US1952736A - Volume control - Google Patents

Description

March 27, 1934. E. E. UNGER 1,952,736 vowus CONTROL I Filed July "27, 1931 2 Sheets-Sheet 1 a Q 7 Z March 27, 1934. E. E. UNGER VOLUME CONTROL Filed July 27, 1931 2 Sheeiis-Sheet 2 Patented Mar. 27, 1934 UNITED STATES PAT VOLUME CONTROL Illinois Application July 27, 1931, Serial No. 553,248

2 Claims.

This invention relates to radio receiving means in general and more particularly to meansfor controlling the volume of reception of radio broadcast programs and other radio communications. In its broader aspect this invention is concerned with the control of the volume output of thermionic tubes, especially in a communication circuit, regardless of the function that the particular tubes have in the circuit.

It has been heretofore proposed to control the volume output of radio receiving sets by controlling the grid bias on the amplifying tube or tubes. There are conditions under which this is unsatisfactory because such a high value of grid bias may be necessary as to cause the amplifier to function as a detector, and thus considerably distort the signal. To reduce this difiiculty it has been proposed to combine with the grid biasing means a variable shunt for the input side of the tube, to shunt out a portion of the received signal. This takes the form of a potentiometer with one end connected to the grid bias lead, the other end connected to the antenna lead, and the rider, or movable arm grounded. In using the potentiometer type volume control I have found that as the grounded movable arm is shifted along the resistance to vary the grid bias it will also vary the antenna to ground resistance and this variation, together with variation in contact resistance as the contact arm slides along, will produce a variation on the input side of the tube. This is amplified through the set and ultimately appears as noises emanating from the loud speaker. This is commonly known as noisy volume control. It is an object of the present invention to correct this defect in potentiometer type volume controls. I

I accomplish this end by fixing the potential of one point on the potentiometer. The potential at which this point is fixed is preferably ground potential, although it is within the purview of the present invention to fix this potential at any other desirable value. By this arrangement, as the slider moves toward the ground point to vary the grid bias it will produce no variations in the input circuit to the tube. As the slider moves past the ground point towards the antenna it will not affect the grid bias.

It is a further object of the present invention to provide an improved potentiometer suitable for use in circuits as above set forth. The potentiometer comprises a variable resistor, which may be varied in any preferred manner, said resistor having a tap intermediate its ends,

which tap is grounded directly or through a fixed resistor.

The attainment of the above and further objects of he'present invention will be apparent from the following specification, taken in conjunction with the accompanying drawings forming a part thereof.

In the drawings:

Figure 1 is a fragmentary circuit diagram illustrating the principles of my invention;

Figures 2 and 3 are similar circuits, illustrating modified forms of my invention;

Figure 4 is a plan view of one .form of potentiometer suitable for use in the circuit shown in Figure 3;

Figure 5 is a view similar to Figure 4 with the contact making spring removed;

Figure 6 is a fragmentary sectional view taken along the line 6-6 of Figure 4;

Figure 7 is a fragmentary circuit arrangement illustrating a modified form of my inven-- tion wherein the antenna signals are attenuated before the grid bias is increased;

Figure 8 is a fragmentary circuit diagram illustrating the application of myvolume control to the plate circuit of an intermediate tube of a radio set; V V

Figure 9 is a fragmentary circuit diagram illustrating the application of my invention to the grid input side of an intermediate tube of a radio set; and i Figure 10 is a fragmentary circuit diagram illustrating my invention as applied to the coupling between the plate and the grid of two adjacent tubes of a radio set.

Reference may be had now more particularly to Figure l of the drawings, wherein I have shown one tube of a typical amplifying circuit to which my invention has been applied. An antenna or pick-up device 1 is coupled to a thermionic tube 2 by means of a coupling 3. The coupling 3 may be of any preferred type insofar as concerns the present invention and I have herein shown, by way of illustration, a well known coupling comprising an air core transformer whose primary 5 is connected between ground and the antenna, and whose secondary is in multiple with a variable condenser 6 and connected between ground and the grid '7 of the tube 2. The tube 2 may be of any preferred type and includes a cathode 8 and an anode 9 in addition to the grid 7. The cathode may be heated in any preferred manner. As herein illustrated the tube 2 is of the heater type wherein the cathode is raised to proper temperature by a suitable heater, not shown.

