US2656487A - Voltage supply control circuit - Google Patents

Description

Oct. 20, 1953 w. o. FULLERTON VOLTAGE SUPPLY CONTROL CIRCUIT 2 Sheets-Sheet 1 Filed June 29, 1950 M/l ENTOP W O. FULLERTON ATTORNEY Oct. 20,1953 w. o. FULLERTON 2,

VOLTAGE SUPPLY CONTROL CIRCUIT Filed June 29, 1950 2 Sheets-Sheet 2 //v l/ENTOP W0. FULLERTON ATTORNEY Patented Oct. 20, 1Q53 NI ED PATENT OFFICE William 0. Fullerton, West signer to Bell" Telephone no a News Orange, N. J., as- Laboratories, Incor- Y., a corporation of New Application J une 29,1950, Serial No. 171,080

1 This invention relatesto control circuits and, more partifiularlyytjo'control circuits of the interlocked type. 'The invenucna sa relate to'flse quentially operatingfcontrol'circiiits'of th interlocked type'whereinohe am nt mu t be en 5 ized before another elemen't'is 'ene'r' zed.

' The invention ma xbel'iised with v ioiisty-pes of eouipnientand apparatus, such as electron dis charge devices. When Used with electron discharge devices, the invention isparticu1ai'1ynsee fill for controlling automatically in' proper" quence the applicatfon of electric energy to electrodes of Slwh'devides. Accordingly, for th'e'pur-' pose of explaining'it's pfr" cipleis'and features of operation, the invention'will he desc rilcedwith reference to its use ith felcctronfldischarige 'de} vices. The desirability of automatically delaying the app ca ve e t t ei yitc'joine ele' trod of an lelectronfdischarge cey cemran ass I terval of time aftertth e applicationp jectr'ic energy to anotherelect rodje cf thesa'me d'e cc is Well known. "seduentialfapplication of energy is imDQrtantin the case of elec on discha e d ice of he therm eni alhade vpefp'r automatically delaying thejapplicationjof electric potential to he anode of. suchad'e ceuntil its cathode has been heatedt'o" m a ur Th s seq jer I i 1 ularly important when the grid {of n e,fctrcn discharge device is closelyj'spaced withire'spe'ctjto. its anode and i evenmoreclosel s aced w t lr spect to its cathode, hecau sef if"eitlier'fthe anode or cathode potentials should b applied to their respe t ele r e be'ftoreith icath prope ly h a e thee id ii -934 I A i damaged.

Accordingly, it is an object of this invention to provide a control circuit withinterloclging means whereby the deenergizationof one portion circuit causes another'portion of "the c become deenergi ze'd. Another object of the invention is to provide a sequentially operating control circuit w 7 whereby onelportion' of the ,circuit s n gized until another .porticn oi the circu t been energized. A further object is toprovide a, control circuit for automatically delaying the application "of anode and cathode potentials to anielectron discharge device T for a preassigned interval 'of time afterthe application of cathod h'eating current to. the device. Anadditionalobject isto provide acontrol cir- Qu for is ntmuin ;,.the application; of anode and cathode potentials to an electron discharge 3 (listings. 01. 315-104 device in the event of an interruption in its cathode heating circuit.

Still another object is to provide a circuit with control means for applying cathode and anode potentials from common supply sources to a plurality of electron discharge devices only after the expiration of an assigned interval of time after a circuit has been closed tov connect cathode heating current to the devices and to further provide the two circuits with interlocking means whereby an interruption of the cathode heating current supplied to any one of said devices effects an interruption of the up ly of anode and cathode. potentials to all of the devices.

These and other objects of the invention are accomplished by connecting the energizing cir cult of a control relay in parallel With the cathode heating circuit of an electron discharge device. h ner i n circuit includes delay means of the thermal bimetallic type. An armature of the control relay controls a circuit for applying an electric potential totheanode of the device. Another relay is connected in the anode circuit ior controlling the application cf a different electric potential wane cathode of the device. 'I hus, when current is supplied to the cathode heating circuit; the s meeurrent will also be delivered to the control relay'which will operate its armature at the expiration of the delay period to apply voltage to theariode of the electron discharge device." As so'onas the anode current rises to an assigned value, the relayf'in the anode circuit will operate its armature to apply voltage to the cathode of the same electron discharge device. Since the 'circuitior energizing the control relay is interlocked with the circuit for heating the cathode, any interruption or the cathode'heating current will cause the control relay to release ma mature thereby opening the anode circuit which in turn, causes thean'ode relay to release its armature to open the cathode circuit. When the cathode heating current isrestored, the sequential applicaticnof potentials to the anode and. cathode will not occur again until after the expiration of the delay periodrequired for energizing the cQntrOl relay. .In certaintypes of equipment, a plurality of electron discharge'device's are employedwith common sourcesof anode and cathode potentials. and cathode heating current. In such cases, additional interlocking means'are provided for interrupting the supply of anode and cathode potentials to all or the electron discharge devices whenever there is an interruption in the cathode heating current supplied toany one of the electron discharge devices.

