US2439844A - Amplifier and oscillator circuit - Google Patents

Description

April 20, 1948. R. J. DAVIS AMPLIFIER AND OSCILLATOR CIRCUIT Filed oct. 29, 1943 Patented Apr. 2O, 1948 AMPLIFER AND osoILLAToR cIncUrr Robert J .o Davis,y Luthervlle, Mtl., assigno'r to Bendix Aviation Corporation,V New York, N. Y., a

corporation of Delaware Application October' 29, 1943, Serial No. 508,177'

13 Claims. (Cl. Z50-17) v This invention relates to. vacuum tube signal amplifiers and more particularly tol vacuum tube signal amplifiers which may be optionally employed as generators of alternating current energy.

Present day radiotransmitters are normally designed to emit a number of differentl types of radiation, among which are: voice modulated continuous waves, keyed continuous waves and keyed continuous waves with a steady modulation tone impressed. The generation of waves ofthe third type requires that a steady audio tone, be. generated within the equipment which is then utilized to modulate. the transmittedl energy. This is usually accomplished by the use of a separate oscillator tube generating the desired frequency which is set into operation and connected to the modulating amplifier when keyed MCW, as the third type of. radiation is often designated, is to be transmitted. During periods of Voice modulated transmission, the modulating oscillator is deenergized and thefmodulating ampliiier is actuated by voice currents from the speech amplifier. In some cases, the modulating,

frequency is generated by disconnecting the speech amplifying or modulator tubes fromttheir normal circuits and connecting them to a new set of circuit components arranged to provide sustained oscillations. In each of the above instances, the provisions made for generating the tonemodulation frequency have added materially to the bulk, expense, and/or weight of the equipment as it has been necessary to provide additional tubes and circuit components,

One of the principal objects of this invention is to provide new and novel means for optionally utilizing a signal ampliiier as an oscillation generator.

Another object of the invention is to provide new and novel means for optionally utilizing a vacuum tube as a degenerative signal amplifier and as an oscillation generator.

Still another object of the invention is to provide a radio transmitter capable of emitting both voice modulated continuous waves and keyed MCW having less bulk and Weight than existing apparatus of this type.

TheA above objects and advantages of the invention are accomplished by providing means forV selectively impressing energy from the output circuit of a vacuum tube ampliiier on thek input circuit of the same amplifier with a positive or negative feedback relationship. The positive feedback is of a magnitude producing sustained(- oscillations whose frequency may be controlled 2v by a tuned circuit simultaneously connected in the output circuit or in-putY circuit of the ampliiier tube; and the negative feedback reduces. the amount of distortionl produced in the amplica.- tion process in the well known manner whenothe tube is. acting as a signal amplier.

Other objects and advantages will in part be disclosed and' inpart be obvious when the following speciiication is read in conjunction with the drawing which. is a schematic diagram of a transmitter modulator unit incorporating the in,-

vention. r

Referring to the drawing, the control grid ll ofthe speech amplifier 2 is connected to ground through the gridleak, resistor 55 and to a source of voice currentsas. indicated. Energy from this source is applied to the speech ampliiier only when. it l isv desired to transmit voice modulated. ca ;rriier waves. Theanode 3 of amplifier 2l is. connected throughv theY anode load 'resister 5i to a contactll' of the switch section di, and an arin 16, which is a part ofthe same switch section is connected to the positive terminal ofthe intermediate voltage source 4 and is selectively mov'- able between the contact. 84 and a second contact 13; The source 4 may supply adirect current po tential of 380 volts. The anode circuit of tube 2 is completed by the cormectionl of the negative terminal or the source 4 directly to ground, and theI connection of the cathode 6 to ground through the paralleli combination of resistance 8 and capacity 9 to supply operating bias for the control grid i; A heater l elevates the temperature ofY cathode 6` as required for functioning of the tube 2. The heater energizing circuits for this and the remainder of the thermionic tubes in the apparatus will be described in detail inA a later paragraph.

The anode terminal of the resistor 5 is coupled by the capacitor I3 to the control grid ll of the oscillator and driver tube l2', here shown to be ofthe tetrode type,V and a grid resistor lli isconnected'between the control grid il and the mov-n able arm 25 of the switch section 2%, selectively engageable with the contacts liti and 61'. Accelerating grid 20 of the amplifier i2 is excited from the sourcer throughl dropping resistor 85 and the contacts l5, I8v ofv the keying relay I9, while the anode I4 of this amplifier is connected with the source A through the primary I6 ofthe transformer I l. Connected iny parallel between grid 20 and ground are the bl'eeder resistor 86` and bypass capacitor 35 to provide for' adjustment of the' direct grid current voltage and grounding'for alternating-currents. The cathode 3 2| is connected to ground through the resistor 23 and capacitor 24.

