US2491391A

US2491391A - Electronic transducer

Info

Publication number: US2491391A
Application number: US716297A
Authority: US
Inventors: Jr George M Rose
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1946-12-14
Filing date: 1946-12-14
Publication date: 1949-12-13
Anticipated expiration: 1966-12-13

Description

Dec. 1949' e. M. ROSE, JR

ELECTRONIC TRANSDUCER 2 Sheds-Sheet 1 Filed Dec. 14, 1946 3nvento1 fieazge M Baltic/12 W 4 Gttorngg Dec. 13, 1949 G. M. ROSE, JR

ELECTRONIC TRANSDUCER 2 Sheets-Sheet; 2

Filed Dec. 14, 1946 Zhmentor 60mg? Mkwe, c/l" W (Ittorneg Patented Dec. 13., 1949 ELECTRONIC TRANSDUCER George M Rose, Jr., East Orange, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application December 14, 1946, Serial No. 716,297

6 Claims.

This invention relates to devices for converting mechanical movement into electric current or voltage and more particularly to devices for varying the plate current of vacuum tubes by mechanical movements in phonographs, microphones and analogous devices.

Various constructions have from time to time been proposed for varying the output of vacuum tubes by varying the relative positions of the electrodes.

In these prior art devices there has been an attempt to limit mechanical resonance of moving parts but the constructions have been such that this cannot satisfactorily be accomplished.

An electronic transducer suitable for use as a phonograph pickup or as .a. microphone must fulfill certain mechanical dernands before it can be considered satisfactory.

1. Its size must be-small to permit its accommodation to the rest of the. acoustic system.

2. Its moving element must be small andlight in weight because the mechanical forces delivered from a phonograph record or a microphone diaphragm are very small and vary at frequencies up to several thousand cycles per second. The moving element of the transducer must be capable of following the movements of stylus or diaphragm faithfully.

3. The mechanical resonant frequency of the moving element should be higher than the highest frequency that is to be reproducedctherwise excess mechanical damping has to be applied in order to obtain a uniform frequency response characteristic. Such damping can cause a large reduction in electrical output. When there is an internal member which cannot be sufiiciently damped, its free resonance should occur considerably higher, by. afactor of 2 to 3,v than the highest frequency to be. reproduced. Otherwise, spurious outputs can arise from shock excitation.

4. The electrical output. should vary linearly with the mechanical displacement of the moving element. Otherwise distortion is introduced.

It is an object of this invention to provide an electronic transducer to meet all these requirements.

It is another object of this invention to provide atransducer that is of small size with a single moving partof. low inertia, is free from mechanical resonance in the operating range and has a linear output.

Other objects of the invention will appear in the following. specification. reference being had to the drawing, in which;

Fig. 1 is a section through the axial plane of a tube of my invention;

Fig. 2 is a section taken on the line 2-2 of Fig. 1;

Fig. 3 illustrates diagrammatically a circuit in which the invention is preferably used;

Fig. 4 illustrates a modification of the invention together with its circuit connections;

Fig. 5 is an axial section of a modification;

Fig. 6 is a section taken on the line 6-45 of Fig. 5;

Fig. 7 is a view. of the control electrode of Fig. 5;

Fig. 8 illustrates the way the electrodes in the device of Fig. 5 are connected;

Fig. 9 illustrates the tube and attachment to, a phonograph needle;

Fig. 10 is an end view of the tube of Fig. 9 indicating the direction of vibration;

Fig. 11 illustrates the use of my improvement with a microphone; and

Fig. 12 illustrates the tube as a modulated oscillator.

Referring to Fig. 1 of the drawing, the envelope or shell I of the tube is made of small size for phonograph use and is about of an inch outside diameter with a slightly enlarged end 2 for reception of the base or stem 3 which may be of glass. The overall length of the tube may be about three-quarters of an inch. The cathode 4 may be of any desired type'but I prefer to use the unipotential' type having a heater, the terminals of which are indicated by reference character 5. The anode 6 is the moving element and it consists of a single rod such as music wire or drill rod, both of which are high carbon steel. The rod is silver soldered at the center of a flexible diaphragm 1 which is silver soldered to the shell I. Both the diaphragm and the shell may be made of mild steel.

