US2211421A - Cathode heater testing circuit for a vacuum tube network - Google Patents

Description

Aug. 13, 1940. R H. HERRICK 2,211,421

CATHODE HEATER TESTING CIRCUIT FOR A VACUUM TUBE NETWORK Filed Jan. 10, 1938 1 C I j T "W n m INVENTOR. L3 i ROSWELL H.HERRICK BY M M ATTORNEYfi Patented Aug. 13, 1940 UATHODE HEATER TESTING CIRCUIT FOR A VACUUIW TUBE NETWORK Roswell H. Herrick, Oak Park,

111., assignor to Associated Electric Laboratories, Inc., Chicago,

111., a. corporation of Delaware Application January 10, 1938, Serial No. 184,254

8 Claims.

The present invention relates to vacuum tube networks and more particularly to improvements in circuits for determining the condition of vacuum tubes embodied in signal amplifiers and the In certain applications such, for example, as small communication systems, it is advantageous from the standpoint of economy of construction and operation to use an available current source for a number of different purposes. Ordinarily, the voltage of such a source is too high to permit the connection of vacuum tube heaters directly in parallel therewith. Hence, when a vacuum tube network forms a portion of the apparatus to be energized from the source, it becomes desirable to connect the cathode heaters of the respective tubes in series with each other and with a current determining resistor across the source. When the series arrangement is used, however, it is quite difiicult to locate a tube having an opencircuited cathode heater.

It is an object of the present invention to provide improved and exceedingly simple apparatus for quickly determining the condition of the '25 cathode heaters embodied in the vacuum tubes of a vacuum tube network.

More specifically, it is an object of the inven tion to provide improved apparatus for rapidly locating a tube having an open-circuited cathode heater in a vacuum tube network wherein the cathode heaters are connected in series across a current supply source.

The invention as described in detail hereinafter is illustrated in its embodiment in a signal amplifier of the general form used in public address systems. In brief, the objects as set forth above are attained in accordance with the present invention by providing a test or indicating lamp, and means comprising a manually operable ro- 10 tary switch for selectively connecting the indicating lamp in parallel with any one of the cathode heaters respectively embodied in the vacuum tubes of the amplifier. More particularly, the heater selecting switch comprises two sets of contacts, and two uni-controlled wipers having the indicating lamp connected therebetween and each associated with one of the contact sets. Circuit connections are provided which extend between the contacts of the switch and the heater terminals and are so arranged that the indicating lamp is successively connected in parallel with difierent ones of the series connected oathode heaters as the wipers of the switch are operated successively to engage different ones of their 3 associated contacts. The voltage rating of the indicating lamp is chosen substantially to equal the voltage of the source and, hence, its resistance is relatively high as compared to that of each of the cathode heaters.

By the arrangement de scribed, and when the lamp is connected in parallel with the cathode heater of a selected tube, it is only energized to radiate its rated illuminating power when the parallel connected heater is opencircuited.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawing, in which there is illustrated a signal amplifier having incorporated,

therein the features of the present invention.

Referring now to the drawing, the improved signal amplifier is illustrated as comprising three amplifier stages In, H

and I2 coupled in tandem in the order of their enumeration and arranged to amplify signal voltages impressed between the amplifier as indicated and to supply the amplified output loud speaker or translating device "5. More particua three-elechaving its input electrodes l8 andl9 coupled to the input terminals I 3 and I5 by means of a transformer 20, and its output electrodes I9 and 2| coupled by means of an output circuit including a resistor 22 to the input circuit 23 of a second three-electrode vacuum tube 24 included in the second amplifier stage II. The control grid in the input circuit of serially connected grid biasing cells 25 biasing network provided the tube l1 comprises two and 26,

which are connected between the cathode l9 and the lower terminal of the secondary winding of the transformer 20, and are by-passed for signal frequency currents by a capacitor 27.

In order to balance the primary winding 01 the transformer 20 with respect to ground, thereby to minimize stray coupling to the input circult of the tube l1, there are connected in series across this winding two resistors 8 and 9 having the junction point therebetween grounded as indicated. The cathodes of the tubes I! and 24 are maintained substantially at ground poten-. tial, as regards signal currents, through the provision of a coupling condenser 1 connected as shown. The coupling network connected between the two amplifier stages II and I 2 comprises a transformer 28 having a primary winding 29 forming a part of the output circuit of the tube 24 and a secondary winding 30 comprising a portion of the input circuit for the push-pull connected tubes 3| and 32 of the power stage I2. Each of the two last-mentioned tubes is of the well-known pentode type comprising an anode, a cathode, a control grid, a suppressor grid and a screen grid. The two control grids of the tubes 3| and 32 are respectively connected to opposite terminals of the secondary winding 30 and the cathodes thereof are connected together and to the center point of the winding 30 through a cathode biasing resistor 33 which functions in a well-known manner to maintain the proper potential relations between the input electrodes of each of the two tubes noted. Also in accordance with conventional practice the suppressor grids of the tubes 3| and 32 are maintained at cathode potential through the connections indicated in the drawing. The respective anodes of tubes 3| and 32 are connected to opposite terminals of the primary winding 33' of a transformer 3 having its secondary winding 35 connected to the input terminals of the loud speaker IS. The midpoint of the primary winding 33 is connected to each of the screen grids included in the two tubes 3| and 32.