A resistive voltage dividing assembly 68, 69 is connected between the anode I4 and ground inseries with the blocking capacitor 10, and the ungrounded terminal of the grounded resistor 69, junction .point is attached to the terminal 61 of switch section 26. The values of the resistors 63, 69 are selected to be large as compared with the impedance presented by the transformer I1, and the voltages appearing at the resistor junction point |00 are substantially 180 degrees out of phase with the voltages impressed on the control grid Anode I4 is additionally connected to the terminal 13 of switch section 6I by the parallel circuit of inductance 1I and capacity 12, which are selected to be resonant at some desired frequency in the audio spectrum, usually approximately 1,000 cycles per second. This circuit determines the frequency at which the carrier is modulated when Vmodulated continuous wave transmission is employed.

The output energy from the amplifier I2 isimpressed on the transformer I1 having the center tapped secondary winding 21, whose outer terminals are connected individually to the control grids 28, 29 of the amplifiers 30, 3|. The anodes 36, 31 of these ampliers are connected to the outer terminals of the center tapped primary Winding 38 of the modulation transformer 39 and the center tap 'of this winding is directly connected to the positive terminal of source 4. Excitation for the accelerating grids 40, 4| of the amplifiers 30, 3| isv derived from the source 4 in series with the dropping resistor 18, and these grids are also connected to ground by the bleeder resistor 19 paralleled by capacitor, whose impedance is negligible at the operating frequencies. The exact values of the accelerating grid supply resistors are determined by the Voltage of source 4 and the characteristics of the particular tube employed. Electron emission for the tubes 30, 3| is provided by the cathodes 32, 33 located adjacent the heaters 8|, 82, respectively, and the cathodes are connected to the positive terminal of the heater 82, which is also connected to the positive terminal of the 28 volt source 62 through the filament dropping resistor 34. As the center tap of the winding 21 and theA negative terminal of the source 62 are grounded, this connection provides the necessary bias for the control grids 28, 29. The two tubes 30, 3| serve as the conventional push-pull power amplifier delivering the necessary power to modulate the final amplifier stage of the radio frequency amplifier.

One of the terminals of the winding 21 of transformer I1 is connected by the blocking capacitor 83 and the resistors 9| and 92 in series to the cathode 2| of amplifier I2, and the potential at the junction point |02 of the two resistors is impressed on the contact 66 of the switch section 26. The connection of this combination to the winding 21 is selected to provide at contact 63 potentials which are substantially in phase with alternating current potentials impressed on the control grid I, and the magnitude 0f the resistors is chosen to provide negligible loading of the secondary 21. The low potential end of resistor 9| may be considered at ground potential for alternating currents because of the bypassing action of capacitor 24, but is above ground potential for direct current because of its connection to the cathode 2 I.

Located in the transmitter, and excited byV aV radio frequency source not shown in the drawings, is the final radio frequency power ampliiier 90. The radio frequency energy from the driver is coupled to the control grid 93 by the capacitor 44, and the resulting flow of grid current through resistor 43, connected in series with the choke 42 between grid 93 and ground, provides the operating bias. Choke 42 prevents excessive loss of radio frequency driving power to the resistor 43. Electron emission for the amplifier is provided by the filament 46 having one terrninal connected to ground and the other connected to the terminals of the heaters 25 and 8| marked A. The heater and filament circuits thus comprise heaters 1 and 25 in series, connectedin parallel with heaters 8| and B2 in series, which series-parallel combination is then connected to the source 62 in series with the dropping resistor 34 and the power amplifier lament 45.

