniform degree of modulation for all settings. current telephone apparatus. Other objects, features and purposes of my in- The principal object of my invention is the vention will be pointed out as the description proprovision of apparatus suitable for use on rail- 5 ceeds.

roads or elsewhere for emergency communica- I shall describe two forms of apparatus emtion between two local stations or between such bodying my invention, and shall then point out stations and a central ofiice over existing telethe novel features thereof in claims. phone, telegraph or signal line circuits without Referring to the accompanyin drawings, Fig. interference with the services normally using 1 illustrates one form of apparatus embodying such circuits, or under conditions involving the my invention in which the switching from the failure of the usual communication circuits. receiving to the transmitting condition is accom- In its preferred form my invention is embodied plished, in part, by the provision of a relay R in in a set of electron tubes and associated appathe unit. while Fig. 2 illustrates another form in ratus mounted in a readily portable unit to which which the switching is o p ed o y by a telephone hand set is attached. Power may changing the potentials appli d t the t e be supplied to the unit by connecting it to an al- Similar reference characters refer to similar ternating current lighting circuit, and the unit parts in both Viewshas two line terminals by which it may be con- Referring to Fig. 1, the reference character I nected between a pair of wires on a pole line, or designates a twin tube containing in a single bulb between a line wire and ground, to provide voice two voltage amplifier triodes VI and V2, while communication between the set and one or more tube V3 is a power amplifier tetrode. Direct cursimilarly connected units at distant points to rent is supplied to the positive terminal B of the which the line wires extend. unit, and to the negative or ground terminal G, The units are adapted to transmit and receive connected to the frame of the unit, by a power amplitude modulated carrier currents of a suitsupply it of v n l m comprising a able frequency such as 35 kilocycles per second. transformer h g its p i y Winding 2 All units employ the same carrier frequency, and ranged for connection to the local alternating each hand set is provided with push-to-talk current lighting circuit as indicated by the legend button to change the unit fr m th receiving t A. C. on its terminals. The end terminals of the transmitting condition. No calling device'is e o d y Winding 3 are connected to t e needed, as the apparatus is intended principally plates of a f Wave rectifier t d the for use under conditions where an operator will middle terminal of secondary winding 3 is conbe listening for calls. nected to ground. The filament of tube V4 is en- One object of my invention is the provision of 35 ergizcd m a Secondary Wihdihg 4 of transformmodulated carrier current transmitting and red s co e t o h a filt r c il 5 to ceiving apparatus which ha a, minimum nu b terminal B, with a filter condenser C4 connected of parts, and which can be quickly adjusted to across terminals B and function efficiently either as a receiver or as a The h at s of tubes V2 and V3 are e transmitter under widely different line condigiZed from another s n a y gs of transtions. formerT l, the heater circuits being omitted to A feature of my invention is the provision of simpl y the d novel means for controlling the unit by a push Terminal B Supp h v lta e to the screen button so as to enable the button to be located grid of the tetrode V3. d t o h Coil 1 of a in the hand set but without requiring additional transmitting filter coupling C0i1T3 t0 the'plate wires leading thereto. In accordance with this of tu t cat f whi h'is connected to feature of my invention, a voltage divider is proground through a biasing resistor 8 and a byvided in the unit by which the potentials appass condenser plied to the electron tubes are controlled, which The resistor and the Winding o relay R is so arranged that the unit may be changed are connected in series across the supply termifrom the receiving to the transmitting condition nals B and G and constitute a voltage divider for by closing a circuit over a push button contact providing a reduced voltage for energizing the which shunts a portion of the voltage divider. tubes VI and V2, relay R being normally ener- Another feature of my invention resides in gized, as shown, in parallel with tubes VI and V2. improved means for regulating the output of The output or plate to cathode circuit of tube VI extends from terminal B over resistors I and I I, front contact a of relay R to the plate of tube VI, the cathode of which is connected directly to ground. The input or grid to cathode circuit of tube VI includes resistor I2 and condenser I3 and the winding Id of a receiving filter or coupling coil TI. The Winding I4 is tuned to resonance by a condenser C2, and is coupled inductively to the line winding I5 which is tuned to resonance by the condensers CI and connected over front contacts I) and d of relay R and condensers CI to the line terminals LI and L2.