Anode potential is supplied to the anodes of the tubes H, 24, 3| and 32 and to the screen grids of the tubes 3| and 32 from a supply source l4. A filter network indicated generally at 3B is provided for preventing transient changes in the voltage of the source M from introducing noise in the amplifier channel. Specifically, this network comprises a filter circuit including a choke inductance element 38 and two capicitors 35 and d connected between the source it and the lower terminal of the resistor 22, and a similar filter circuit including the choke inductance element 4|, a capacitor 12 and the capacitor 50 connected between the source l4 and the anode of the tube 24. The last-described filter circuit is also connected between the source It and the screen and anode electrodes of the tubes 3! and 32.

As illustrated in the drawing, each of the tubes embodied in the amplifier is of the indirectly heated cathode type, the heaters of which are connected in series across the voltage source This series circuit also includes a circuitinterrupting switch 6| and a current limiting resistor 00 for maintaining the current flowing through the circuit at the rated current value of the tube heaters. As pointed out in the introductory portion of the specification, when the plurality of tube heaters are energized in series it is quite diificult to locate in a short time a tube having an open-circuited heater. Accordingly, there is provided, in accordance with the present invention, a novel heater testing unit indicated generally at 62 for quickly locating a tube rendered defective by a burned out or broken cathode heater.

Considering now the system as a whole and disregarding for the present the details of the input circuit 23 and the testing unit 62, it is believed that the operation of the amplifier as described above, will readily be understood by those skilled in the art. In brief, a signal voltage impressed between the input electrodes 13 and i5 is stepped up by the transformer 2.0 and impressed on the input electrodes of the tube ll. The amplified voltage output of this tube is impressed by means of the input circuit 23 on the input electrodes of the tube 24, which tube further amplifies the voltage and delivers its output to the transformer 28. Further voltage gain is obtained in the transformer 28 which delivers the driving voltage to the control grids of the tubes 3| and 32 of the power stage i2. Finally, the amplified power output of the stage I2 is delivered to the input terminals of the loud speaker I6 which converts the signal currents into sound energy.

In certain applications the voltage of the source It is necessarily limited to a relatively low value. In railroad train power systems, for example, this voltage is usually of the order of sixty-five volts and, hence, must be utilized to the fullest extent if any considerable voltage gain is to be realized in each of the driver stages l0 and II. To this end, the separate control grid biasing network comprising the cells 25 and 20 is used in the input circuit for the tube ll so that substantially the entire voltage of the source I4 is efiectively applied to the anode 2! of this tube. For the same reason independent biasing cells are used in the input circuit 23 for the tube I'l. The particular arrangement of this input circuit forms the subject matter disclosed and claimed in applicants above referred to co-pending application. In order to minimize trouble occasioned by deterioration of the biasing cells included in the input circuits for the tubes l1 and 24, these cells are preferably of the type known in the trade as The Mallory grid bias cell. These cells, which are manufactured by the P. R. Mallory 84 Company, Incorporated, are described in a descriptive bulletin issued by this company June 15, 1935, and are known to have a life which exceeds the ordinary life of any commercially available vacuum tube.

Briefly described, the circuit 23 comprises a voltage dividing resistor 43 having its low potential terminal directly connected to the cathode 44 of the tube 24 and its high potential terminal capacitively coupled by means of a capacitor 45 to the anode 2| of the tube N. This resistor has an adjustable portion 46, the value of which is determined by the setting of an adjustable terminal 4-1, which terminal is slidable along the resistor 03 to vary the signal voltage input to the tube 24 and, hence, the output volume of the loud speaker l6. Biasing potential is applied to the control grid 48 of the tube 24 through the provision of three serially-connected cells 19, 50 and 5|, which are connected between the control grid 48 and the adjustable terminal M of the voltage dividing resistor 00. For reasons pointed out above it is desirable to use cells 49. 50 and 5| of the Mallory type as the bias voltage source. Since, however, each of these cells has an internal resistance ranging from 11,000 to 50,000 ohms, it is impractical to connect the same between the cathode 44 and the control terminal of the resistor 43. Thus, if this were done, from 33,000 to 150,000 ohms of resistance would be included in the adjustable portion of the resistor 03. This resistor, in the particular arrangement illustrated, is of approximately 500,000 ohms resistance and, hence, if the biasing cells were connected in the manner described, the minimum signal voltage level attainable would be limited to from approximately six to twenty-two per cent of the level for maximum signal voltage input to the tube 24. It will be seen, therefore, that the presence of the control grid biasing cells in the cathode leg of the input circuit would definitely limit the minimum voltage setting of the volume control signal voltage divider. With of the cathode heaters.

the arrangement illustrated, however, the adjustable terminal 41 of the voltage dividing resistor may be moved to a point where the signal voltage input to the input electrodes of the tube M is reduced to zero.

