US2375099A - Transmission system - Google Patents

Description

May 1, 1945.

GQM. GIANNINI TRANSMISS ION SYSTEM 3 Sheets-Sheet 1 Filed Nov". 14, 1941 INVENTOR.

Mia/megs y 1945. G. M. GIANNINI TRANSMISSION SYSTEM Filed Nov; 14, 1941 3 Sheets-Sheet 2 .OQOCOOOIII.

INVENTOR- 'unnnou A mttuQb mu mm BY Gabriel MGiannim} M'W y 1, 1945. G. M. GIANNINI 2,375,099

TRANSMISSIbN SYSTEM Filed Nov. 14, 1941 3 Sheets-Sheet 5 HQVENTOR.

Gabriel. M Giiznmm) Attorneys.

Patented May 1, 1945 signor,. by mesne assignments, to Automatic Electric Laboratories, Inc., a corporation of Delaware Application November 14, 1941, Serial No; 419,167

10 Claims. The present invention relates generally" to-improvements in signal-current transmission sys-- terms of the type in which signal controlled switching circuits are provided for partially or completely blocking certain of the signal current channels included therein and; more particularly,

to or improvements in transmission systemsof this character as utilized inintercommunicating systems-of the loudspeaking type.

Itxisan object" of thepresent inventicntopro vide an: improved transmission system ofthe character described,- wherein the automatic gain control of the signal-transmission channelsis ac-- complished in a simpleand reliable manner with a minimum of. circuit equipment.

According to another "object of the invention,

an improved signal responsiveg-ain control cir-- cuit is provided which is electronically coupledto anassociated signal current channel and utilizes a normally conductive gaseous discharge tube "in its operation to control the gain settingofthe associated channel. i

In accordance with still another object oftha invention, an alternating current source isutilized for supplying space current to the dischargetube of the control circuit, whereby a non-conductive condition may instantly be established in-the-tube by utilizing grid control and without'resorting "to" complicated relay orelectronic-switching arrange ments for accomplishin this purpose;

The invention both as'toits organization and method of operation, together with further ob jects' and advantages thereof; will best be understood by reference to the specification taken in connection with the accompanying drawingsin which Figs. 1 and 2 illustrate an improved transmission system constructed and arranged" in accordance with the present invention; and Fig; 3

illustrates a modifiedarrangement ofthe channel amplifier which may be incorporated in the systemshown in Figs. 1 and 2; 1

Referring now more particularly to Figs. 1 and Z-of the drawings, the transmission systemthere illustrated isshown as being connected'between an east station and a west station of anintercommunicating line; These stations-respectivewhich functions to increasethe sig-nalcurrentamplitude-in the direction of signal cur rent transmissiontherethrough: Fonthe purpose of rendering the: east west channel it! active'an'd the west east channel i l" inactive when signal currents are transmitted from the translating device 8 through the channel 1 10 for reproduction bythe translating device 9, again control circuit 12 1s provided-L A similar circuit, indicated generallyati. I3; is provided for reversely controlling the gains of the two channels I wand l i iii-response to signal. current-transmission from the translating device 9 through the channel I i to the translating deviceB, l

cults respectivelyprovided in association with thetwochannels 1n and II areidentical in arrangement, and accordingly, like reference nu metals; distinguished only; by theletterse and 20, have been used-to identify corresponding circuit elements; Briefly considered, the: amplifier provided intheeast-west channel] 0 comprises three am plifying stages Me, i5'eiuand- Hiawhich respectively include the vacuum tubes Isle; IBeand l9e'. Theiinput terminals of this amplifieriareconnectedt c the terminals: of thewtranslating. device'fland the output terminals are connected to theterm-inalso'f the translating device. 9 1 More specifically, the nrstxamplifiertubexl leis of thecom-- "mercial 6J7 pentodatype and. comprises input el'ectrod'es zfle andll ei which are coupledthrough an input transformer 22e!to theterminals of the.

translating'devi'ce 8 over alinewhichma'y be: cone nected between the" east station andthe transmissi'onsystem; The output electrodes 22c and 2 le of the tube [1e are coupled to the inputelectrodes 23eand 24c ofwtl'ie secondl amplifier tube lBe through a resistance-capacitance: coupling network which. comprises a coupling resistorflie .l andar direct currentlblocking; condenser 2 6e. The

correct operating. potentiall is. maintained upon thecathode Zlewo'fl the tubeslle throughxtheprovision of a cathode: biasing network which: in-

