US2487455A - Telephone circuit - Google Patents

Description

New. 8, 1949 A. LESTI TELEPHONE CIRCUIT 2 Sheets-'Sheet l Filed Jan. 29, 1945 Nov. 8, 1949 A. LES-rl 2,487,455

TELEPHONE CIRCUIT Filed Jan. 29, l1.945 f 2 sheetssneet 2 M/CROPHO/Vf HMPL /F/[R 3 /7- ,9+ 5- 5+ 6 6 4 afP/L/o/vf 5 J Pfc. wf

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BY 29m/@@1122 Patented Nov. 8, 1949 Arnold Lesti, Rochester, N. Y., assigner to Stromberg-Carlson Company, Rochester, N. Y., a corporation of ew York Application January 29, 194,5, Serial No. 575,096

9 Claims.

' This invention relates to telephone circuits for a microphone and a'receiver such as are utilized at'subscribers telephone substationsl and the like.

rThe carbon microphone or transmitter has beenA almost'unversally used in subscribers telephone Sets because, in addition to modulating current in accordance with sound, it also possesses lthe unique property of acting as an amplifier. However, the carbon microphone is subjectto highly undesirable defects, such as carbon noise, packing, deterioration with age, positional defects, non-linear `distortion, and distortion which results in modulation products when two frequencies are impressed thereon. Extensive research on the carbon type of microphone has failedV to overcome these defects. It is therefore the purpose of thel present invention to eliminate the carbon microphone in substation circuits and to substitute therefor a high quality microphone which may be any one of several kinds, commonly designated electro-magnetic, moving coil, crystal, condenser or velocity types. However, such high quality microphones have no inherent amplification and consequently do not deliver sufhcient output to actuate to the desired level, the receiver atA a distant station. It therefore becomes necessary to associate an amplifier with the microphone, preferably at the substation, so that the amplified output of the microphone will be at the desired` level.v However, the use of such an amplierA introduces the problem of providing a power supplyl source without disturbing hum.

Furthermore, substation circuits of the type under consideration are preferably arranged to have anti-side-tone characteristics and it therefore becomes a real problem to provide an anti-sidetone network into which the output of the amplifier'can be efficiently introduced.

Where an amplifier is provided at a subscribers substation, it is highly desirablethat the vacuum tube or tubes in the amplifier be operated only when this substation is in use. However, vacuum tubes whosel cathodes are heated from a rectified andfiltered alternating current source require a substantial interval of time before their cathodes are heated to the degree where there is ample electron emission therefrom to render these tubes operative. The required heating'interval necessary to render the cathodes of such vacuum tubes effective, is longer than the time interval between lthe start of the lifting of the handset from the desk-stand and the placing of the transmitter'up to the mouth. Consequently, if conventional vacuum tube amplifiers are used, it is 'possible for the subscriber to speak into the transmitter before the amplifier is operative, with the result that a portion of the conversation can be 10st,

The main feature of the present invention therefore relates to a novel arrangement for effecting high speed heating of the cathodeor cathodes of the vacuum tube or tubes inv anamplien; so lthat the amplifier can bel used at va subscriberssubstation,and will be operative'before the subscriber starts talking.

A further -feature of the invention relates to an arrangementfor deriving the vpower 'for a telephoiieisubstation from the commercial lighting circuit available at the substation.

yAn `additional 'feature' of the invention relates to a telephone substation including a high quality microphone and anr amplifier therefor ypowered from: the 'commercial lighting circuit, together with'a normally ineffective standby carbon microphone automatically rendered yeffective and poweredover the telephone line from'the centralexchange, whenever the lighting source fails.

' Other featuresfand advantages of the invention will appear from the detailed description and claims-when taken with the' drawings in which; l,

Figpil is a diagrammatic showing of a novel substation circuit arrangement 'in accordance with thepresent invention;' Fig. 2'is'a diagrammatic showing of a modified 'form of rectifier which can be substituted for the gas tube rectifiers in Fig; l;

' Fig. 3` isa simplified diagram of a portion of the filter network of Fig. l;

Fig. 4 is a diagram useful in describing the connection of the amplifier into the anti-sidetonecircuit arrangement of Fig. l; and

Fig. 5 illustrates a simplified showing of a circuit whcrein a highl quality microphone and amplifier are powered from the commercial lighting current source at the substation` and wherein there is provided a carbon microphone in standby 'relation ready to be cut into service and to be powered over the telephone line from the battery at' the exchange when the commercial lighting current source fails.