from a 25 -volt battery B, negative bias potential for the cathodes |3-23 is supplied from an ll-volt battery C, and heating current for the cathode heating filaments |424 is also supplied from the battery 0. For the purpose of explanation,

the tubes H3 and 2e may be considered as constituting a two-stage broad-band microwave amplifier in which signals from any suitable source, such as the wave guide W, are applied to the control grid If! for amplification in the tube iii and are then supplied over another wave guide Y extending from the anode l to the control grid 22 for further amplification in the tube 28.

In this particular embodiment of the invention, the tubes l and 2B are of the type disclosed in Patent 2,502,531 issued April 4, 1950, to J. A. Morton et al., and have their electrodes closely spaced; that is, the grids !2--22 are located very close to the anodes ||-2| and are located even more closely with respect to the cathodes |323. Due to this close spacing of the electrodes, it is important that the application of operating potential to the anodes ||2| be delayed until the cathodes |323 have been heated to their proper operating temperature by their respective heaters |424. It is also important that the application of bias potential to the cathodes |323 be delayed until the anode current in their respective tubes |020 has risen to a proper operating value. If either of these precautions is not observed, electrostatic charges are liable to accumulate between the anodes ||2| and the grids |2--22 or between the grids |2-22 and the cathodes |323 with the result that the grids |222 would be physically damaged. It is therefore desirable to employ a control circuit for automatically applying in proper sequence electric energy to the electrodes of the tubes I0 and 20.

In the control circuit shown in Fig. 1, the electrodes, including the heater filaments, of each of the tubes |3 and are connected individually to electric conductors, the conductors associated with each tube |020 being grouped in the form of a cable 3340. Each of the cables 40 is provided at its end with a multi-element terminating plug 3|4|. It is to be noted that each of the terminating plugs 3|4| has two of its elements electrically connected together by a short-circuiting conductor 3242 to form interlocking means described hereinafter. The plugs 3|4| are adapted to be detachably inserted into multi-element receptacles 33-43 10- cated adjacent thereto, each of the elements of each of the receptacles 33-43 being adapted to make electric connection with a corresponding element in its respective associated plug 3|-4|. The elements of the receptacles 3343 are connected individually to electric conductors which supply electric energy thereto from batteries 13 and C in a manner that will now be described.

In operating the control circuit of Fig. 1, the

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first step is to insert the plugs 3|-4| into the receptacles 33-43 thereby electrically connecting the conductors in the cables 33-40 to the conductors extending from the elements of the receptacles 33-43. The next step is to operate manually a conventional toggle switch 50 for closing a normally open control contact 5|. The closing of the contact 5| completes a circuit extending from battery C, over the filament elements of receptacle 33 and plug 3|, through the heating element l4, and then over conductor 52 to ground 53. The actuation of contact 5| also closes a similar circuit for current from battery C to flow along conductor 54, over the filament elements of receptacle 43 and plug 4|, through the heating filament 24, and then over conductor 55 to ground 53. Thus, heating current from. battery C is applied substantially simultaneously over parallel circuits to the filaments l4 and 24- for heating the thermionic cathodes l3 and 23.. Since these parallel circuits are not interlocked, the withdrawal of one of the plugs 3|-4| from its respective associated receptacle 3343 will affect the opening of only the filament circuit associated therewith thereby permitting the other filament circuit to remain closed.