The coupling coil Tl serves as a filter to selectively pass modulated carrier current, and tube VI is connected to function as a detector of the voice frequency components of the carrier current as will be readily apparent. The carrier frequency component of the current in the output circuit for tube VI is by-passed through condensers I6 and I! to ground, while the voice frequency components pass through condenser l8 and a gain control resistor RR to ground, resistor RR having an adjustable connection to the grid, or input circuit of tube V2.

The output circuit of tube V2 extends from terminal B over resistor ID, and the primary Winding of transformer T2 to the plate of tube V2, the cathode of which is connected through a biasing resistor I9 to ground, so that tube V2 amplifies the voice frequency currents and supplies them to the telephone receiver H connected to the secondary winding of transformer T2,

The receiving system of Fig. 1 thus comprises the selective filter TI, the detector VI, audio amplifier V2, transformer T2 and the telephone receiver H.

To establish the transmitting condition, the operator presses the button P on the hand set which shunts relay R by a low resistance circuit portion comprising primary Winding 20 of transformer T3 and the microphone M of the hand set, through which sufficient current fiows to render the microphone active, but reducing the voltage across relay R to such a low value that tubes Vl and V2 are disabled, and relay R releases.

When relay R releases, its back contacts 0 and 6 connect the line Winding 2I of the transmitting filter T3 through the condensers C3 across the line terminals LI and L2, in place of the receiving filter TI, and the output circuit of tube V: is supplied with current from terminal B over resistor 22, winding 23 of the receiving filter TI, resistor 24 and back contact a of relay R. The winding I4 of filter TI which is tuned by condenser C2 to the carrier current frequency and is connected to the input circuit of tube VI, is thereby set into oscillation, and serves as a tank circuit to cause tube VI to supply current to winding 23 of filter TI to maintain the oscillations. In the transmitting condition, tube VI thus serves as an oscillator to generate carrier current, but inasmuch as the circuit for winding I5 of filter TI is now open, the current in coil I l and condenser C3 will tend to oscillate at a slightly different frequency from that to which they are resonant in the receiving condition. It is desirable that the received and transmitted frequencies should be substantially the same, and to this end compensation is provided by including the resistor 24 in the circuit of winding 23 to shift the phase of the voltage fed back to the tank circuit of the oscillator by the required amount.

The carrier frequency voltage from the lefthand terminal of resistor 22 to ground, due to the oscillations generated by tube VI, is impressed upon the gain control resistor RT through con densers 25 and 26, which are of relatively small capacity. Resistor RT has an adjustable connection for applying a portion of this voltage to the input circuit of tube V3, the output of which is supplied through the coupling coil T3 and condensers C3 to the line terminals LI and L2. The secondary winding 21 of transformer T3 is connected across resistors RT and 28, in series, and it will be seen that as the sound Waves to which microphone M responds cause current variations in winding 20, a voice frequency voltage is generated in winding 21 which varies the bias of the grid of tube V3 to correspondingly modulate the carrier current supplied by tube V3 to the line circuit.

The resistor 28, by-passed by condenser 26, is included in the secondary circuit of transformer T5 in order to permit the gain control resistor RT to be set at any position without altering the degree of modulation of the carrier frequency delivered by tube V3. To make this clear I have shown a condenser CT in the drawing, representing the stray capacity of the grid of tube V3, which it will be seen shunts the upper portion of resistor RT, and so tends to cause the change in the carrier voltage applied to the input circuit of tube V3 in response to a given change in adjustment of resistor RT to be less than the change in voice frequency voltage, since the latter is not afiected materially by the p.esence of the small capacity CT. On the other hand, the presence of resistor 28 in the circuit has but slight effect upon the carrier voltage due to the pro vision of the by-pass condenser 26. Resistor 28 reduces the effect of a change in adjustment of resistor RT with respect to the voice frequency voltage, to correspond. with the reduced change in carrier voltage due to the effect of the stray capacity CT. That is to say, for any setting of resistor RT the ratio of the modulation voltage to the carrier voltage is substantially constant.

The transmitting system of Fig. 1 thus comprises a source of carrier current comprising the oscillator tube VI, the tank circuit I4C2, and the coupling coil 23; a source of voice fr;quency current comprising the microphone M and transformer T6; a grid-modulated transmitting amplifier V3, and a transmitting filter T3.