1y comprisesound translating devices 8- and 9 or" the-electrodynamic soundpowered type, each of' which serves both as a, transmitting elementand" a loudspeaking receiver element. The transmission system or repeater network connectedb'' tween the two soun'dtranslating devices 8 and '9 comprises an east west transmission channel" "1* and awest-east transmission channel H; Each of these channels includes a plural stage amplifier eludes a resistor- M er shunted: by a by-pass con- The vacuum tube We: included theiamplifleri of east-westohaznnel in? at: the second stage thereof is a twin: triode of: the commercial 6N7. type having its. two c'ontrolTe1ectrodes' tied together so that the electron: streams respectively terminating at the' two: anodes thereof are? con-- trolled" like manners: by signal currents im-- pressed between the connectedcontrol grids and the cathode 242: With thisuarrangement it will t. e that: theatub'ewlte: provided with two The amplifiers;- controllcircuits and power cir sets of output electrodes. One set of these electrodes, namely, that comprising the cathode 24c and the anode 336, is coupled to the input electrodes 34c and 35e of the tube I9e through a resistance-capacitance network which comprises a voltage dividing resistor 36c, a coupling condenser 3'Ie, and a direct current blocking condenser 38e. The output electrodes 39c and 35a of the tube I92 are in turn coupled through an output transformer 406 to the terminals of the translating device 9 provided at the west station. For the purpose of maintaining the cathode 35c at the correct operating potential, a cathode biasing network is provided which includes a resistor lIe shunted by a signal current by-pass condenser Me. A similar network comprising the parallel-connected resistor 43c and condenser Me is included in the space current path of the tube We in order to maintain the proper potential difierence between the cathode 24c and the connected control grids 23c.

.For the purpose of supplying positive screen and anode potentials to the screen and anode electrodes of the three tubes Ha, I 8e and I96, a full wave rectifier, indicated generally at 45c, is provided. This rectifier comprises a full wave rectifying tube 466 to which alternating current of commercial frequency is delivered through a transformer file. More specifically, the transformer 41a is provided with a high voltage secondary winding 48e having a grounded center tap and having its outer terminals connected to the two anodes-of the tube Mic. The cathode of this tube is energized from a low voltage secondary winding 49c of the transformer He, and

is connected to the grounded center tap of the high voltage winding 486 through a filter network which includes the series-connected iron core choke 50c and a pair of shunt connected filter condensers Ie and 52a. The high voltage terminal of the filter network is connected directly to the screen electrode of the amplifier tube We and is connected to the anode 39c of this tube through the primary winding of the output transformer 40c. Similarly, the required positive potential is impressed upon the anode 336 of the amplifier tube I86 through a pair of current limiting resistors 53c and Me, the last of which is by-passed for alternating currents by a condenser 55e. Positive operating potential is also supplied to the anode and screen electrodes of the first amplifier tube I'Ie through a resistance network which includes the three current limiting resistors 56c, 51c and 586, the resistors 51c and 58e being by-passed for alternating currents by a condenser 59c.

The control circuit I2 is electronically cou pled to the east-west transmission channel In and comprises a diode rectifiertfile and a gaseous discharge tube 6Ie of the commercial 885 type, connected in tandem in the order named. More specifically, the second set of output electrodes 62c and 24e ,of the second amplifier tube I8e is coupled to the diode 60c through a transformer 63s. The space current path of this diode is connected across the secondary winding of the transformer 63c in series with a small battery 64c and a voltage dividing resistor 652 which is shunted by a smoothing condenser tile. The bias voltage developed across a portion of the resistor 65e is negatively applied to the control grid 61a of the discharge tube GIe, which tube is connected to operate as, a grid controlled rectifier. As indicated above, this tube is of the gas filled type such that current flow therethrough cannot occur so long as the negative bias on the control electrode 61c exceeds a predetermined value. When, however, the tube becomes ionized the grid 61c loses control, so that the discharge through the tube can only be arrested by reducing the voltage between the output electrodes 68c and 696 substantially to zero. These electrodes are connected in circuit with a biasing resistor IIle which is shunted by a smoothing condenser II e and is connected through a resistor 12c and the resistor 36c to the control electrode 34c of the tube I9e included in the final amplifier stage I6e of the channel I0. Space current i delivered to the gas-filled discharge tube ISIe from an alternating current source of commercial frequency through a voltage transformer 13c. This transformer is provided with a secondary winding Me which is bridged by a voltage divider 15c, the adjustable portion of which is included in circuit with the biasing resistor 10c and the space current path of the tube 6 le. Current for energizing the cathode heater of the tube 6Ie and the heaters of the vacuum tubes IIe, IBe, I 9e and e is supplied from a low voltage winding 16c forming a part of the transformer 13c.