A comparatively high resistor 10 is connected between the antenna or input circuit 1 and the cathode 8, said resistor being provided with a movable grounded contact 11. The resistor is provided wth a grounded tap 12 intermediate its ends, for a purpose to be more fully set forth as this description proceeds. A resistor 13 is provided for preventing the grounding of the cathode by the contact 11.

I have not illustrated the output circuit of the tube 2, including the anode voltage supply, since this may be of any preferred type. It is sufiicient here to state that the circuit from the anode 9 extends through the usual arrangements back to the cathode 8 by Way of ground, whereby the circuit is completed through the grounded contact 11 or the ground tap 12 and the portion of the resistor 10 to the left thereof. A potential gradient'is thereby set up across the resistor with the result that the cathode is maintained positive with respect to ground. Since the grid 7 of the tube is connected to ground the cathode of the tube is therefore positive with respect to the grid, or the grid is negative with respect to the cathode. This is the proper condition for efiicient operation of the tube as an amplifier. It is well known that the amplification of the tube varies inversely as the negative grid biasing voltage varies. Therefore, as the contact 11 is moved in a counter clockwise direction from the position shown, the grid bias is increased and the amplification of the tube is decreased, thereby decreasing the volume output of the tube.

As is Well known, the variation in grid bias can not, alone, be used to produce the full volume control, since, to get a very great reduction in volume it might be necessary to have such a high grid bias that the tube 2 will cease acting as an amplifier. For this reason the antenna is connected to the potentiometer so as to provide a variable shunt for a portion of the received signal that would otherwise flow through the primary of the transformer 5. As the contact 11 moves towards its extreme position in a counter clockwise direction it gradually decreases the amount of resistance shunting the primary of the transformer 5 and thereby reduces the current flow through the transformer, thus varying the volume output of the tube.

I have found that the provision of the grounded tap 12 on the potentiometer resistance 10 alters the mode of operation of the volume control in a decidedly advantageous manner and produces a new and highly desirable mode of operation in a circuit such as has been described, and it is with this feature that the present invention is primarily concerned.

Consider first the mode of operation of the volume control in the absence of the grounded tap. Assume that the contact is at its extreme right hand position and is being moved in a counter clockwise direction to decrease the volume output of the tube. At the commencement of the movement the grid biasing resistance is low and the resistance shunting the primary of the transformer is very high. Therefore, the initial movements of the contact 11 will produce an appreciable variation in the grid biasing resistance and only a very small proportionate change in the resistance shunting the primary of the transformer 5. This will control the volume prmarly by control of the grid bias. As the contact 11 arrives at positions further advanced along the resistor 10, additional movement will not only vary the grid bias but will also vary, to an appreciable extent, the resistance shunting the input to the tube 2. This introduces two rather difiicult problems. First, it is quite difiicult to produce a resistor of the above type which will afford straight line attenuation of the volume, or anything even approaching straight line attenuation. Secondly, it is very difficult to produce a volume control of this type which will not be noisy. Both of the above difficulties arise from the fact that variation of the grid bias also produces a variation in the resistance shunting the input circuit to the tube. This last mentioned variation in current flow is amplified through the set and finally appears as noise in the loud speaker, or output side of the set.