Direct current excitation for the accelerating grid 41 is supplied from the source 4 through the keyed contacts l5, I8, secondary winding 49 of the modulation transformer 39, and a dropping resistor 48 whose grid end is connected to ground through capacitor 50 to insure the necessary shielding efciency for stable amplification. The grid 45 of the tubel shown in the diagram is a suppressor grid and is connected to ground for further improving internal shielding and diminishing secondary emission effects. The anode 5| is provided with power from the high voltage source 52, which may have a, potential of 1000 volts, by connection through the output choke 53, lter choke 54 and the secondary winding 59 of the modulation transformer 39. The eciency of choke 54 in eliminating radio frequency potentials from the winding 59 is improved by the connection of the capacitor 60 between the junction of chokes 53 and 54 and ground. The radio frequency power existing in the output circuit of the amplifier 90 is conveyed to the radiating antenna 'I5 by the blocking capacitor 11 and the variable loading inductor 16, which is varied to neutralize the residual reactance of the antenna proper.

I-t may be seen by inspection that amplifiers I 2 and 90 are operative only when contacts I5 and I3 are in the closed position, as they are when the winding of keying relay vI9 is energized. Operation of this relay may be controlled by the key 63 connected between the positive terminal of source 62 and the ungrounded terminal of the operating winding for relay I9. Since it is desired to have ampliers I2 and 90 continuously in operational uring telephone transmission, a switch section 60 is provided, having the arm 65 and contact 64 connected to the respective terminals of the key 63. During the time that switch section 60 is in the closed position, the operating Winding of relay I9 is continuously excited, thereby maintaining amplifiers I2 and 90 in the desired continuously energized state. As shown in the diagram, switch sections 26, 60 and 6I vare mechanicallycoupled by the member 81, and the switch sections are therefore simultaneously moveable. The composite switch thus formed may be termed the emission selector switch, as its position determines the type of intelligence radiated by the apparatus. When in the right hand position, the circuits are those required for the 'production of keyed modulated continuous waves, while with the emission selector switch in the left hand position, the circuit arrangement is that necessary for the radiation of telephonie signals. The operation of the switch assembly may` be readily visualized from the Code-Phone markings in the` drawing.

VWith the foregoing in mind, the opera-tion of the apparatus may now be readily understood. AssumingY the emission selector in the Code position, the anode circuit for amplifier 2 isbroken at the contact 84, thereby disabling the telephone input system, andthe arm M is in engagement with the contact 13, thereby connecting the. tuned circuit H, I2 in shunt with the primary I6 ofl the transformer H. Simultaneously, the cathode end of the grid resistor lI!! is connected tothe tap- I-2 on the. voltage dividerv 9 l, 92A by the switch sectionV 2,l coupling anode circuit disturbances. of the amplifier I2 back to the control grid II in aiding relationship to an extent causing the tube I2 act as anindependent generator of oscillations when accelerating grid voltage is supplied. Optimum voltage conditions for oscillation Vare secured; by the connection of the voltage divider to. the cathode 2l,v which removes from the grid Il the potentialgenerated by the how of anode current through the cathode bias resistor 23 and allows the control grid to assume an optimum value of bias, generated by the now of gridA current through the grid resistor It which now functions in the same manner as the grid leak in conventional oscillator arrangements, and the frequency of oscillation is determined by the insertion of; the resonant circuit 1l, 12, peaking the transmission gain at its resonant frequency. As is Well known, the oscillation frequency is that frequency at which the necessary phase relationships are obtained and the loop circuit gain is. maximum.

In the' Code position of the emission selector, switch section 6D presents an open circuit, so that contacts I5, I8 of the keying relay IS. are open except for those intervals when the key 63 is vdepressed; With key 63 down, relay I9`is energized to; close the contacts I5; IB and connect the grid 201 to the source 4, whereupon tube l2 oscillates. During transmission, this oscillation exists in termittently, corresponding to the manipulation of the key 63, Considering the key condition for the moment, audio frequency oscillation energy from the tube I2 appears across the secondary 21 of.v thel transformer I:'I and is amplified in the usual manner by the tubes 30, 3l to provide in the output circuit thereof energy of the same frequency, but materially increased in power. This power is now used to vary or modulate the anode and screen potentials of the radio frequency amplifier 9U, thus amplitude-modulating the radio frequency energy radiated from the antenna 15. Impression of the modulating energy on both the anode and screen has been found to effect a material improvement in the linearity of themodulation characteristic. It will be noted that grid 4T also derives its power from the source 4 through the contacts of relay I9, so that with key 63 up, radiation of high frequency energy ceases together with the interruption of the modulating. oscillations.