The cathodes |3 and 23 do not become heated to their proper operating temperature immediately upon the closing of the filament circuits but instead must be heated for a short interval of time in order for their temperature to be raised to the proper point. It is therefore desirable to provide means for delaying the application of anode and cathode potentials for a corresponding interval of time so that these electrodes will not be energized before the cathodes have been heated to their proper operating temperature. In this embodiment of the invention, the delay means for accomplishing this result includes a relay 60 having its energization controlled by thermal delay means of the bimetallic type comprising a heat-responsive bimetallic strip 6| composed of any two suitable electrically conductive metals having different thermal expansion coefficients. Associated with the bimetallic member 6| is a heating element 62 of any suitable construction, such as a resistance wire which becomes hot in response to the flow of electric current thereover. This heating element 62 is disposed in any appropriate heat-transfer relation with respect to the heat-responsive member 6| for transferring heat thereto. The member 6| is designed in such a manner that, upon being heated by the heat transferred thereto by the element 62, it gradually bends down-ward until it finally engages its associated contact 63 to complete an electric circuit described hereinafter. The element 62 and the member 6| are so constructed and proportioned that the interval of time between the energization of element 62 and the engagement of member 6| with its contact 33 is substantially the same as the interval of time required to heat the cathodes |323 to their proper operating temperature.

The circuit for energizing the heating element 62 extends from battery C in parallel with the cathode heating circuits. Thus, when the control contact is closed in the manner described above, a circuit is completed for current from battery C to flow along conductor 56 and associated elements of receptacle 33 and plug 3|, along the short-circuiting plug conductor 32, through .plug 3| and receptacle 33, along conductor 51, over elements of receptacle 43 and plug 4|, along the second coupling conductor 42, out through plug} and receptaae It, along conductors to and over the ron armature and outer contact 'of relay F8, and then through the heating element 6'2 to ground 61. is to he that the elements of the plugs B l -ll that are con;- nected by the short circuiting conductors ab-42 constitute interlocking means whereby the energ'ization of the thermal delay deli-ice Fl -'62 is prevented if one of the cathode h'afin'g circuits extending to eitherof the filaments 1 4 open due to the fact that its "associated plug M44 is not inserted in its respectiveassociated receota'cle 33 The closing of the above-described circuit for energizing the heating element "62 "causes the elerhe'n't 62 to generate heat which is transferred to the 'hea t respohsive inmbr 61. At the end of the assigned delay period, the bi'rnetelllic member 61 engages its assoeiafted een'taet 53 to close a path for "energizing the ielay to. This energizin'g "circuit extends from the junction pointcs located in the circuit for energizing the heating e1eine'n't'62, along conductor 66, over *member "6ft, contact 63, and then "through winding of relay 6D to ground 64. Upon the closing of this energizing circuit, relay 60 operates both of its armaturesto engage their inner contacts.

When the top armature of relay "60 moves down out of engagement with its outer contact, it'dis'connects battery C from the heating element 62 thereby causing both element 62 and member 61 to cool. Theengagement of the "top armature of 'r'elay'iib with its inner contact closes another path for energizing re1a'y"60. This secondpath extendsf'from the junction point "'65, along-conductor 59, top armature and inner contact of relay ta, 'andthe'n through the winding'of relay 6!! to ground "64. Relay *80 *Will nowbesupplie'd with energizing 'curr'erit over "these two parallel paths simultaneously for "a "short period of time. When the bimetallic member 6 I has cooled sulfici'ently to movejoutofeng'agement with its associated contact '63, "it "thereby opens [the 'upper parallel path which includes the "conductor 66. The lower 'parallel'path, 'Whlchincludes the 'conduct-or 59, will remain "closed until "either the control contact 5| is opened'or one of "the Iilugs 3 I 4 I is withdrawnfrom itsrespe'ctive associated receptacle 33 43.

The enga'gement of the bottom armature of relay "6D with its contact applies electric energy from "battery 13 over obvious parallel circuits extending -throughthe windin'gs of relays ll- 12 and over elementsof receptacles ttg fl andolugs 3l-'4I to the anodes PI- 2]. Thus, o erating potential Iromsbatteryli is applierl to the anodes I l-2 l anflassigned interval of time :aiiterelectric energy from battery "C has -been applied to the cathode heating filaments 11-2-4.