It has been stated that the units are intended for use over existing communication circuits without interference with the services normally using such circuits. This is accomplished by the use of filters such as TI and T3 which are signed to present a high impedance to all frequencies outside the band of frequencies em ployed by the units. The tuned impedance presented to the line by the receiving filter TI to currents Within that band is also made relatively high in order to permit several units to be bridged across the line without causing serious attenuation of the transmitted signals.

Referring now to Fig. 2, this form of my invention differs principally from Fig. 1 in that the change from the receiving to the transmitting condition in response to the operation of the push-to-talk button P is obtained wholly by the resulting changes in the potentials applied to the tubes without the use of a relay R as in Fig.1. I

In Fig. 2, the power supply unit comprising transformer T5 and tube V4, etc., is the same as in Fig. 1 except that its output terminals are designated B and N, the lower terminal N being made negative with respect to ground by connecting resistors 3!, 32 and 33 across the terminals B and N and grounding the junction of resistors 32 and 33 as shown. Tubes Vi and V2 function as a detector and audio amplifier in the receiving condition, and tube V3 as a grid modulated amplifier in the transmitting condilion, as in Fig. 1, and since Fig. 2 lacks means for changing the connections of tube Vi provided in Fig. 1 by relay R, a separate oscillator tube V5 is provided having a tank circuit comprising the coil T5 and condenser 05, similar to the winding i l of filter Ti and condenser Cl, and the transmitting filter T3 is arranged to serve as a receiving filter as well, by the provision or a winding 34 tuned by a condenser C6 and loosely coupled to the line winding 2!.

The output circuit of tube V3 in Fig. 2 extends from terminal B through winding 1 to the plate of tube V3, and from its cathode through resistor 8 to ground, and thence through resistor 33 to terminal N, and the grid of tube V3 is connected through resistors RT and 28 to terminal N, which maintains a negative biasing potential on the grid of tube V3 suflicient to cut off the plate current and render tube V3 inactive so long as the unit is in the receiving condition.

The output circuit for tube V5 extends from terminal B through resistor 35 to the plate of tube V5, and from its cathode through the lower portion of coil T5 to ground and thence through resistor 33 to terminal N. The grid of tube V5 is connected through resistor 36 to terminal N so that tube V5 is also biased negatively to a point which renders it inactive when the unit is in the receiving condition.

In Fig. 2, the potential difference across resistor 33 is applied to the detector tube Vi, the plate to cathode circuit of which extends from terminal 13 over resistors 3i and 32 to ground, and thence in parallel with resistor 33, from ground through the gain control resistor RR to the plate of tube Vi, and from its cathode to terminal N. When modulated carrier current is received from the line by the filter T3, the resonant voltage induced in its winding 34 is supplied through a small condenser 30 to the grid of tube Vi, which is also connected through a high resistance 3? to terminal N, tube Vi serving as a detector, as in Fig. 1. The carrier frequency components of the output of tube Vi are by-passed to terminal N through condensers 38 and 39, and the voice frequency components are impressed on the gain control resistor RR and adjustably applied through condenser ifi to the input circuit of tube V2. The output circuit of tube V2 in Fig. 2 extends from terminal B through transformer T2 to the plate of tube V2 and from its cathode to ground, and thence through resistor 33 to terminal N, and the voice frequency components of the output of tube V2 are supplied through transformer T2 to the telephone receiver H.

In Fig. 2 the microphone M and winding 29 of transformed T6 are connected across resistor 32 and are supplied with a low value of current in the receiving condition through resistors 31 and 33.

When button P is pressed to establish the transmitting condition, its contacts connect terminals N and G together, short circuiting resistor 33. This disables the detector tube Vi, the plate to cathode circuit of which is connected across resistor 33, and the current through the microphone M is increased to a suitable operating value, while the negative bias on the grids of tubes V5 and V3 is decreased to a value proper for their operation. The tank circuit comprising coil T5 and condenser C5 is set into oscillation at the carrier frequency to energize the input circuit for tube V5, the output of which is supplied to the lower half of coil T5 to maintain the oscillations and is impressed on the transmitter gain control resistor RT through condensers 25 and 26. The voice frequency voltage developed by microphone M is impressed by transformer T6 on resistors RT and 28 so that tube V3 functions as a grid modulated transmitting amplifier as in Fig. 1, to supply its output through the filter T3 to the line terminals Li and L2.