'Referring especially to Fig.. 1, the substation telephone set therein disclosed includes a microphone 5 of al high quality type such as aV crystal microphone, or if coupled to the amplifier in a manner well known to the art an electromag- -netic'microphone, a moving coil microphone, a

traveling springs.

is preferably of the crystal type which is especially suitable because of its small size. The substation set also includes a receiver 6 preferably of the watch case type so that the microphone E and the receiver 6 can be mounted in the handle of the usual hand set 1, more commonly referred to as the French type of telephone. The substation A.set also includes a conventional ringer` 8 and equipped with or with.- out a dial impulse sender Si, as occasion demands. There is also included in the substation set `a lso-called hook-switch actuated to its alternate positions in the conventional marmer bythe removal from and the replacement respectively, ofl the hand set "l on the cradle yof the telephone stand (not shown). The hook-switch comprises sets of contact springs iO-ll, I2-I3, I4--l5V and Iii-H. The operation of traveling contact springs of each set is delayed by an air dashpot comprising a fixed cylinder I8 and a plunger i9 movable therein under the inuence of these Contacts Iii-H close first to complete an alternating current power circuit which, in combination with-a suitable rec- .tier unit to be described, conducts pulsating .rectified current through the filaments of the amplifier tubes to be referred to,.and this heats `up these tubes quickly. The air dash-pot increasesthe time from the closure of spring contacts ltl-Il to the closure of spring contactsl 14..- of the hook-switch. During this interval the rectified alternating current haslample time to heat up the filaments ofthe tubes. When contacts llil5 close, they thereby connect certain" capacitors across all filter circuits for hum `elimina-tion f as will be described. This occurs faster-,than the time required to remove the hand Vsetfroi'n the hook-switch and place the receiver minimizes the 'duration of momentary noise ,onl

the line. 7

i Since the present substation. arrangement is intended to be used where a high signal output is desired, it is essential that the response of thev microphone -5 be amplified. This is effectedby a two-stage amplifier comprising the, electron tubes 2li and 26, which areof the directly heated pentode type, and may be of the type now commonly sold under the code numbers 1L4 and 3Q4,

respectively.- The tube is providedwith electrodes including a filamentary cathode 2|, a control grid 22, a screen grid 24, a suppressor grid 23 and an anode or plate 25. Likewise, the tube 26 is provided with corresponding electrodes designated y27, 28, '29, 3l) and 3i. The amplifier tubes 20 `and -213 are resistancefcoupled by the capacitor 32 and the resistors 61 and 33 the last-named of which is connected potentiometer fashion to the `,control grid 28l to serve as, a` volume control.'

These amplifier tubes are powered by rectified and filtered'current derived from the alternating current leads 34 and 35 of the commercial'source wof electric lighting current available at the telephonev subscribers residence where they substa-v tion setis-located; i l