In further reference to the arrangement of the control circuit I2, it is noted that this circuit not only control the bias normally impressed upon the control electrode 34a of the associated channel amplifier tube Mic, but in addition is arranged to control the negative bias voltage impressed upon the control electrode 2010 of the amplifier tube I'Iw. To this end a connection is provided between the voltage dividing resistor c and the control electrode 2020 which includes a resistor 'I'Iw shunted by a by-pass condenser 18w. Similarly, the control circuit I3 is arranged to control both the gain of the final amplifier tube I9w included in the west-east channel I I and the gain of the first amplifier tube I'Ie included in the east-west channel III. To the latter end, the voltage dividing resistor 65w is connected to the control electrode 20c of the tube I'Ie through a resistor IIe which is shunted by a by-pass condenser 18c.

In considering the operation of the system, it may be assumed that the signal amplifiers provided in the two channels Ill and I I and the control networks I2 and I3 respectively associated with these amplifiers are conditioned for operation. In this regard it will be noted that when alternating current is delivered to the primary winding of the transformer 4'Ie, current alternately traverses the two anodes of the tube 46c to produce a high direct voltage between the upper terminal of the choke coil 50c and ground. This voltage is positively applied to the screen and anode electrodes of the tubes IIe, I8e and I9e over the paths previously described. With the power transformer 73c energized, current is delivered to the cathode heaters of the tubes I'Ie, I8e, I9e, 60c and (He over parallel circuits which may include current limiting resistors, if required. In

a similar manner the tubes I'Iw, I8w, I9w, 6010- and 6 Iw included in or associated with the westeast transmission channel I I are conditioned for operation when current is delivered to the parallel connected primary windings of the two transformers 4110 and 13w.

When electron emission starts in the diode Elle, direct current is passed from the battery 64c through the space current path of this tube, the biasing resistor 65c and the secondary winding of the transformer 63c so that a direct voltageis developed across the resistor 65c. This voltage is asaaoee I V negatively: applied". to. 'the control; electrode file; v

charge: tubeyfile, but serves tolpolarize. this tube. so. that when a: relatively small increase; in. the negative bias. voltage occurs the. tube, is rendered IIOHr-GODdHCIiVB. In this regard it is notedthat tentialtvalue I once. during each half-cycle, thereof, and accordingly when the bias-voltage impressed betweenthe two electrodes 61c and, We exceeds the space currentcut-ofi value, space current flow the alternating voltage. impressedbetween the two; electrodes 68c; and: 69e passes through :a zero: poa through the tube is arrested at the next follow:

ing. zero. potentialpoint of the applied voltage.

Since the-tubeB-Ie acts as a grid controlled. rectifier, it passes currentin only onedirection to set;

up; a. pulsating current through the biasing. resistor We. The resulting voltage appearing across this resistoris smoothed throughtheaction of the a condenser He and is negatively applied. to the control electrode.,34e of the final amplifier tube I9ein the east-west. channel It! overa path which includes the resistor. I2e and voltage dividing resistor 36c.

current transmission.

In a manner similar to that justt describedwith reference to-the action of the control circuit I2} the rectifierfillw and the gas-filleddischargia tube Blw, in cooperation with their. respective assoa portionof the The extent of .this

voltage isisufiicienttobias the amplifier tube I9e beyond anode current cut-off so that the. eastwest channel I 0 is normally blocked against signal ciated circuits, serve normally to provide a direct voltage across the biasing resistor 10w which is negatively applied. through the, resistor 1210 and t a portion of the voltage dividing resistor 36w to the control electrode 35w of the final amplifier tube ISw included in the west-eastchannel II.

This voltage serves tobias the tube I 9w beyond itsanode current cut-01f point so that the. westeast channel I I. is normally blockedagainstjsignal current transmission ,therethrough.