Upon placing the emission selector switch in the Phone-position, the amplifier 2 is connected to its source of anode potential, and at the same time, the resonant peaking circuit is disconnected from its shunting position across the primary of transformer I1, restoring to this stage its normal uniformity of frequency response. Also, the cathode end of grid resistor I0A is disconnected` from the point |02 and connected instead to the point |00, which causes anode circuit disturbances of the tube l2 to appear in the control grid circuit, in opposing relationship` to.

provide degeneration or negative feedback; thereby improving the linearity of the amplifier response. As resistor 69 is connected to ground. there is now provided for the control grid II a negative operating bias equal in. magnitude to the voltage drop across the cathode resistor 23 to fulfill the requisites for the faithful reproduction of the input wave form. Switch section 6u is moved into the closed position during this operation and shunts the key 63 to maintain the relay I9; in the energized position continuously.

Under this condition of operation, amplifier 2, functioning. in, the conventional manner, impresses on control grid Il an enlarged replica of the wave present at grid I, and this is, in turn, further amplified by the tube I2 with the excellence of linearity contributed by the presence of negative feedback, and fed to the modulators 39, 3 I. The output of these modulators is impressed, as before, on the iinal radio frequency amplifier 90, and provides a carrier amplitude modulated with the waveform of the original telephone or voice frequency input, which energy may then be radiated in the usual manner.

While this circuit,. which may be alternatively employed as an amplier and oscillator with the same elements at all times connected to the electrodes of the tube, has been described in conjunction with radio transmitting equipment and shown in use as a tone modulator, it will be recognized that its employment is of equal value in any apparatus where the alternate functions of signal amplification and the generation of a sustained wave ofl predetermined frequency are required.

It will be obvious that many changes and modications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description.

I claim:

1. In signal responsive apparatus, an electric discharge -device having input and output circuits, means for selectively feeding back energy from said output circuit to said input ycircuit in phase with and in phase opposition with the energy in saidV input circuit, and means for peaking the response of said output circuit at a predeterminedv frequency when the output energy is fed back with an in phase relationship and eliminating said peaked response when the output energy is fed back in phase opposition.

2. In signal responsive apparatus, an electric discharge device having input and output cir cuits, means for selectively feeding back energy from said output circuit to said input circuit in phase with and in phase opposition with the energy in said input circuit, said in phase feedback being of a magnitude sufficient to cause sustained oscillations in said input and output circuits, and means for peaking the response of said output circuit at a predetermined frequency when the output energy is fed back with an in phase relationship and eliminating said peaked response when the output energy is fed back in phase opposition.

3. In signal responsive apparatus, an electric discharge device having a cathode, a control grid and an anode, means for impressing input energy on said control grid, a load impedance connected to said anode, a source of direct current energy connected between said rcathode and the end of said load impedance remote from said anode, means for impressing a fraction of the alternating voltage between said anode and said cathode on a contact member, means for impressing a fraction of the alternating voltage between said anode and said cathode on a second contact member with a phase substantially opposite to the phase of the voltage on said rst mentioned contact member, a third contact member selectively engageable with said rst mentioned and said second mentioned contact members, an impedance connected between said control grid and said third mentioned contact member, a ycircuit comprising parallel inductance and capacity, and means for connecting said circuit across said anode load impedance during the engagement of said second mentioned contact member by said third mentioned contact member and for disconnecting one terminal of said circuit from said anode load impedance during the engagement of said rst mentioned contact member by said third mentioned contact member.

4. In radio transmitting apparatus, an amplifier tube having a cathode, a -control grid and an anode, a transformer having primary and secondary windings, a connection between said primary winding and said anode, a resistive impedance connected to said anode and to ground, means for impressing a portion of the voltage across said resistive impedance on a contact member, means connecting one terminal of said secondary winding to ground, a resistive impedance connected between a second terminal of said secondary winding and ground, means for impressing a portion of the voltage across said second mentioned resistive impedance on a secon-d contact member, a third contact member selectively engageable with said first mentioned contact member and said second mentioned contact member, a resistive impedance connected between sai-d control grid and said third mentioned contact member, a parallel circuit 'of inductance and capacity resonant to a predetermined frequency, and means for connecting said parallel circuit in parallel with said primary winding when said third mentioned Contact member is in engagement with said second mentioned contact member.