Since the amplifier tubes are lighted only duringthe time that the subset is in use, these tubes as well asv the rectiflers must become effective in. a period -o'f 'about one-quarter .of a second which is shorter than the interval between the lifting of the telephone handset from its cradle and the time when the subscriber first speaks into the microphone. The use of indirectly heated cathode tubes is therefore precluded, since they require up to twenty seconds to heat up from a cold start. Instead, directly heated fllamentary type tubes-are necessary, but this alone is not enough. A'circuit with these latter tubes and a resistance capacitor filter adequate to lter the hum, takes about 1.1 seconds to warmup. The long time-constant of the filter, together with some thermal inertia of the filaments slows the action. In accordance with the present invention, there is provided a high speed circuit, which reduces the warming-up time to about 1% second. In this circuit, cold-cathode 'y gas rectifier tubes 36 and 31 are used to insure The latter prevent noise in yconductor t5 throughresistors S6 and 61 to the fast starting. However, a single selenium half wave rectifier 38 may be substituted for the gas rectifier tubes as shown in Fig. 2. Capacitors 'iti and 'H merge into one unit, if this substitution is made. Two separate filter networks F-,l and SEL-2 for the filament and anode supplies respectively, are provided. The filter F--I includes the resistors 39 and 4i) in series in the positive conductors 6I and 62 of the filament circuit to be described. This filter also includes capacitors 4l and 42, capacitor 4I having one terminal thereof connected to the common terminals of resistors 39 and 4D. Capacitor 42 has one of its terminals connected to conductor 6,2. The plate filter F-Z includes the resistors 49 and 5t in series with the plate supply conductor 5i and the capacitors 53 and 54. The capacitors 4|, 42 and 53, 54 have their negative leads 55 and 'l5 connected to the hook-switch contact spring l5 and must be extended through springs itl-I5 to complete the circuit to the common negative return lead 57. These capacitors are in an opened circuit when the amplier is not in use, and also during the interval of time between the closing of contacts lll-l l of the hook- .switch and the closing of contacts lli-I5, which interval begins substantially when the hand set is lifted by the subscriber. During this interval, the rectifiers 3S and 3i are functioning so that pulsating current iiows through the filament circuit to heat up the filaments ofV the amplifier tubes 20 and 26. This filament circuit may be traced from power lead 34, conductor A59, hookswitch contacts ifi-ll, conductor 6B, through rectifier 3?, conductor 6l, filter resistors v39 and 4Q, conductor t2, maments 2l and 2|, conductors 63 and 5T, to the power lead 35. The plate supply for the amplifier tubes extends from the power lead 34, conductors 59 and 6i! through the rectifier 36, conductor 5l, through the resistors 49 and 5d in series, to the junction 64. From this junction, the circuit to amplifier 2l) extends over screen. grid 24 anode 25 respectively of tube 20. From junction 64, the conductor 68 extends to the screen grid of tube 26 and through coil 69,` to the anode 3l of tube 26,

Condensers 10 and 'il respectively connect the conductors 6l and 5l fromthe rectiiiers 31 and 365 to conductors 'l2 and 5l, leading to the power lead 35,

`,thereby providing a return circuit for the alter- `and 'it dueto the time required to charge them which tous etrierwis teiqiiird. The intertai estime jtween the @rising @i spariscontacts iti-"ii anali- 15j ofthe rigor-switch is increased by the. air dash-pot connected to the traveling springs, but this;V sio'vw intervaiis shorter than the longest permissible heating- 11p time. lrhs slew interi/.ai aiiows enough .puisations, which are separated by 1/f'io Second, .to 1iiow fhroughthe filaments o f the ampliertubesto insure their heatinglup. Sulo1sequently, when "slhiitingeirct o 'f the capacitbrs ai aridi-2 aiises a momentary drop inv filament vcurrentI which, ecaus'e of thermalinertiacf the ilarnents 2| and 21, is insufficient to lowerthetemperature of these filaments sufficiently to V stop electronic emission. ,iOther factors outlined' hereinafter, are effective which contribute to an even faster action.

, During theinterval oi spring travel, thecapaciicml ,which is foniletedto theiunctiou f the resistors 39 and 40 isin series withthe capacitor as illustratedin Fig. 3. Consequently,

during such an interval, the resistor 4|] isshurited by a capacita nce combination in series v 4I, 4 2 and addition .to the current flowing through the resistor 4|.'l there is at first also a current surge through the capacitors 41, 42., These cornbineilcurrentsi'lowin the filaments of the ampliuer tubes, andhence decrease the interval required toraise the temperature of these -:lilarnents to ,the value where electron emission occurs. Moreover, both these lament circuit capacitors, 4|, 4 2 andthep1ate circuit capacitors 5 3, m54 remain charged after tl'iehaiids.et l .iS lgplacedonthe i hooksys/itch, becausethe common return lead 5 6 oi ,these capaciterais open at that time. Thestoredcharge.speeds up subsequent starting times yeven further, for a considerable periodaiter the handset has been used a iirst time. In the first operation, from' a cold uncharged conditiongachargeiis developed on the capacitors which minimizes the shunting action when contacts 4 and 'I5 of the hookswitch close. The warming-up of the filaments from ac old first startto a point where, the 'speech is transmitted is about T-.Second Repeatedon and off operation of the hook-switch to flash the operator charges the capacitors 4I., 42 and pre-.-