With the system in the condition described above, if a user of the west station speaks into the translating device 9, a signal voltage is developed across the primary winding of the coupling transformer 2210*. This voltage is reproduced across the secondary winding of the transformer 22w; islimpressedbetween the input electr'ode's 27w and Zlw of the first amplifier tube I'Iw. When amplified by this tube the signal voltage is impressed through the resistance-capacitance coupling network between the. input electrodes 23w and 24w of the second amplifier tube I811). The amplified signal voltage appearing across the output circuit of the tube I8w is impressed through the resistance-capacitance coupling network between the input electrodes 34w and 3510 of the final amplifier tube Illw. Initially, however,

the tube law is blocked against signal current transmission. therethrough due ,to the existing negative bias voltage between the input electrodes thereof.

through the first two stages Me and I5e of the At the second stage I5w of the west-east channel amplifier, the signal voltage as amplified through the tubes I'Iw and I8w also appears between the output electrodes 33w and 24w of the tube I8w. This voltage i applied across the primary winding of the coupling transformer 63w,

and. during. alternate. half-cycles thereof, addional-sw a Passed bvihfi de ifi rfiqw todncrefllsethe current flow: through the biasing.

resistorwfibw. ,.'I he resulting increase in voltage. across this resistor i smoothed through the ac- I tion of thecondenser 6610 and servesto increase the negative bias between thev input electrodes 6110 and 68wcfthe discharge tube. 6Iw. In response tothis increase in biasvoltage, the discharge tube Bilw is rendered. non-conductive dur ingthenextfollowingizero potential point in the voltage. applied across the output electrodes 69w and 368w thereotwwhensthis tube is rendered non-conductive the voltage across the resistor 10w. drops to zero to remove the negative bias normally: applied to the controlgrid 34w of the.

finaluamplifier tube Iu9w. Thi decrease in the bias betweenthe-input electrodes of the tube I9w' serves to render thexfinal amplifier stagefllfiw ac-.

tive, whereby the incoming signal currents are amplified therethrough. and are transmitted through the output transformer 40w to the translating device 8lfor reproduction.

When the signal being transmitted from the translating device9 through thewest-east channel. II to the translating device 8 is interrupted;

thecurrent traversing the space current path of the diode rectifier Sflwis reduced to its normalv value so that the bias voltage impressedbetween the input electrodesof the discharge tube' fi lw -isdecreased sufiicientlyto iagain. permit space cur rent. flow through this tube. In this regard it willqbe; noted that the action of the condenser- BBw prevents an immediate collapse of the bias voltage between the inputelectrodes ofxthe tube GIw, and the action of the condenser 11w prevents an immediate, rise of the bias voltageacross the resistor 'I'Ilw. to its normal valueafter space current flow through the tubev 'IIw iswreestablished. Thusashort periodelapses betweenthe: interruption of the signal andthe increase in the.

bias voltage acrosstheresistor Iliw tohaqvalue above and will be clearly apparent from the-pre- I ceding explanation.

It will be noted that the signal a amplified through the west-east channel H and transmitted to the translating device 8 is also applied to the primary winding of the input transformer Me of the first amplifier stage Me included in the eastwest, channel. II]. In the absence of some means for. blocking the transmission of the signal.

east-west channelamplifier IILthissignal voltage is free to act upon the controlcircuit I2 in the above. described manner to render the final amplifier-stage I lie operative to pass the signal through the three stages Me, Ice and Ito and the transformer 48c to the input transformer 22w of the first amplifier stage I4w. In other words, a closed circuit is necessarily provided. which includes the combined gains of the amplifiers reelly excee the l s s f h c os c and.

hence an unstable condition of the circuit is produced. It is to obviate this diificulty that the control circuit I3 is arranged to block signal transmission through the east-west channel II) at the first amplifier stage Me thereof during each interval when the west-east channel is active. To this end the voltage developed across the adjustable portion of the resistor 65w is negatively applied through the resistor He and the secondary winding of the input transformer 22c to the control grid 20a of the amplifier tube I'le. Incident to the increase in this voltage which occurs when a signal is impressed upon the primary winding of the input transformer 22w, the first amplifier tube He is biased beyond anode current cut-off so that the east-west channel I is blocked against signal current transmission therethrough. Obviously this channel is rendered active each time a signal transmitted through the west-east channel II is interrupted to cause the voltage across the biasing resistor 65w to be reduced to its normal value.