5. In radio transmitting apparatus, an amplier tube having a cathode, a control grid, an accelerating grid and an anode, a transformer having primary and secondary windings, a connection between said primary winding and said anode,

a resistive impedance connected to said anode and f to ground, means for impressing a portion of the voltage across said resistive impedance on a contact member, means connecting one terminal of said secondary winding to ground, a resistive impedance connected between a second terminal of said secondary winding and ground, means for impressing a portion of the voltage across said second mentioned resistive impedance on a second contact member, a third contact member selectively engageable with said rst mentioned contact member and said second mentioned contact member, a resistive impedance connected between said control grid and said third mentioned contact member, a parallel circuit of inductance and capacity resonant to a predetermined frequency, means for connecting said parallel circuit in shunt with said primary winding when said third mentioned contact member is in engagement with said second mentioned contact member, a source of direct current potential having positive and negative terminals, means connecting said negative terminal to said cathode, means connecting said positive terminal to the end of said primary winding remote from said anode, relay means connecting said accelerating 8 grid to said positive terminal, and keying means controlling the operation of said relay means.

6. In radio transmitting apparatus, an ampliiler tube having a cathode, a control grid, an ac'- celerating grid and an anode, a transformer having primary and secondary windings, a connection between said primary winding and said anode, a resistive impedance connected to said anode and to ground, means for impressing a portion of the voltage across said resistive impedance on a contact member, means connecting one terminal of said secondary winding to ground, a resistive impedance connected between a second terminal of said secondary winding and ground, means for impressing a portion of the voltage across said second mentioned resistive impedance on a second contact member, a third contact member selectively engageable with the said first mentioned `contact member and said second mentioned contact member, a resistive impedance connected between said control grid and said third mentioned contact member, a parallel circuit of inductance and capacity resonant to a predetermined frequency, means for connecting said parallel circuit in shunt with said primary winding when said third mentioned contact member is in engagement With said second mentioned contact member, a `direct current source having positive and negative terminals, means lconnecting said negative terminal to said cathode, means connecting said positive terminal to the end of said primary winding remote from said anode, relay means connecting said positive terminal to said accelerating grid, keying means controlling the operation of said relay means, and means for disabling and keying means when said third memtioned contact is in engagement with said iirst mentioned contact.

7. In signal responsive apparatus, an electric discharge device having a cathode, a -control grid and an anode, a transformer having primary and secondary windings, a connection between said primary winding and said anode, a direct current source having positive and negative terminals, a resistor connecting said cathode and said negative terminal, means connecting said positive terminal to the end of said primary winding remote from said anode, a resistive voltage divider connected to said negative terminal and connected to said anode by a capacitor, means connecting an intermediate point on said voltage divider to a contact member, a second resistive voltage divider connected to said secondary winding and connected directly to said cathode, means connecting an intermediate point on said second voltage divider to a second contact member, a third contact member selectively engageable with said first mentioned contact member and said second mentioned contact member, and a resistive impedance connected between said control grid and said third mentioned contact member.

8. In signal responsive apparatus, an electric discharge device having input and output circuits including a plurality of electrodes, an impedance connected at one end to said input circuit, means for applying electric potentials to said electrodes,` means for selectively impressing energy from said output circuit on the other end of said impedance in aiding and opposing phase relationship with respect to the energy in said input circuit, and means for varying the potential applied to one of said electrodes from one value when said energy is cou-pled back in aiding relationship to another value when said energy is coupled back Vin opposing relationship 9. In signal responsive apparatus, an electric discharge device having input and output circuits including a plurality of electrodes, means for applying electric potentials to said electrodes, means for selectively impressing energy from said output circuit on said input circuit in aiding and opposing senses with respect to the energy in said input circuit, and means for varying the potential applied to one of said electrodes from one value when said energy is coupled back in said aiding sense to another value when said ener-gy is coupled back in opposing sense.

10. In signal responsive apparatus, an electric discharge device having input and output circuits including a plurality of electrodes, means for applying electric potentials to said electrodes, means for selectively feeding back energy from said output circuit to said input circuit in phase with and in phase opposition with the energy in said input circuit, said in phase feed back being of a magnitude sulicient to cause sustained oscillations in said input and output circuits, and means for varying the potential applied to one of said electrodes from one Value when said feed back energy is in phase with the energy in said input circuit to another value when said fed back energy is in phase opposition with the energy in said input circuit.