vents surges. The filaments are thus protected and the circuit is fool-proof. The surgethrough the ,bavio'r 4| and ileso-compensatesiar the Alowered starting ,voltage gluel to the ncrease d 6,0 lcycle irripedancel the rectiiierJcircuit when the capacitors are opened byspringsL l4 -fl 5 of the .,hdokrw-tlcu-, This. increased impedance low rs the starting ,surge through the rectifier t1i b es36;. 31 and insures that they will have The above .high speed circuit requires special precautionsto insure duiet operationbofthe ampliier. Referring to Fig. l, the, capacitor i 4 2 must filter zthe, filament circuithtoythehpoint yvh're noA can be h'eard. 'Ihe return lead. of. this capacito'rA is ,connectedby conductor: 1 5

directly to spring l5. of the lloo'kJ-s v vitch ar 1d is separate from thereturn lead,5 6. of the other capacitors which isalso connected to spring 15.

to such a 10W iev'ei that it is ot'noticed 6r use Tire crystal microphone s iii themndst 1 is completely, shielded and is connected tothe volt,- age ampiinertubezo by a shieiuedcableietith shielded coupling 11. The shielding grmrtiiiscabie must be closely meshed.A tT he microphone is connected directly to the control grid 22 of the tube 20, with the shielding connected to the cathode 2| thereof, to provide a return path for the micrqf phone current and to insure quietriefss`. y The tube- 20 is a pentode voltage amplifier and is resistance coupled to the pentodepower tube 26....with volum'e control potentiometer as previously meritioned. A slight amount of inverse feed-back on tube 26 and its input network is provided by the resistor {0 0, The plate of the ltube .2.615, oo nnected to the primary Winding 69 of the iii'dction coil designated as '|9 This 6011 is analogous t"o the local battery 'anti-sid-tone coil of conventional substation circuits, but in this case the carbon transmitter thereof is replaced ^by Ythe tube 26; and the coil impedancesar suitably altered as will be described. The, secondary windings 82 and 83 as well as the resistors and 81 together with capacitor 88 are the Qihr impedancesofthe antiside-,ton'e netti/ Ork hereinafter discussed. In the hook-switch', Sp' iig terminals I'2--I3 close the telephone linecircuit through to the induction coil, while. the coil designated 80 is a choke which providesia low resistance direct current .pathfor central office supervisory purposes. This latter arrangement, together with the opening of the receiver circuit at springs Iii-l1 of the hook-switch; and the connections to the dial sender 9 arey conventional. Springs III-II of rthe hook-switch control the power circuit as mentioned and close first Awhen the handset is taken from its cradle. The hous- This reduces thevcommon resistance between the l quiet capacitor 42 andthe other capacitors which are connected to points', in the circuitiny which' the. voltage pulsations are high,v and hencewould i'riduj Vpillsatina .voltages in the capaci11011. 4,2.

ing, (notl shown) of theinstrument is not` connected directly to any point of the circ t; but the .005 mf. capacitor 8l is connected t thereturn conductor 51 of vthe power circuit. The housing is safe and.no electrical shock can' be obtained from it. The capacitor 8l eliminates ine noise. Conductor SI1 and terminal 9i provide a convenient arrangement for testing the filament voltage.

Amplifier tubes, such as 26, do not necessarily have their output -plate load impedance equal to their plate impedance. For example, thetubeZB 'should Work into a load often thousand ohms for 'maximum undistorted power output, but its plate impedance is rated at one hundredtwenty thousand ohms. In the conventional aritifsdetone substation, the induction coil used thereinis designed in accordance with welll-known formulas since the load impedance ofthe telephonetransmitter or microphone is equal toits internal irnpedance. However, the fact that the plate load of tube 26 is ten thousand ohms while its plate impedance (internal impedance) is one hundred twenty thousand ohms, made it` appear impossible to substitute apower Iamplifier tube, such as 26, for the usual transmitter in, $11.6. conventional anti-sidertone Vnetwork and stillretaih ,.a maximum output network. For desirable operation it a first step. This iixes Equations 4 and 1.