It will be seen that the filter T3 continues to supply voltage to the grid of the detector tube VI, when button P is pressed, but the plate, grid and cathode of tube Vi are then grounded, and tube VI is thereby disabled and rendered incapable of energizing the input circuit of tube V2. The plate circuit of tube V2 is constantly energized in Fig. 2, and it would be capable of energizing the telephone H if tube Vi were not fully suppressed or if voltage were supplied to the grid-cathode circuit of tube V2 by a stray field, and to prevent the response of the receiver H under such conditions, button P is provided with an additional contact in Fig. 2, by which transformer T2 is short circuited when button P is pressed.

In Fig. 2, the tuned impedance which the apparatus presents to the line is made relatively high, as in Fig. 1, but since in Fig. 2 the same coupling coil is used for transmitting, its impedance will generally be higher than the optimum value at which tube V3 delivers its maximum output, in which case the apparatus of Fig. 2 would not be capable of transmitting as much power as that of Fig. 1. For this reason the amplifier tube V2 is arranged to be supplied with a higher plate voltage in Fig. 2 to increase the power supplied to the receiver H thereby compensating for the decreased power of the received signals.

Although I have herein shown only two forms of apparatus embodying my invention, it is to be understood that various changes and modifications may be made therein within the scope of the appended claims Without departing from the spirit and scope of my invention.

Having thus described my invention, what I "claim is:

1. Carrier current communication apparatus comprising a telephone receiver and a microphone, an electron tube detector, an electrical filter including resonant circuit elements tuned to a carrier current frequency having input terminals for connection to a line circuit and output terminals connected to the input circuit of said detector tube, means for energizing said telephone receiver in response to the voice frequency components of the output of said detector tube, a normally inactive transmitter tube having an output circuit energized from a direct current source, said circuit including elements tuned to a carrier frequency and adapted to deliver the output of said transmitter tube to said line circuit,

an input circuit for said transmitter tube connected to a normally inactive source of carrier frequency current and also to a source of voice frequency current derived from said microphone, a Voltage divider connected across the terminals of said direct current source for supplying a reduced voltage to said detector tube and said microphone, manually operable contact means for shunting a portion f said voltage divider "to disable sa-id detector tube and to render the iniemphene effective 't'o impress voice frequency variations"o'n'the input circuit of 'said't'r'ansmitt'e'r tube, and means responsive '-to the shunting 'of said voltage divider by said push button circuit for rendering said source of'ca'rrie'r current efiective "to energze the input circuit of said transmitter tube and to render said transmitter tube "effective to deliver its output to said line circuit.

2. Carrier current communication apparatus comprising a telephone receiver and 'a 'microphone, an electron tube detector, an electrical filter including resonant circuit elements tuned to a carrier current frequency having input terminals for connection to a line circuit and output terminals connected to the input circuit of saiddetector tube, means for energizingsaid telephone receiver in response to the voice frequency components of the output of said detector tube, a normally inactive transmitter tube having an output circuit energized from a direct current source, said circuit including elements tuned to a carrier frequency and adapted to deliver the output of said transmitter tube to said line circuit, an input circuit for said transmitter tube connected to a normally inactive source of car-' rier frequency current and also to a source of voice frequency current derived from said microphone, a voltage divider connected across the terminals of said direct current source -forsupplying a reduced voltage to said detector tube, comprising a resistor and the Winding-of 'arelay in series, manually operable contact means for shunting the winding of said relay to effect its release and to disable said detector tube, and means controlled by said relay when released for rendering said source of carrier current effective to energize the input circuit of said transmitter tube and to render said transmitter tube effective to deliver its output to said line circuit.

3. Carrier current communication apparatus comprising a telephone receiver and a micro- -phone, an electron tube detector, an electrical filter including resonant circuit elements tuned to a carrier current frequency having input terminals for connection to a line circuit and output terminals connected to the input circuit of said detector tubes, means for energizing said telephone receiver in response to the voice frequency components of the output of said detector tube, a normally inactive transmitter tube having an output circuit energized from a direct current source, said circuit including elements tuned to a carrier frequency and adapted to deliver the output of said transmittertube to said line circuit, an input circuit -for saidtransmitter tube connected to a normally inactive source of carrier frequency current and also to a source of voice frequency current derived from said microphone, a voltage divider connected across the terminals of said direct currentsource for supplying a reduced voltage to said detector tube, means responsive to the operation of said push button for shunting a portion of said voltage divider to disable said detector tube, and means including said voltage divider when shunted for rendering said-source of carrier current and said microphone efiective to energize-the input circuit of said transmitter tube andto render said transmitter tube eiiective tosupplymodulated carrier current to said line circuit.