The manner in which the control circuit I2 functions to render the east-west channel I0 operative to pass signal currents from the sound translating device 8 to the sound translating device 9 during sound wave transmission to the translating device 8, and to block the west-east channel II at the first stage I411), is exactly the same as described above with reference to the operation of the control circuit I3 during signal current transmission in the opposite direction and will be clearly apparent from the explanation pertaining thereto.

Referring now more particularly to Fig. 3 of the drawings, the modified channel amplifier arrangement there illustrated is substantially similar to the amplifier arrangement utilized in each of the two channels I0 and II of the system shown in Figs. 1 and 2. Accordingly, corresponding elements of the two amplifier arrangements have been identified in the drawings by the same reference numerals, but the sufiix c has been added to the reference numerals in Fig. 3 in order to distinguish the two arrangements. In the arrangement illustrated in Fig. 3 a triode He is used at the first amplifier stage I40, which tube is provided with input electrodes 20c and 2 I c that are coupled to the secondary winding of the input transformer 220 through a voltage dividing resistor 80 in order that the proportion of the available signal input may be adjusted to the desired value. At the second stage I5c of the amplifier, a pentagrid mixer tube of the well-knowncommercial 6L7 type and provided with two control electrodes 23c and 84 is utilized.

In order to substitute the amplifier shown in Fig. 3 of the drawings for the amplifier provided in the west-east channel II of the system shown in Fig. 2, the terminals A, B, C, D, E, F, G, and H of the circuit shown in Fig. 3 are connected to the correspondingly labeled terminals shown in Fig. 2. After these connections are made it will be noted that the anode and screen supply voltage as developed through operation of the rectifier 45w is impressed across the voltage dividing resistor 82, and a small portion of this voltage is positively applied to the cathode 240 of the tube I8c in order to maintain the correct potential difference between this cathode and the associated control electrode 230. Substantially the full voltage appearing across the resistor 82 is also positively applied to the screen electrode of the tube I8c through the resistors 53c and 540 and to the anode 62c of this tube through the resistors 53c and 85. Suitable by-pass condensers 84 and 86 are provided for preventing audio frequency components of the voltage across the resistor 82 from being impressed upon the anode and screen electrodes of the tube I8c. Current for energizing the cathode heaters of the tubes IIc, I80 and I9c may be supplied from the low voltage winding 16w of the transformer 13w.

In considering the operation of the amplifier as shown in Fig. 3 of the drawings when connected in the manner pointed out above, it will be noted that the voltage normally developed across the resistor 1020 of the control circuit I3 is negatively applied to the control electrode 84. The magnitude of this bias voltage is sufficient to prevent the tube I8c from passing signal currents and thus signal current transmission through the amplifier is blocked. With the apparatus in this condition, if a signal voltage is applied to the primary winding of the input transformer 220, this voltage after amplification through the first amplifier stage I40 is impressed through the condenser 8I upon the primary winding of the transformer 63w. The resulting increase in.the bias voltage across the resistor 65w causes the gas-filled discharge tube 6Iw to be rendered non-conductive and a blocking bias potential to be applied to the first stage of the inactive east-west channel I0, all in the manner previously explained. Incident to the extinction of the space current flow through the tube BIw, the bias voltage as developed across the resistor 10w and negatively applied to the control electrode 84 is reduced substantially to zero, whereby the tube I8c is conditioned to amplify signal currents impressed across the signal input electrodes 23 and 240 thereof. The amplified signal output is, in turn, impressed between the input electrodes of the final tube I9c and transmitted from this tube through the coupling transformer 400 to the translating device 8 for reproduction. When an interruption in the signal voltage applied across the primary winding of the input transformer 22c occurs, the amplified signal voltage as impressed through the first amplifier stage I40 upon the input transformer 63w of the control circuit I3 is arrested, so that the gas-filled discharge tube 6Iw is again rendered conductive to restore the high negative bias voltage on the cut-off grid 84 of the second amplifier tube I8c. Incident to the decrease in the voltage across the resistor 65w which occurs when the signal is interrupted, the blocking negative potential applied to the control electrode of the first amplifier stage Me in the inactive channel I0 is reduced sufiiciently to permit signal currents to be passed through this stage and the succeeding amplifier stages to the control circuit I2.