-As was stated above, it is important that the application =01 bias potential to the cathodes Iii-4 3 be delayed until "the anode "current in their respectivetubes iiihaszrisen to a proper operating value. This is-eccoinplished by means of-the relays It-I2 which'have their windings connected in the 'anode circuits. When the equipment is in an uno'perated condition, the relays J' l -12 are not energized-and;-consequently, their armatures are in --engagement with :their outer contacts -thereby :closi'ng parallel "circuits extending: from 'thecathodes Ii -'23; through elements of the-. plugs a i -M andtreceptacles m n, and then al'ong 'c'on'dnctors -13-'N, respectively, to ground 15. Thhs, #the cathode's l 3- 23 are at ground potential tvhen the equipment is in an unoperated condition. The relays 11-42 are marginal in that they are so adjusted that they will not operate their armatures until the electric energy flowing through their windings exceeds an assigned value. In the present embodiment offthe invention, each of the relays 11-12 is adjusted to operate its armature when the current of its associated tribe Ill- 20 irises to a value or approximately 5 mill'i-amperes.

when either one of the relays i l-12 =becomes sufficiently energized, its armature is away from theouter contact to engage the inner contact. The removal of the armature tram its outer contact disconnect the associated cathode ls- 2 from "ground 1-5. The engagement of the actuated armature with its inner contact closes a circuit for cathode bias pote'ntia'l to applied "from battery through the 'respwtive associated series-connected potentiometer IS- H, along the respective conductor T3 11, and then over the elements of the associated receptacle =43 -et1'1dplug m to the respective cathode T8 23. Since each of the tubes Ill-Z0 is of the type having it plate current determined by potential *difi're'nce between its grid 12- 22 and cathode 13- 23, the application of bias potential to the cathodes l t-23 tends to stabilize the plate current in the tubes 20.

It "is to be noted that, since the sequentially operatingcon-trol circuitbf thi's inventi'o'n includes interlocking means -'comprising the shortoircuited elements *of the plugs 31-41, any "interruption of the cathode heating current supplied to either one of the heating filament ll24 willeffect an interruption of fihesupplyofanode and cathode potentials "to both of the tubes 111*2'0. For "example, if the supply of heating current to the filament 1 4 of the tube H1 is interrupted 'by the withdrawal of its associated filug 3! from the receptacle 33, "then the circuit for' energizing relay 60 will be open at those elements of receptacle 33 "that "are connected to the conductors 56 and 51. Thus, since the shortcircuiting conductors 32 and "'42 are connected in series, the removal-of either one from the-energi'zi'n'g circuit of relay 60 will cause the relay 60 to release its'armatures eventhough the-other remains connected "to the associated elements of its respective associated receptacle 33- 43. release of the-bottom armature of relay 60 615- con'nects battery B "from the circuits extending through the windings of relays 'l'l and 12 "to the anode ll-21. This, in "turn, effects ='a -'rlease of 'the "arrnatureso'f relays 1-1 and 1-2 thereby disconnecting "the cathode conductors 13 and 14 from thefbia-sing circuit extending f'rcm 'ba'ttery C through the potentiometers T6 and -11 and connecting the cathode conductors 13 and M to ground 15. At the same "time, the release of the top armature of relay to disconnects the conductor 58 from the winding-of relay '60 "and connects it to the heating element 62 in the thermal delay device 6 I 62.

when the cathode heating circuit is "again closed by "the "reinsertion of "the plug H in the receptacle -33, the sferieseircult' through the coupling 'con'ducto'rs 32- 12 will again be closed to apply currentfrom battery*Bnver the'c'onductor 59 to the thermal delay device 6 I 62 to recycle its operation. At theexpiration1515'the "delay period described above, relay to "again becomes energized and operates its armatures' to efiectia repetition of the above 'described sequential application oi potentialsto"theianodesfl;g r -a cathodes |3-'23.