The provision of the grounding tap 12 removes the above mentioned difficulties. In this case, as the grid bias is varied by movement of the contact'there is no appreciable corresponding variation in the resistance shunting the input side of the tube 2, therefore the noises which were heretofore produced on this account are eliminated. When the contact 11 moves in a counter clockwise direction past the ground point 12 to decrease the resistance shunting the input side of the tube 1, there will be no corresponding variation in the grid bias. The potentiometer resister 10 is not of uniform resistance throughout its length. The resistance is correlated to the circuit in which it is used so that equal increments of length at different points on the resistor will produce proportionately equal variations in the volume output of the set. The problem of suitably tapering the resistance to correlate the same to the circuit within which it is to be used is appreciably simplified by the presence of the grounding tap 12 since by this arrangement the portions of the resistor on either side of the tap may be considered as separate units, each independent of the other. One circuit may be tapered considering only the results produced by varying the grid biasing resistance, and the other portion tapered only to take care of incoming signal attenuation. In the absence of the grounding tap independent consideration of these two factors is not possible.

Reference may now be had to the circuit diagram of Fig. 2, illustrating a modified form of my invention. The volume control shown in this figure differs from that previously described only in that the potentiometer resistance is provided with a gap adjacent the tap 12. This means that as long as the contact 11 is to the right of the tap 12 the resistance shunting the input side of the tube 2 will be infinite, or, stated in other words, no part of the signal will be shunted to ground. The resistance of the resistor portion 21 is made rather high, so as to avoid an abrupt change in volume as the contact 11 first moves past the gap 20 in its counter clockwise motion. This form of volume control is very efficient in controlling the amplification of initially weak signals, since no part of the original signal is lost when high amplification is desired, as when the contact 11 is in its extreme right hand position.

In Fig. 3 I have shown a circuit diagram similar to that of Fig. 1, differing therefrom only in that the tap 12 on the potentiometer is connected to ground through a fixed resistance 25. The resistor 25 isv therefore always in shunt with that portion of the potentiometer resistor that lies between the tap 12 and the contact 11. When the contact is in either of its extreme positions the iiiO value of the poteniometer resistance in shunt with the resistor 25 is so high that it is of ne li i ble effect, and therefore the resistor 25 is the controlling resistor. As the movable contact approaches the tap 12 the effect of the resistor25 diminishes, this resistor being of zero effect when the movable contact is at the tap 12. As the movable contact moves from one side to the other of the tap 12 the resistor 25 is connectedeither in the grid biasing circuit or in the antenna pick up circuit depending upon the position or the contact 11. The resistance 25 is of a rather low value whereas the potentiometer resistance is of a rather high value. When the contact 11 is moved counterclockwise from its extreme right hand position to increase the grid bias there is no effect upon the antenna to ground resistance due to the fact that the low resistance 25 is shunting the varying portion of the potentiometer resistor between the tap 12 and the contact 11. Therefore there will be no rheostat noises introduced into the input circuit at this time, when the amplification or" the tube is the maximum. As the contact 11 approaches the tap 12 and the amplification oi the tube 2 has been reduced, further movement of the contact toward the tap 12 increases the grid bias and also reduces the antenna to ground resistance. As the contact moves to the left of the tap 12 it removes the resistance 25 from the antenna circuit and includes it in the cathode or grid bias circuit in shunt with the portion of the potentiometer resistance between the contact and the tap 12. The efiect of this portion of the potentiometer resistance in the grid bias circuit gradually decreases as the contact 11 moves further and further from the tap 12. Thus there is a gradual attenuation of the antenna to ground resistance and of the grid biasing resistance as the contact moves to one side or the other of the tap 12. The resistance 25 however, serves to reduce or substantially eliminate the rheostat noises and afiords a ready means for producing a properly attenuated resistance of the potentiometer. I