When the opposite channel, i. e., the east-west channel I0, is rendered active to transmit signal currents from the translating device 8 to the translating devic '9, the control circuit I2 functions in the manner previously pointed out to impress a relatively high negative bias potential through the resistor 11c and the lower portion of the voltage dividing resistor upon the control electrode 200 of the first amplifier tube He. The magnitude of this bias voltage is sufiicient to block the first amplifier stage I40 against signal current transmission therethrough, whereby the west-east channel II is rendered completely inactive.

While one embodiment of the invention has been described, it will be understood that various eemed modificationsmay be made 'therein it without departing irom the true "spii -it and scope oi thein- 1. "In a transmission system, a signal current channel including a plural 'stage' ampliiier; a

ducing the bias on said intermediate, or lastfstage to a predetermined normalvalueto prevent signal current ftransmission thereby when :said tube is rendered non-conductive, and means coupled to said amplifierat'apoint preceding saidlintermediate or last :stage for normally maintaining current flow "through "said tube andionpreventing current flow through said :tube inres'ponse .to a

signal inputto said'amplifier. d

2. "In .a transmission "system, .a signal current channel including 'a plural stage amplifier, a

. gaseous discharge tube having. a control electrode,

a biasing'network controlled by saidtube and includirigmean's for biasing an intermediate or last stage of 'saidamplifier to prevent signal current transmission thereby when said tube; is conductive and for reducing the bias ,onsaid intermediate or iaststageto alpredetermined normal value to I permit signal current transmission thereby when said tube isrendered non-conductive, a rectifier coupled to .said amplifier at a ,point preceding said intermediate or last stage,,.albiasing resistor connectedto impress afbiasing voltage on the control electrode of said dischargetube, and a current source connectedto pass'current through .said resistor and rectifier in series so that theresulting biasvoltage which .is impressed'on said control electrode 'is ,normally below the space,

current cut-off value of. said tube thereby permitting current flow therethroughuand .isiincreased to a value exceeding the space current cut-off value of said tube thereby preventing current flow therethrough when a signal is impressed on the input side of said amplifier.

3. In a transmission system, a signal current channel including a plural stage amplifier, said amplifier including at one stage thereof a ther-' mionic tube which comprises two sets of output electrodes, one set of said electrodes being included in said signal current channel, a gaseous discharge tube, a biasing network controlled by said discharge tube and including means for biasing a succeeding stage of said amplifier to prevent signal current transmission thereby when said discharge tube is conductive and for reducing the bias on said succeeding stage to'a predetermined normal value to prevent signal current transmission thereby when said discharge tube is rendered non-conductive, and means coupled to said other set of output electrodes for normally maintaining said discharge tube conductive and for rendering said discharge tube non-conductive in response to a signal input to said amplifier.

4. In a transmission system, a signal current channel including a plural stage amplifier, said amplifier including at one stage thereof a thermionic tube which comprises two sets of output electrodes, one set of said electrodes being ,included in said signal current channel, a gaseous discharge tube having a control electrode, a biasing network controlled by said discharge, tube and including means vfor biasing a suc- V seeding stage of said amplifier to prevent signal current transmission thereby when said discharge tube is conductive and for reducing the bias on said succeeding stage to a predetermined ;normal value to permit signal current transmission thereby when said discharge tube is rendered non-conductive, a rectifier coupled tothe other set of said output electrodes, a hi- I asing resistor connected to impress a biasing voltage on said control electrode, and a current source connected to pass" current through said resistorand rectifier in series so that theresulting bias voltage acrosss aid resistor is normally below the space current cut-off value of said discharge tube thereby permitting "current flow therethrough and is increased to a value exceeding the space current cut-off value of said, discharge tube thereby preventing current flow "therethrough when a, signalyoltage app ars across saidother set of output electrodes.

5. jIn'a transmissiongs'ysternya pair of channels each including a lural stage amplifier, a gaseous discharge tubeindividualto one of said amplifiers,

a biasing network controlled byjsaid tubeand including means. for biasingan intermediate or last stage of said one amplifier'to preventsignal current transmission thereby when said tube is con ductive and for reducingthe bia jon said intermediate or last stage; toQapredeterminednormal value to permit signal current transmission thereby when said tube is rendered non-conductive and means coupledto said one amplifierat apoint preceding said intermediate or last stage jornormallymaintainin current flow throughsaid tube and for decreasingjthe gain of theoth'er channel and preventing current fiowthroughsaidjtube in responsetoasignal input'to said one amplifier.