The invention is particularly useful when employed with equipment having a number of electron discharge devices which require interlocking of their electric energy supply circuits. Such an embodiment of the invention i shown in Fig. 2, in which an equipment rack R is provided with a plurality of transverse shelves SI, S2, and S3 for supporting several related equipment units. The top shelf Si supports a transmitter-modulator unit M; the middle shelf 52 supports three amplifier units Al, A2, and A3 constituting a three-stage amplifier; and the bottom shelf S3 supports a transmitter control unit T. As is indicated in Fig. 2, the transmitter-modulator unit M and each of the three amplifier units Al, A2, and A3 include electron discharge devices 20!, 202, 203, and 234, respectively, which may be of the type disclosed in the above-mentioned patent to J. A. Morton et al. In each of these units, the anodes Pl, P2, P3, and P4 and cathode heating filaments Fl, F2, F3, and F4 or the tubes 20l, 202, 203, and 204 are connected to elements of respectively associated multi-element receptacles 2| i, M2, M3, and 2 Hi. In order to simplify the drawing, the grid circuits of the tubes 20|, 202, 203, and 204 have not been shown in Fig. 2. For the same reason, the cathode biasing circuits of the tubes 20l, 202, 203, and 204 have not been shown, as it is to be understood that they are of the same type as those shown in Fig. 1; that is, each cathode is connected to ground over the armature of a relay, similar to the relays ll and 12 shown in Fig. 1, which, upon being energized by its respective associated plate current, operates its armature to connect biasing current from battery B2 to the cathode. It is to be noted that two of the elements of each of the receptacles 2||, 2|2, 2|3 and 2|4 are electrically connected together by coupling conductors 2l5, 2H3, 2|'|, and 2|8 to form interlocking means similar to those described above.

Associated with each of the receptacles 2||, 2|2, 2|3, and 2|4 is a multi-element plug 22l, 222, 223, and 224 adapted to be detachably inserted therein. The elements of the plugs 22l, 222, 223, and 224 are connected individually to electric conductors which are grouped in the form of cables 225, 226, 221, and 223 and which are connected at their other ends to elements of double plugs 229 and 230. The plugs 22!) and 230 are adapted to be detachably inserted into multi-element receptacles 23| and 232 associated with the transmitter control unit T. Two of the elements in each of the receptacles 23| and 232 are connected together by coupling conductors 234 and 233, respectively, to form additional interlocking means as will be described hereinafter.

The transmitter control unit T also includes a thermal delay device 26|-262, similar to the device 6|--62 shown in Fig. 1, for controlling the energizing circuit of a relay 260.

When all of the plugs 221, 222, 223, 224, 229 and 230 have been inserted in their respective associated receptacle 2H, 2|2, 2|3, 2 l4, 23l, and 232, the operation of the equipment can be initiated by manually actuating a conventional toggle switch 250 to close a normally open control contact 25|. The closing of the contact 25| causes heating current from battery C2 to be applied over obvious parallel circuits through the individual cathode heating filaments Fl, F2, F3, and F4, and then to ground 253. At the same time, current from battery C2 is also applied along conductor 256, over the elements of recep- Fl, F2, F3, and F4.

tacle 2| I that are connected by the short-circuiting conductor 2|5, over elements of receptacle 23| that are connected by conductor 234, similar elements of receptacle 2l2 connected by conductor 2|6, along conductor 25?, along the coupling conductor 2|| associated with receptacle 2|3, over the coupling conductor 233 associated with receptacle 232, over coupling conductor 2|8 associated with receptacle 2l4, along conductor 258, over the top armature of relay 280, and then through the heating element 282 to ground 234.

After being heated for an assigned period of time, similar to that required for operating the thermal delay device 6|52 described above, the bimetallic member 26| engages its contact to close a circuit for energizing relay 280. Relay 260 operates its armatures in the same manner as that described above for relay 00, the actuation of its bottom armature applying potential from battery B2 over obvious parallel circuits to the anodes Pl, P2, P3, and P4. Thus, operating potential from the battery B2 is not applied to the anodes P l, P2, P3, and P4 until an assigned interval of time after the application of current from battery C2 to the cathode heating filaments As was stated above, it is to be understood that when the anode current in any one of the tubes 2Bl, 202, 233, and 224 rises to an assigned value, a relay, similar to relay H and 72 in Fig. 1, is caused to operate its armature to disconnect its associated cathode from ground and to connect it to the battery C2.

Since this sequentially operating control circuit includes the interlocking means described above and since these interlocking means are all connected in series, any interruption in the supply or" current from battery C2 to any of the cathode heating filaments Fl, F2, F3, or F4 will effect an interruption of the supply of anode and cathode potential to all of the tubes 20l, 202, 203, and 204. For example, if the supply of current to the filament Fl is interrupted by the withdrawal of the plug 22l from the receptacle 2| I, then the energizing circuit for relay 260 will be open at the two bottom elements of plug 22l 'with the result that relay 260 will release its armatures. The release of the bottom armature of relay 250 opens the circuit from battery B2 to the anodes Pl, P2, P3, and P4 and this, in turn, efiects the disconnection of battery C2 from the associated cathodes in the manner explained above in connection with the description of the circuit shown in Fig. 1. When the plug 22l is reinserted in its receptacle 2| I, the thermal delay device 26|262 recycles its operation to efiect a repetition of the above-described sequential application of potentials to the anodes and cathodes of the tubes 20|, 202, 203, and 204.