4. In combination with a telephone handset "comprising a receiver, a microphone, and apush button; a detector tube having an input circuit coupled to a line circuit and an output circuit 'eouple'd to said telephone receiver, a normally inactive source of carrier current, a normally in- -'active 'transmi'tter'tube for supplying modulated said voltage divider across which said detector tube is connected to disable said tube, and means respons'ive'to the shunting of said portion for rendering said carrier current source efiective to energize the input circuit of said transmitter tube and for causing said transmitter tube to supply modulated carrier current to said line circuit.

5. Carrier current communication apparatus comprising a telephone receiver and a microphone, a detector tube, a transmitter tube, a source of direct current for energizing said tubes, a relay normally energized from said source in series with a resistor, an input circuit for the detector tube including resonant circuit elements tuned to a carrier frequency and coupled to a line circuit over front contacts of said relay, a

first output circuit for said detector tube connected across the winding of said relay over a front contact thereof and arranged to supply voice frequency components of its output to said telephone receiver, a second output circuit for said tube connected to said source over back contacts of said relay and arranged to supply carrier frequency output of said tube to its input circuit to cause oscillations of a frequency determined by said resonant circuit elements, means for sup plying oscillations from said second output circuit to the input circuit of said transmitter tube and for modulating said oscillations in accordance with current supplied by said microphone, an output circuit for said transmitter tube connected'to a tuned circuit connected across said line circuit over back contacts of said relay, and means including a normally open manually operable contact in series with said microphone for shunting said relay to effect its release and to render said microphone effective to modulate the oscillations supplied to said transmitter tube.

6. Carrier current communication apparatus "comprising a telephone receiver, a microphone and a push button, a relay controlled by said push button, a detector tube having an input circuit coupled to a line circuit over front contacts of said relay and an output circuit coupled to said telephonereceiver over a front contact of said relayya source of carrier current controlled 'over a back contact of'said relay, a transmitter tube having an input circuit coupled to said source of carrier current and to said microphone, and an output-circuit coupled to said line circuit over back contacts of said relay, a source of direct current for energizing said tubes across which the winding of said relay is connected in series with a'resistor, a circuit controlled by said push button including said microphone for shunting said relay to effect its release and at the same time energizing the microphone to render it ef- -fective to modulate the carrier current supplied to "-said transmitter tube.

[Carrier current communication apparatus comprising a telephone receiver and a microphone, a-detector tube having an input circuit coupled to a line circuit andan output circuit coupled to said telephone receiver, a transmitter tube having an output circuit coupled to said line circuit, an oscillator tube for supplying carrier frequency current to the input circuit of said transmitter tube, means controlled by said microphone for modulating the current supplied to the input circuit of said transmitter tube, a source of direct current for energ z ng said tubes and said microphone, a voltage divider connected across the terminals of said source which normally provides a reduced voltage for energizing said detector tube and applies a biasing voltage to the input circuit of said transmitter tube sufiicient to render such tube inactive when the detector tube is energized, and manua ly operable means for shunting a portion of said voltage divider to disable said detector tube and to remove said bias to render said transmitter tube effective to supply modulated carrier current to said line circuit.

8. Carrier current communication apparatus comprising a telephone receiver, a microphone, a detector tube having an input circuit coupled to a line circuit and an output circuit coupled to said telephone receiver, a normally inactive transmitter tube having an output circuit for supplying modulated carrier current to said line circuit, means for supplying carrier current to the input circuit of said transmitter tube and for modulating said current by said microphone, a source of direct current for energizing said tubes and said microphone, a voltage divider connected across the termina s of said source which normally provides a reduced voltage for energizing said detector tube, means including said voltage divider for preventing said transmitter tube from supplying current to said line circuit when said detector tube is energized, and manually operable means for shunting a portion of said voltage divider to disable said detector tube and at the same time render the transmitter tube effective to supply current to said line circuit.