6. In atransmissionsystemna pair of channels each including a plural stage amplifierf'a gaseous discharge tube individual to "one of said amplifiers andlhaving a control electrode, a'biasing'network controlled by said tube and including .means for biasing an intermediate or last stage of said one amplifier to prevent signal current transmission thereby when said tube is conductive and for rev ducing the bias on said intermediate or last stage to a predetermined normal value to permit signal current transmission thereby when said tube is rendered non-conductive, a rectifier coupled to said one amplifier at a point preceding said intermediate or last stage, a biasing resistor connected to impress a biasing voltage on said control electrode, a current source connected to pass current through said resistor and rectifier in series so that the resulting bias voltage which is impressed on said control electrode i normally below the space current cut-off value of said tube thereby permitting current fiow therethrough and is increased to a value exceeding the space current cut-off value of said tube thereby preventing current fiow therethrough when a signal is impressed on the input .side of said one amplifier, and means for impressing the voltage developed across at least a portion of sa d resistor on one of the stages of the other amplifier to decrease the gain of said other amplifier when a signal is impressed on the input side of said one amplifier.

'7. In a transmission system, a pair of channels each including a plural stage amplifier, one of said amplifiers including at one stage a thermionic tube which comprises two sets of output electrodes, one set of said electrodes being included in the signal current channel through said amplifier, a gaseous discharge tube individual to said one amplifier, a biasing network controlled by said discharge tube and including means for biasing a succeeding stage of said one amplifier to prevent signal current transmission thereby when said discharge tube is conductive and for reducing the bias on said succeeding stage to a predetermined normal value to permit signal current transmission thereby when said discharge tube is rendered non-conductive, and means coupled to the other set of said output electrodes for normally maintaining current flow through said discharge tube and for decreasing the gain of the other amplifier and preventing current fiow through said discharge tube in response to a signal input to said one amplifier.

8. In a transmission system, a pair of channels each including a plural stage amplifier, a gaseous discharge tube individual to one of said amplifiers, a biasing network controlled by said tube and including means for biasing an intermediate or last stage of said one amplifier to prevent signal current transmission thereby when said tube is conductive and for reducing the bias on said inter mediate or last stage to a predetermined normal value to permit signal current transmission thereby when said tube is rendered non-conductive. means coupled to said one amplifier at a point preceding said intermediate or last stage for normally maintaining current flow through said tube and for preventing current flow through said tube in response to a signal input to said one amplifierand mean responsive to a signal input to the other amp1ifier for blocking said one amplifier against signal current transmission at a point preceding said last-mentioned point.

9. In a transmission system, a pair of channels each including a plural stage amplifier, a gaseous discharge tube individual to one of said amplifiers and having a control electrode, a biasing network controlled by said tube and including means for biasing an intermediate or last stage of said one amplifier to prevent signal current transmission thereby when said tube is conductive and for reducing the bias on said intermediate or last stage to a predetermined normal value to permit signal current transmission thereby when saidtube is rendered non-conductive, a rectifier coupled to said one amplifier at a point preceding said intermediate or last stage, a biasing resistor connected to impress a biasing voltage on said control electrode, a current source connected to pass current through said resistor and rectifier in series so that the resulting bias voltage which is impressed on said control electrode is normally below the space current cut-off value of said tube thereby permitting current flow therethrough and is increased to a value exceeding the space current cut-off value of said tube thereby preventing current flow therethrough when a signal i impressed on the input side of said one amplifier, and means responsive to a signal input to the other amplifier for blocking said one amplifier against signal current transmission at a point preceding said last-mentioned point.

10. In a, transmission system, a pair of channels each including a plural stage amplifier, one of said amplifiers including at one stage a thermionic tube which comprises two sets of output electrodes, one set of said electrodes being included in the signal current channel through said amplifier, a gaseous discharge tube individual to said one amplifier, a biasing network controlled by said discharge tube and including means for biasing a succeeding stage of said one amplifier to prevent signal current transmission thereby when said discharge tube is conductive and for reducing the bias on said succeeding stage to a predetermined normal value to permit signal current transmission thereby when said discharge tube is rendered non-conductive, means coupled to the other set of said output electrodes for normally maintaining current flow through said discharge tube and for preventing current flow through said discharge tube in response to a signal input to said one amplifier, and means responsive to a signal input to the other channel for blocking said one amplifier against signal current transmission at a point preceding said one stage.

GABRIEL M. GIANNINI.

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