Reference may now be had more particularly to Figures 4, 5 and 6 wherein I show a preferred mamier of incorporating the tap 12 and the resistor in a well known type of potentiometer. The potentiometer comprises a frame of suitable insulating material in which is mounted a resistance element 31 which may comprise 'a strip of cardboard whose upper surface is covered with carbon, graphite, ink, or other desirable conducting material, of varying density throughout the length of the strip so as to provide a proper resistance attenuation. Terminals 32 and 33 connect the ends of the resistance strip 31 with terminal lugs 34 and 35, respectively. A

thin, flexible metal disc 38 is secured at its hub and adapted to be flexed at its rim into engagement with the resistance strip 31. A swinging arm 40 is secured to the rotatable element or shaft 41 of the potentiometer and. makes wiping engagement with the rim of the disc 38 to press the portion of the rim immediately under the wiper arm 40 into engagement with the resistance strip. As the arm 40 wipes along the strip 33 the point of contact between the rim or" the strip 38 and the resistor is shifted thus varying the amount of resistance between either of the lugs 34 or 35 and the contact strip. The contact strip 38 is itself held against rotation and is electrically connected through its hub to a terminal 42. This much of the potentiometer herein shown is well known in the art and does not constitute my invention. The resistance strip 31 .includes'a short inwardly extending portion 45 which is also coated with the resistance material and forming a continuation of the resistance material on' the strip 31. A contact 46 presses against the extension 45 and makes electrical engagement therewith. The contacting strip 46 is in engagement with the disc 38 and thereby establishes a circuit to the disc 38. The resistance of the portion of the strip 45 between the contact 46 and the portion of the strip 31 engaged by the contacting disc 38 opposite thereto represents the amount of fixed resistance represented at 25 in, Figure 3. II" the contacting portion 46 is brought out further to the rim of the strip 31- there will be a resulting decrease in the value of the hired resistance 25. If this contact 46 is brought out to the center of the strlp 31 the value of the resistance 25 will be reduced 'to zero thereby corresponding to the system shown in Figure 1. If it is desired to provide a gap in the potentiometer resistance, as indicated at 20 in Figure 2, this may be accomplished by scraping away the resistance material from the cardboard strip 31 along a radim 100 line adjacent the terminal portion 46.

In Figure 7 I have illustrated, in diagrammatic form, an arrangement wherein the antenna signal is attenuatedbefore the grid bias is increased. The potentiometer comprises two re- 1105 sistance portions 50 and 51 over which the contact 11 moves. The resistance 51 isthe'grid biasing resistance whereas the resistance 50 is the antenna to ground shunting resistance;

While I have herein shown these two resistances 110.

as laid out in a straight line, it is to be understood that they may be arranged in the form of a continuous curve over which the contact 11 wipes or swings. There is provided'a grounding switch 55 that is actuated by the contact 11, or 113' by the means which actuates the contact 11, so that it selectively grounds either the contact 56 or a contact5'7, depending upon the position of the contact 11. When the contact 11 is to the right of the switch 55 this switch grounds the liq contact 56. To produce the minimum volume on the output side of t e tube 2 the contact 11 is moved to its extreme right hand position. At this time there is a maximum grid bias due to the fact that the maximum amount of resistance 12 5 51 is included in the cathode circuit. This reduces the cathode-anode conductance to its minimum value. At the same time the switch 55 grounds the contact 56 thereby producing a ground path for the antenna signal. To in- 30 crease the volume output of the tube 2 the contact 11 is moved to the left as seen in Figure 7. As this contact approaches the end of the resistance 51 the gridbias is being reduced with a resulting increase in the conductance between 35 the cathode and the anode thereby increasing the volume output of the tube 2. As the contact 11 moves past the switch 55 to the resistance portion 50 it actuates the switch to remove ground from the contact 56 and to place ground upon the contact 57. Further movement of the contact 11 increases the antenna to ground resistance thereby resulting in a further increase in the output of the tube 2. The resistance 50 is of a rather high value so as to afford a rather high 1 5 antenna to ground resistance when the contact 11 is at its. extreme left hand position. If desired the arrangement may be such that the contact 11 moves ofi of the resistance 50 at the end of its travel, thereby entirely removing the an tenna ground. When the contact 11 moves to the right past the switch 55 it returns this switch to the position shown in the drawings.

The switch 55 is incorporated within the potentiometer structure. Since this switch carries a negligible current the entire arrangement may be made very compact.