The electrical sensitivity and the acoustic properties of the tube are dependent upon the size and shape of the anode rod 6. The electrical sensitivity is proportional to the rod length but of course the mass of the rod increases with the length and the resonant frequency correspondingly decreases. The mechanical resonant frequency should be preferably as high as 15 kilocycles per second. Iobtain a rod of this frequency by tapering it from a maximum diameter at the diaphragm to a minimum at the internal tip. This shape gives maximum length and minimum mass for a given resonant frequency. With av conically tapered rod anode having a frequency of substantially 15 kilocycles per second the internal rod length is around of an inch 3 with a base diameter of .040 inch and a tip diameter of .005 inch.

To prevent electron flow from the cathode to the shell, which is at anode potential, a shield 8 is placed between the cathode and the adjacent side of the shell. This may be a thin plate oi nickel or any other suitable metal and may be given cathode potential. The cathode is off center and is parallel to the axis of the wire anode from which it is spaced at the nearest point about .025 inch. The phonograph needle arm 9 is $01- dered or otherwise fastened to the outer end of the anode rod 6 (Fig. 9) and is of such length that the maximum tip deflection of the rod is about .0005 inch toward and .0005 inch away from the cathode.

The arm 9 rests on the top of the tube, or on a layer 9, such as synthetic rubber as shown in Fig. 9, to minimize torsional force of arm 9. The desired movement of the end of the rod t is perpendicular to the plane of the paper in Fig. 1 so as to move the anode toward and away from the cathode. This construction may be dispensed with if desired as the rod 5 is brazed to the end of the tube, which resists the torsional force but the I flexibility of the end '3 permits the desired tilting of the rod.

While the pick-up tube may be a transformer coupled to the amplifiers typified by triode iii (Fig. 3), I prefer to use capacity coupling with the anode or load resistance 1 l of about ten times the internal resistance of the pick-up tube. The pick-up tube in such a circuit operates as a constant current device with the anode-cathode voltage varying with the anode displacement which r in phonographs isonly a few ten-thousandths of an inch. This variation of voltage for the small displacements involved is for all practical purposes exactly proportional to the mechanical displacement and any non-linear distortion is so small that it can be completely neglected.

Instead of using a single cathode one may use two cathodes connected to a resistance 1! as shown in Fig. 4. In this figure parts similar to those of Fig. 1 have been given similar reference characters. The voltage source is connected from the anode 6 to the center of this resistance which is sufliciently high to cause the tube to operate as a constant current device. The amplifying devices are connected to the conductor I2, 13. In

this modification the spacing between the anode B and one cathode decreases as the space between it and the other cathode increases and the variable output across the conductor l2 and I3 is substantially twice that with the single cathode in Fig. 1.

I have shown in Fig. 5 the invention embodied in a triode having a grid or control electrode I 3 having a slot 15 about .010 inch in width and about it; inch long. The grid is equidistant from the cathode and anode. The slot lies in the plane of the cathode and anode. The grid is connected directly to the cathode but if desired it may be given a negative bias. The other parts in this embodiment are similar to those in Fig. 1 and they have been given similar reference characters. This type of pick-up tube has about ten times the output of the tube in Fig. 1. A wire mesh type of grid may be used when desired.

Moving the grid instead of the anode is feasible but the variation of the output current is less than with a movable anode. The cathode may be the movable member and it produces a very sensitive pick-up but due to the weight of the heater and other parts in a unipotential cathode this is not so satisfactory as moving the anode. Also, the oxide or other coating on the cathode would be subject to damage by vibrating the cathode.

The tube may also be used as an oscillator as shown in Fig. 12. Here the grid 19 shown diagrammatically and the vibratile anode 6 are connected through blocking condensers 20, 2| to inductance 22 shunted by tuning condenser 23, as in a Hartley circuit. The usual grid leak 24 may be used between cathode 2 and grid 19. The direct current supply may be connected across anode 0 and cathode (i as indicated. In this embodiment the tube oscillates and the vibrating anode E5 actuated by a mechanical vibrating means, not shown, having a frequency as high as 15 kc. per second, modulates the plate current which may be amplified and detected in a known way.

Fig. 11 shows how the tube may be utilized as a microphone. In this figure the cone diaphragm i5 is connected to anode 5 by a link it so that sound waves such as produced by the voice vibrate the diaphragm and produce current in the same way as described in connection with the devices in the other figures.