is important that the tube 26 should work into its optimum load (10,000 ohms), the subset circuit should be of the anti-side-tone type, and receiver circuit efciency should be as high as possible. It was only after extended study and experimentation that the present anti-side-tone network (Fig. l) was discovered. The desirable operation outlined above is achieved in the antiside-tone network of this invention, as will be seen from the following analysis of the circuit diagram of Fig. 4 which is a simplified showing of the anti-side-tone network of Fig. 1. In this circuit the condition of side-tone balance is achieved in the first conjugacy condition given by the relation.

g=Z-N (1) The second conjugacy condition ordinarily existing in anti-side-tone circuits, namely, that an E. M. F. acting in ZL produces no current in ZN holds approximately when N/ZCZILZR zT-2 2 2) Maximum power is delivered from ZT when ZCzDJfa/zcz' D=ZL+ZN (3) ZA ZT The power distribution ratio from ZT to ZL has the reciprocal property and equals that from ZL to ZT; that is, with the source in ZT the ratio of power delivered to ZL to the total that can be delivered by ZT is equal to the ratio of the power received by ZT, with the source at ZL, to the totalfpower that can be delivered by ZL.

The power distribution ratio Y is equal to the ration of power received by ZT to the power received by ZR with the source in the line. In the above circuit when the condition given by Equa tion l is satised Y is given by In the present arrangement the transmitting output can be increased by the volume control, that is, by increasing the amplification. This feature can be `taken advantage of to increase the receiving eiciency, as compared to conventional sets, even though there is no amplification for receiving. The ratio Y can be made small so that the power transmitted to the line, with an E. M. F. in the transmitter, will go down, but the power in the receiver, with an E. M. F. in the line, will increase, and by utilizing amplication to step up transmitting emciency, both high transmitting and receiving e-liciencies may be attained.

The choice of Y for the present arrangement is Then Equation 3 can be applied to connect the tube to its required load of 10,000 ohms by using this value of rated load impedance in the equation in place of ZT (internal impedance). While this will not be a maximum output circuit, it will be the best working condition for the tube. The rst conjugacy condition for side-tone balance, as given by Equation 1, will be maintained.

If the tube impedance were actually 10,000 ohms and delivered maximum power by Equation 3, while Equation 1 held, and if the second conjugacy condition held as given by Equation 2, then it can be shown that the network is a maximum output network, and an E. M. F. at ZL will deliver its maximum power to ZT and ZR while no power is given to ZN from ZL. The total power delivered from ZL is divided between ZT and Za by the relation 4, and/Za will receive all of the power that can be given to it from ZL with the chosen value of Y.

From the first conjugacy condition and the reciprocitytheorem, it can be shown that the receiving eiciency, measured by the value of the current in ZR with an E. M. F. in ZL, is not affected by changes in the resistance of the transmitter. Therefore, if the best value of ZR is calculated when the network is treated as if it were of the maximum output type, with the receiving power the highest that can be given to it with the chosen value of Y, the receiving efficiency will still be highest when the tube impedance is 120,000 ohms. Of course, this destroys the second conjugaey condition and the condition that the total power delivered from ZL is maximum, but the power transferred into ZR is not lessened and is still equal to the full amount that it would be if the network were of the maximum output type.

The foregoing conclusion may be summed up mathematically by eliminating ZA and ZT from Equations 2 and 3, for the receiving eiiciency is independent of ZT and ZA.

Rearranging (2) gives Zim/202D Z T Z R and rearranging (3) gives Equating the right hand members of (5) and (6) and solving for Za yields Having chosen a value of Y and xing ZN by Equation 4, Equation 7 then gives the value of ZR for delivery to ZR from ZL of the full receiving power called for in the ratio Y. When Y vanshes all the power that can be transferred from ZL is delivered to ZR, but this would leave no possibility of power delivery to ZL from ZT. With a value Y=.52 used in the present arrangement, ZR receives from ZL power which is 1.82 decibels below all the power that could be transferred from ZL, and the power delivered into ZL is 4.65 decibels below the maximum that could be transferred from ZT. A gain in receiving of 2.8 decibels, which equals the loss in transmitting, is calculated in relation to an induction coil of the conventional type with a value of Y=1.9.