Reference may now be made to Figure 8 showing a modified form of my invention wherein the volume control is located in the plate circuit between two adjacent coupled thermionic tubes of a radio receiving set. In this case the potentiometer is indicated at 62 and comprises a contact 63 adapted to move over a resistance comprising the sections 64 and 65 A resistance 67 shunts the resistance 65, being connected to the juncture of the resistances 64 and 65 at the tap 68. The potentiometer resistance is connected at one end to the plate or anode of the tube 60 and at its other end is connected to the usual anode voltage supply which may comprise a B battery, and B battery eliminator, or its equivalent.

The contact 63 is coupled to the grid or input side of the tube 61 in any preferred manner. I

have herein shown a standard coupling means comprising an air core transformer 71 and a tuning condenser 72. It is however to be understood that any other desirable coupling arrangement may be provided. The degree of amplification of the unit comprising tubes 60 and 61 may be varied by shifting the contact 63 along the potentiometer resistance, the amplification being maximum when the contact is at the upper portion of the resistance 64 and minimum when the contact is at the lower portion of the resistance 65. The provision of the resistance 6'? in shunt with the resistance 65 greatly facilitates the proper attenuation of the potentiometer resistance.

In Figure 9 I have shown an arrangement corresponding to the arrangement shown in Figure 8 but difiering therefrom in that here the volume of control is placed on the input side of the tube 61 rather than on the output side of the tube 60 as previously. The tubes 60 and 61 are coupled by a coupling means which may be of any preferred construction and has been here illustrated as comprising an air core transformer. The primary of the air core transformer is connected between the plate of the tube 60and the plate voltage supply. The secondary side of this transformer is connected across the potentiometer 62, one side of the potentiometer being grounded, and the movable contact being con.- nected to the grid of the tube 61. It is apparent that movement of the contact 63 will vary the output of the tube 61.

In Figure 10 I have shown the output side of the tube 60 coupled to the input side of the tube 61 in a well known manner, as by means of an air core transformer and a variable tuning condenser. The volume is controlled by means of the potentiometer 62 connected between the plate circuit and ground, with the movable contact 63 of the potentiometer connected to the grid of the tube 60 through a small fixed condenser 85. This condenser is provided in order to prevent short circuiting of the plate voltage supply through the primary of the coupling transformer, the potentiometer resistance, the wiper 63, and the secondary of the coupling transformer to ground. In the absence of the condenser the B battery supply might be short circuited when the contact 63 is moved to its extreme left hand position because of the fact that one terminal of the B Voltage supply is grounded. This condenser 85 may, if desired, be interposed between the potentiometer and the connection to the anode or plate of the tube 60.

From the description thus far given it is apparent that my invention may be employed in a wide variety of forms and is not limited to the precise embodiments herein shown for illustrative purposes. What I consider new and desire to secure by Letters Patent is:-

I. In a thermionic tube system employing a hot cathode tube having a grid, and wherein the volume output of the tube is controlled both by varying the grid bias and by variably shunting the grid input circuit, a resistor having one terminal connected to the grid input side of the tube and another terminal connected to the grid bias lead of the tube, a grounded contact movable over the resistor, and a ground connection to the resistor ata fixed point intermediate the limits of travel of the contact, said resistor being continuous between the ground connection and the terminals connected, respectively, to the grid input side or" the tube and to the grid bias lead.

2. Incomblnation with a pick-up device and a thermionic tube having an anode, a cathode and a grid coupled to the pick-up device, means for controlling the volume output of the tube, said means comprising a grounded resistor shunting the signals from the pick-up device, a grounded resistor controlling the grid bias, both of said resistors comprising a single electrically continuous resistor unit with the ground point intermediate the ends thereof, whereby the resistor shunt across the pick-up device is maintained at all times, and a grounded contact movable over the resistor unit by a continuous motion, first to vary the grid bias without affecting the shunting resistor and then to shift the effective grounding point of the shunting resistor without further varying the grid bias.

ERWIN E. UNGER.

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