9. Carrier current communication apparatus comprising a telephone receiver and a microphone, a detector tube having an input circuit coupled to a line circuit and an output circuit coupled to said telephone receiver, a normally inactive transmitter tube having an output circuit for delivering modulated carrier current to said line circuit, a gain control resistor associated With the input circuit of said transmitter tube for regulating the current delivered by said tube, a source of carrier current, means for supplying carrier current to said gain control resistor from said source and for also supplying voice frequency current thereto derived from said microphone to modulate said carrier current, manually operable contact means for disabling said detector tube and at the same time rendering said transmitter tube active, and means including a fixed resistor in the circuit by Which voice frequency current is supplied to said gain control resistor to reduce the effect of a change in adjustment thereof to compensate for the reduced effect of said change in adjustment upon the carrier current supplied to the input circuit of said transmitter tube due to the shunting of said gain control resistor by stray capacity.

10. Carrier current communication apparatus comprising a telephone receiver and a microphone, a detector tube having an input circuit coupled to a line circuit and an output circuit coupled to said telephone receiver, a normally inactive transmitter tube having an output circuit for delivering modulated carrier current to said line circuit, a gain control resistor having an adjustable connection to the input circuit of said transmitter tube for regulating the current delivered by said tube, a source of carrier current having a connection to the input circuit of said transmitter tube through stray capacity, means for supp ying carrier current to said gain control resistor from said source and for also supplying voice frequency current thereto derived from said microphone to modulate said carrier current, means including a fixed resistor in the circuit by which voice frequency current is supplied to said gain control resistor to compensate for said stray capacity, andmanually operable means for rendering said transmitter tube active to deliver its output current to said line circuit and at the same time disabling said detector tube to render said telephone receiver nonresponsive thereto.

11. Carrier current communication apparatus comprising a telephone receiver and a microphone, a detector tube having an input circuit coupled to a line circuit and an output circuit coupled to said telephone receiver, a normally inactive transmitter tube having an output circuit for delivering modulated carrier current to said line circuit, means for supplying carrier current to the input circuit of said transmitter tube and for also supplying voice frequency current thereto derived from said microphone to modulate said carrier current, and manually operable means for shunting said detector tube by a relatively low resistance circuit portion including said microphone to render the microphone effective to supply voice frequency current to the input circuit of said transmitter tube and to render said tube effective to deliver its output current to said line circuit and at the same time disabling said detector tube to render said telephone receiver nonresponsive thereto.

12. In combination with a telephone hand set comprising a receiver and a microphone, a source of direct current, a voltage divider connected across the terminals of said source, a detector tube having an input circuit coupled to a line circuit and a plate to cathode circuit connected between a point on said voltage divider and the negative terminal of said source and coupled to said telephone receiver, an oscillator tube for generating carrier current and a transmitter tube for delivering said current to said line circuit, each of which has its plate connected to the positive terminal and its grid connected to the negative terminal of said source with its cathode connected to a point on said voltage divider to bias the tube to cut off and render it inactive, a gain control resistor for regulating the input circuit for said transmitter tube which is supplied with carrier current by said oscillator tube when active and by voice frequency current by the microphone for modulating said carrier current, and manually operable means for connecting a point on said voltage divider to said negative terminal to decrease the bias on the oscillator and transmitter tubes to render said tubes active and to short circuit said detector tube to prevent the response of said receiver to the current supplied to the line circuit by said transmitter tube.

13. In combination with a telephone hand set comprising a receiver and a microphone, a source of direct current, a voltage divider connected across the terminals of said source, a detector tube having an input circuit coupled to a line transmitter tube for delivering said current to said line circuit, each of which has its plate connected to the positive terminal-and its grid connected to the negative terminal'of said source, with its cathode connected to a point on said voltage divider tobias the tube to cut off and render it inactive, means for supplying carrier current from the output of said oscillator tube when active to the in-- i put circuit of sa1d transmitter tube, and for mod-- ulati-ng said current by voice frequency current derived from said microphone, manually operable means for connecting a point on said voltage di- 2b" 12 vide'r' t'o said ne a'tive on saidoscillator and said tubes active, and means-for short circuiting said receiver to prevent itsrespo'nse "When current is-being' supplied to the line-circuit by saidtran'smitter tube.

K. CROOKS.

REFERENCES CITED The f'foll owing references are of record in the file of this patent: V

UNIT-ED STATES PATEN -I S Numbe V Name lfiate' V A 2529!);487" Moore scram-"ce se oer. 2'0, r942 .IFQREIGN EATENTS Number country have 105,358 Australia e -i May- 24, 11937 terminal tode'crea's'e thebias' transmitter tubes "to render

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