11. In signal responsive apparatus, an electric discharge device having a cathode, a control grid and an anode, means for impressing input energy on said control grid, a load impedance connected to said anode, a bias resistor connected to said cathode, a direct current source of electric energy having its positive terminal connected to the free end of said load impedance and its negative terminal connected to the free end of said bias resistor, means for impressing a fraction of the alternating voltage between said anode and said cathode on a first contact member, means for impressing a fraction of the alternating voltage between said anode and said cathode on a second Contact member with a phase substantially opposing the phase of the voltage on said nrst mentioned contact member, a third contact member selectively engaging with said rst mentioned and said second mentioned contact members, a return impedance connected at one end to said control grid and at the other end to said third contact member, and means for connecting said other end of said return impedance to the negative terminal of said source when said rst contact member and said third contact member are in engagement, and to said cathode when said second contact member and said third contact member are in engagement. A

12. In radio transmitting apparatus, an electric discharge device having a cathode, a control grid and an anode, said cathode being connected to ground through a direct current impedance, a transformer having primary and secondary windings, a connection between said primary Winding and said anode, a resistive impedance connected to said anode and to ground, means for impressing a portion of the voltage across said resistive impedance on a rst contact member, means connecting one terminal of said secondary winding to said cathode, a resistive impedance connected between a second terminal of said secondary winding and said cathode, means for impressing a portion of the voltage across said second mentioned resistive impedance on a second contact member, a third contact member selectively engageable with said rst mentioned contact member and said second mentioned contact member, a resistive impedance connected between said control grid and said third mentioned contact member, and means for supplying electric energy to the anode circuit of said `discharge device.

13. In radio transmitting apparatus, a first electric discharge device having a cathode, a control grid and an anode, a two terminal source of electric energy, a switch and impedance connected in series between one terminal of said source and said anode, means connecting said cathode to the other terminal of said source, means for im- .pressing an input signal on said control grid, a second electric discharge device having a cathode, a control grid and an anode, a transformer having a primary and secondary winding, means connecting said primary winding between said one terminal of said source and the second of said anodes, resistive means connecting the second of said cathodes to said other terminal of said source, an impedance connected to the second of said anodes and to said other terminal of said source, means for impressing a portion of the voltage across said impedance on a first contact member, means connecting one terminal of said secondary winding to said other terminal of said source, an impedance connected between a second terminal of said secondary winding and the second of said cathodes, means for impressing a portion of the voltage across the second mentioned impedance on a second contact member, a third contact member selectively engageable with said lirst contactmember and said second contact member, an impedance connected between the second of said control grids and said third contact member, a capacitor connected between the rst of said anodes and the second of said control grids, and means for operating said switch synchronously with the operation of said third contact member.

ROBERT J. DAVIS.

REFERENCES CKTED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,894,952 Gebhard Jan. 24, 1933 2,006,440 Chireix July 2, 1935 2,024,489 Van Der Pol et al. Dec, 17, 1935 2,173,426 Scott Sept. 19, 1939 2,173,427 Scott Sept. 19, 1939 2,244,751 Werrmann June 10, 1941 2,289,752 Bruck et al. July 14, 1942 Certicate of Correction Patent No. 2,439,844. April 20, 1948.

in the printed specification of the above It is hereby certied that errors appear sectmn numbered patent requiring correctx read section 26,1'me 17, after the numero.

umu 9 line 24, for feed read jed; and that the sind occurrence insert down; co1

hould be read wlth these correctlons therem that the same may con- Letters latent s d of the case in the Patent O ce.

form to the recor MSigned and sealed this 15th day of June, A. D. 1948.

THOMAS F. MURPHY,

Assistant ommssioner of Patents.

Certicate of Correction Patent No. 2,439,844. April 20, 1948.

in the printed specification of the above It is hereby certied that errors appear sectmn numbered patent requiring correctx read section 26,1'me 17, after the numero.

umu 9 line 24, for feed read jed; and that the sind occurrence insert down; co1

hould be read wlth these correctlons therem that the same may con- Letters latent s d of the case in the Patent O ce.

form to the recor MSigned and sealed this 15th day of June, A. D. 1948.

THOMAS F. MURPHY,

Assistant ommssioner of Patents.

Certificate of Correction Patent No. 2,439,844. April 2o, 1948.

ROBERT J. DAVIS It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Column 5, line 14, for section 2 read section 26'; line 17, after the numeral 12 insert to; line 45, after key second occurrence insert down; column 9, line 24, for feed read fed; 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 Office.

Signed and sealed this 15th day of June, A. D. 1948.

THOMAS F. MURPHY,

Assistant om/mzss'ioner of Patents.

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