In the tubes shown in all the figures all the electrodes, except anode S, are supported at their ends in mica discs ll, it snugly fitting the inside of the envelope i. The disc I! has a central opening to permit the anode ii to vibrate without engaging it.

The output of the transducer may be connected to a transformer as known in the art or in any other way and various other changes may be made without departing from the spirit of the invention.

What I claim as new is:

1. An electronic transducer comprising an evacuated envelope containing, a flexible end, a rod anode extending through the center of said end and sealed thereto, the inner end of said anode being unsupported, insulation discs engaging the inside wall of the envelope, a cathode parallel to the axis of said anode at one side thereof and a metal shield extending parallel to the cathode between the cathode and the wall of the envelope adjacent thereto, said shield being electrically connected directly to said cathode to form an electron mirror, both said shield and said cathode being rigidly supported at their ends by said discs, one of said discs having a central opening permitting the anode to extend therethrough and to move toward and away from the side of the cathode remote from said shield without engagement with the disc.

2. An electronic transducer comprising an evacuated envelope containing, a flexible end, a rod anode extending through the center of said end and sealed thereto, the inner end of said anode being unsupported, said anode being tapered from said flexible diaphragm to its inner free end, insulation discs engaging the inside Wall of the envelope, a cathode parallel to the axis of said anode at one side thereof, said cathode being rigidly supported at its ends by said discs, one of said discs having a central opening permitting the anode to extend therethrough and to move toward and away from the cathode without engagement with the disc.

3. An electronic transducer comprising an evacuated envelope containing, a flexible end, a rod anode extending through the center of said end and sealed thereto, the inner end of said anode being unsupported, said anode being conically tapered from said flexible diaphragm to its inner free end to have a mechanical resonance frequency above kilocycles per second, insulation discs engaging the inside wall of the envelope, a cathode parallel to the axis of said anode at one side thereof, said cathode being rigidly supported at its ends by said discs, one of said discs having a central opening permitting the anode to extend therethrough and to move toward and away from the cathode without engagement with the disc.

4. An electronic transducer comprising an evacuated envelope containing, a flexible end, a rod anode extending through the center of said end and sealed thereto the inner end of said anode being unsupported, said anode being conically tapered from said flexible diaphragm to its inner free end to have a mechanical resonance frequency above 10 kilocycles per second, insulation discs engaging the inside wall of the envelope, a cathode parallel to the axis of said anode at one side thereof and a metal shield between the cathode and the wall of the envelope adjacent thereto, said shield being electrically connected directly to said cathode to form an electron mirror, both said shield and said cathode being rigidly supported at their ends by said discs, one of said discs having a central opening permitting the anode to extend therethrough and to move toward and away from the side of the cathode remote from said shield without engagement with the disc, and a grid between the cathode. and anode supported at both ends in said discs..

5. An electronic transducer comprising an evacuated metal envelope closed at one end by a stem and at the other end by a flexible diaphragm lying in a plane normal to the longitudinal axis of said envelope, an elongated rod-like electrode extending perpendicularly through and being sealed to said diaphragm and supported solely thereby,

whereby a portion of said electrode extends into said envelope in parallel relation to the longitudinal axis thereof, an elongated cathode disposed between said electrode and a wall portion of said envelope and in parallel relation to said electrode, a metallic shield between said cathode and said wall portion for preventing electron travel from said cathode to said wall portion, and means for insulatingly supporting said cathode and shield from the walls of said envelope.

6. An electronic transducer comprising an evacuated metal envelope, a cathode insulatingly supported in said envelope, a lead-in through one end of said envelope for said cathode, an anode supported at and sealed through the other end of said envelope in electrical contact therewith, said other end being flexible, said anode having a portion extending outside of said envelope and another portion extending within said envelope, said another portion being parallel to said cathode, said cathode being closer to the wall of said envelope than said anode, and means between the envelope wall and said cathode for shielding the wall from electrons emitted by said cathode.

GEORGE M. ROSE, JR.

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

UNITED STATES PATENTS Number Name Date 1,554,561 De Forest Sept. 22, 1925 1,864,214 Petty June 21, 1932 2,142,857 McArthur Jan. 3, 1939 2,165,981 Sampson July 11, 1939 2,331,821 Winlund Oct. 12, 1943