In determining the value of the receiver impedance, as previously discussed, it has also been found necessary to decide where the peak response of the reeciving response-frequency characteristic curve should be. A response-frequency characteristic which favors the lower frequencies,

will suffer less in effective transmission than a the higher frequencies. JLIn the latter case, Athe 1" is placed Ori thehieher frequenies'to. y the' intelligibility,'but` these frequencies are a uated the most on the lQnger lines.""I'n accordance with the present invention I 'preferfthat the relative'response on the'lowerv frequencies, for xainpl'e'in the range' from approXiinatelySO() to 1,500 cycles be increased over that of the' conventional substation set. f "The substation circuit arrangement, as disclosed in Fig. 1, is powered exclusively from the commercial lighting outlet' at the subscribers substation. "Where the commercial lighting source is subject to occasional failure, itv may be desirable to`pr`ovide a carbon microphone" 85 in standby relation to the crystal microphone 5, the'carbon microphone being powered from the battery at the' telephone exchange. Such an arrangement is illustrated in Fig. 5 wherein the two-stage ainplifie'i', including the tubes 2l) and 2t,l is energi'zed from the power supply device including Athe rectifiers 36 and 31 with their'related filters Fi and F2, asabove described. However, the Vpower supply includes a relay 89 which is energized as long as the power supply is functioning to supply the' amplifier. While relay 89 is'energiaed, lit opens the connection of the carbon microphone 85' inseries with the telephone line and the bat- ,tory at the telephone exchange, not shown. When the lighting source fails relay 89 releases tc complete the connection of the carbon microphone in series with the mentioned battery. Under such conditions the carbon microphone 85 is ubsiituted. ier the @afstel microg one. i

1. In a telephone subset network of the type comprising""`a` receiver",` telpho'riically" connected thercin for reception of signals from a telephone lineja' microphone connected in said network', an amplifier for amplifying the response of said microphone and for delivering said amplified l:responseto the telephone line, said amplier including anelectron discharge tube having "a filamentary' cathode ui'rectiyheated by the' 'fie-wbr current thr'ethroughjthe combination therewith of means for temporarily causing pulsating current to flow through said cathode when the amplifier is initially placed in operation and means for thereafter causing substantially continuous current to flow through said cathode during the remainder of the operation of the amplifier.

2. In a telephone subset network of the type comprising a receiver telephonically connected therein for reception of signals from a telephone line, a microphone connected in said network, an amplifier for amplifying the response of said microphone and for delivering said amplified response to the telephone line, said amplifier including an electron discharge tube having a filamentary cathode directly heated by the flow of current therethrough, the combination therewith of means for temporarily causing pulsating current and a surge current to flow through said cathode, and means effective shortly thereafter for causing substantially continuous current to ow through said cathode during the remaining operation of the amplifier.

3. In a telephone subset network of the type comprising a receiver telephonically connected therein for reception of signals fro-m a telephone line, a mircophone connected in said network, an amplifier for amplifying the response of said microphone and for delivering said amplified response to the telephone line, said amplifier including an electron discharge tube having a filain hentay caiiiofie directif heated by a@ now, @i current therethrough, "thefcmbinati'on therewith of. means for temporarily causing unnltered rectified current to flowthrough said cathodeand means effective shortly thereafter "for causing 'filtered rectified current to lflow through said cathode during the remaining operation of the ampli er.. 4. A telephonesubset network comprising a receiver telephonically connected therein for the reception of signals from a; telephone line, a microphone connected in said network, an amplifier Afor amplifying the response of said microphone and for delivering said amplified response to the telephone line, said amplifier including an elecytron discharge tube having a filamentary cathode directly `heated by the flow ofcurrent therethrough, a'source of alternating current, a rectifier for converting said alternating current into pulsating current, a filter operably connectible to said rectifier for sultistantiallyv eliminating any alternating current component from said pulsating current whereby substantially continuous current is supplied to said filament, meansfor initially connecting said pulsating current directly to said filament when the ampliner iis first 'set into' operationjand means effective shortly thereafter to operatively connect saidffilter to said rectier whereby continuous current is supplied to s'aid cathode. y

5. A telephone subset network comprising a receiver t'elephonically connected thereinv for the reception of signals from a telephone line, a mi'- crophone connected in said network, an amplifier-'for amplifying theresponse of said microphone and for delivering said amplified response to the telepl'ione'line,I ysaidfainpliner including an electrorf ldischarge tube having av filamentary cathode direc'tlyheated by the flowy of ycurrent therethrough, a source of "alternating current, a rectifier 'for converting said alternating current into pulsating current, a filter operably connectible to said rectifier for substantially eliminating-'any alternating current component from said pulsating current whereby substantially continuous current is supplied to said lament, a hookswitch` having a primary set of contact springs and a secondary set of contact springs operating in the sequence named, means including said primary set of contact springs for initially connecting said pulsating current directly to said filament when the amplifier is first set into operation, and means including said secondary set of contact springs effective shortly thereafter to operatively connect said filter to said rectifier whereby continuous current is supplied to said cathode.