What is claimed is:

1. In combination, a plurality of thermionic tubes each having a filament and an anode, a plurality of cables each comprising a plurality of electric conductors, the conductors in each cable being connected to the filament and anode of a different one of said tubes, each of said cables having a multi-element terminating plug with two of its elements short-circuited, a plurality of multi-element receptacles for receiving said plugs, each of the elements in each of said receptacles being adapted to make electric connection with a corresponding element in the assoc ated plug, a source of filament potential, a first circuit for connecting said source to a filament element in each of said receptacles, a source of anode potential, a second circuit 'for connecting said source of anode potential to an anode element in each of said receptacles, said second circuit being normally open, a relay having a first armature adapted to close said second circuit, a third circuit for energizing said relay, said third circuit having two normally open parallel paths, a thermal delay device for closing the first of said paths for energizing said relay, and a fourth circuit for connecting said source of filament potential to said thermal delay device for energization thereof, said fourth circuit including said short-circuited elements of each of said plugs and the corresponding receptacle elements connected in series, said relay having a second armature actuable in response to the energization of said relay for disconnecting said thermal delay device from said fourth circuit and for closing the second of said parallel paths.

2. In combination, first and second sources of electric energy, a plurality of thermionic tubes each having a heating filament and an anode, a corresponding plurality of detachable pairs of multi-element circuit connectors, each of the elements in each of said connectors being adapted to make electric connection with a corresponding element in the companion connector when the two connectors of a pair are in engagement, one connector in each of said pairs having an element connected to the filament of a respectively different one of said tubes and the other connector in each of said pairs having a corresponding element coupled to said first source whereby the engagement of the two connectors in any of said pairs efiects the connection of the filament in the corresponding tube to said first source, a plurality of circuits each extending from a respec tively diiferent one of said anodes to said second source, a relay adapted when energized to effect the closure of all of said anode circuits and adapted when unenergized to efiect the opening of all of said anode circuits, an energizing circuit extending from said first source for energizing said relay, and control means for limiting the closing of said anode circuits by said relay to periods when all of said filaments have been connected to said first source, said control means comprising two short-circuited elements in one connector of each of said pairs and circuit means for connecting said short-circuited elements and their companion elements in the other connectors of each of said pairs in series with said first source and the energizing winding of said relay whereby the disconnection of any one of said filaments from said first source caused by the disengagement of its associated pair of connectors effects the opening of said relay energizing circuit.

3. The combination recited in claim 2 and having a cathode in each of said thermionic tubes, each of said cathodes having a supply circuit extending through elements of respectively different pairs of said connectors, and a plurality of marginal supply relays each adapted when unenergized to connect a respectively different one of said supply circuits to ground and adapted when energized to connect its associated supply circuit to said first source, each of said supply relays having an energizing lwinding connected in a respectively different one of said anode circuits whereby the energization of all of said supply relays is limited to periods when all of said anode circuits are closed, each of said marginal relays being so adjusted as to operate only in response to the flow in its respectively associated anode circuit of electric current having a magnitude in excess of an assigned value.

WILLIAM O. FULLERTON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,545,886 Dachary July 14, 1925 1,788,513 Fultz Jan. 13, 1931 1,813,541 Lauritsen July 7, 1931 1,816,913 Schelleng Aug. 4, 1931 1,876,320 Trogner Sept. 6, 1932 1,893,223 Burkle Jan. 3, 1933 1,946,607 Afiel Feb. 13, 1934 2,017,126 Kroger Oct. 15, 1935 2,152,472 Fyler Mar. 28, 1939 2,555,689 Gehlsen June 5, 1951 OTHER REFERENCES Emerson Radio Television Service Manual, Figs. 5-15, schematic diagram Model 608, July 14, 1949.

Western Electric, Telephone Apparatus Catalog N0. 6, page 157, January 21, 1926.

RCA Tube Handbook HB-3, vol. 3-4, type 874, two pages (under Misc. Types), January 15, 1937.

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