6. A telephone subset network comprising a receiver telephonically connected therein for the reception of signals from a telephone line, a microphone connected in said network, an amplifiel` for amplifying the response of said microphone and for delivering said amplified response to the telephone line, said amplifier including an electron discharge tube having a lamentary cathode directly heated by the flow of current therethrough, a source of alternating current, a rectifier for converting said alternating current into pulsating current, a lter of the resistancecapacitor type operably connectible to said rectifier for substantially eliminating any alternating current component from said pulsating current whereby substantially continuous current is supplied to said filament, means for initially conl1 necting said pulsating current directly to said filament when the amplifier is rst set into operation, and means effective shortly thereafter to operatively connect said filter to said rectifier whereby continuous current is supplied to said cathode.

7. A telephone subset network comprising a receiver telephonically connected therein for the reception of signals from a telephone line, a microphone connected in said network, an amplier for amplifying the response of said microphone and for delivering said amplified response to the telephone line, said amplifier including an electron discharge tube having a lamentary cathode directly heated by the ow of current therethrough, a source of alternating current, a rectifier for converting said alternating current into pulsating current, a lter operably connectible to said rectifier for substantially eliminating any alternating current component from said pulsating current whereby substantially continuous current is supplied to said filament, means for initially connecting said pulsating current directly to said filament when the ampliiier is rst set into operation, and means including a dashpot eiective shortly thereafter to operatively connect said filter to said rectifier whereby con tinuous current is supplied to said cathode.

8. A telephone subset network comprising a receiver telephonically connected therein for the reception of signals from a telephone line, a microphone connected in said network, an amplifier for amplifying the response of said microphone and for delivering said amplied response to the telephone line, said amplifier including an electron discharge tube having ka lilamentary cathode directly heated by the nolw of current therethrough, a source of alternating current, a rectifier for converting said alternating current into pulsating current, a filter operably connectible to said rectier for substantially eliminating any alternating current component from said pulsating current whereby substantially continuous current is supplied to said filament, a hookswitch having a primary set of contact springs and a secondary set of contact springs operating in the sequence named, means including said primary set of contact springs for initially connecting said pulsating current directly to said filament when the amplifier is first set into operation, means including said secondary set of contact springs effective shortly thereafter to operatively connect said filter to said rectifier whereby continuous current is supplied to said cathode, and a dashpot controlling the speed of operation of said secondary set of contact springs.

9. In a telephone subset network of the type comprising a receiver telephonically connected therein for reception of signals from a telephone line, a microphone connected in said network, an amplifier for amplifying the response of said microphone and for delivering said amplied response to the telephone line, said amplifier including an electron discharge tube having a directly heated filamentary cathode, a source of rectified pulsating current for heating said lamentary cathode, and a filter for converting said rectified pulsating current into substantially constant current, the combination of means for temporarily lowering the effectiveness of said filter, whereby the amount of voltage initially available at the output of said iilter is raised, and means for thereafter restoring said filter to full eiectiveness.

ARNOLD LESTI.

REFERENCES CITED The following references are Aof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,018,464 Nebel Oct. 22, 1935 2,055,921 Baker Sept. 29, 1936 2,275,581 Barton Mar. 10, 1942 2,282,405 Herrick May 12, 1942 2,346,504 Place Apr. 11, 1944 2,369,352 Herrick Feb.` 13, 1945 Certificate of Correction Patent N o. 2,487,455 November 8, 1949 ARNOLD LESTI It is hereby certied that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 7, line 50, for the word ration read ratio; column 8, lines 44 and 45, for that portion of the equation reading ZA Z A j; read Z;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofce.

Signed and sealed this 4th day of July, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oonmssz'olner of Patenls.

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