Turning now to a consideration of the testing unit 62, this unit comprises, in general, an indicating device in the form of a lamp 63 and means comprising a manually operable rotary switch 64 for selectively connecting the lamp 63 in parallel with any one of the tube heaters. More specifically, the switch 64 comprises two sets of contacts 65 and 66 to which the terminals of the respective cathode heaters are connected, and a pair of uni-controlled wipers 67 and 68, associated respectively, with. the contact sets 67 and 68 and having the lamp 63 connected therebetween. The mode of connecting the heater terminals to the contacts of the sets F5 and 66 is such that with the wipers of the switch 64 cperated to any off-normal position, the lamp 6-3 is connected in parallel with one of the cathode heaters. Thus, the opposite terminals of, the cathode heater in the tube ll are connected, respectively, to the respective first contacts of the sets 65 and 66; the opposite terminals of the cathode heater in the tube 24 are connected, respectively, to the respective second contacts of the contact sets 65 and G5; the opposite terminals of the cathode heater in the tube 3! are connected. respectively, to the respective third contacts of the sets 65 and 65; and the opposite terminals of the cathode heater in the tube 32 are connected, respectively, to the respective fourth contacts of the sets 65 and 66. The fifth off-normal position of the switch 64 is reserved as a test position for determining the condition of the indicating lamp 63. To this end, the fifth contact of the set 65 is adapted to be connected by way of a jumper conductor 63 and the switch iii to the negative terminal of the source l4 and the fifth contact of the set 56 is connected by way of a resistor Ill and the resistor lid to the positive terminal of the source. The resistance value of the resistor 11! is chosen to equal the resistance value of three of the cathode heaters connected in series, so that when the wipers of the switch 64 are operated to the test position the normal rated voltage of the lamp is impressed on the filament thereof. In order to eliminate unnecessary wiring, jumper wires between the contacts are used wherever possible. For example, the circuit connections include a conductor ll con-- nected to the second and third contacts of the set 65 by a jumper or multiple wire 72 and to the junction point between the connected terminals of the heaters embodied in the tubes 24 and 3!. Similar jumper connections employed in the manner shown simplify thewiring to a considerable degree.

By providing the above described testing unit, it is possible rapidly to check the condition of any Thus, if' the amplifier suddenly stops functioning the operator may actuate the wipers of the switch 64 to the test position and if the lamp 63 lights up is informed that this lamp is operating properly. Following this testing operation, the switch wipers may successively be operated into engagement with different ones of their respective associated contacts. In each position, other than the fifth oiT- normal position, one of the cathode heaters is connected in parallel with the lamp 63, Now the filament of this lamp is of a relatively high resistance as compared to that of each ofrthe heaters and, hence, if a parallel connected heater is not open-circuited, insuflicient voltage is impressed on the lamp to cause any substantial illumination thereof. If, however, the lamp is connected in shunt with an open-circuited heater, the voltage of the source M, less the small voltage drops through the resistor iii] and the other heaters, is impressed on the lamp filament to cause the lamp to assume its fully illuminated condition. Such illumination of the lamp indicates an open circuit in the heater of the tube corresponding to the particular position to which the switch 64 is operated.

Although the invention has been illustrated in its application to circuits for energizing the cathode heaters of tubes of the indirectly heated cathode type, it will be understood that it is equally applicable to circuits for energizing cathodes of the directly heated or fil ment type wherein the electron emitting material forming the cathode is coated directly on the heater element.

While there has been ent considered to be the of the invention, it will be understood that various modifications may be made in the circuit arrangement, and it is contemplated in the appended claims to cover all such modifications as fall within the true spirit and scope of the in vention.

I claim:

1. In a vacuum tube network, a plurality of vacuum tubes each comprising a cathode heater, a source of heater current, means connecting said heaters in series across said source, a manually operable switch comprising two sets of bank contacts and two uni-controlled wipers each associated with one of said contact sets, an indicating lamp connected between said wipers, and circuit connections extending between the terminals of said heaters and the contacts of said switch, said circuit connections being so arranged that said indicating lamp is successively connected in parallel with diiTerent ones of said heaters as said wipers are operated successively to engage different ones of their associated contacts, and said lamp having a relatively high resistance such that it is substantially illuminated only at the voltage of said source, whereby an indication described what is at prespreferred embodiment is given by said lamp only when it is connected in parallel with an heaters.

2. In a vacuum tube network, a plurality of vacuum tubes each including a cathode heater, a source of heater current, means connecting said heaters in series across said source, a manually operable switch comprising two sets of contacts and two uni-controlled wipers each associated with one of said contact sets, an indicating lamp connected between said wipers, and circuit connections extending between the terminals of said heaters and the contacts of said switch, said circuit connections including a conductor connected to two contacts of one of said contact sets and to a junction connection between two heater terminals, and being so arranged that said indicating lamp is successively connected in parallel with different ones of said heaters as said wipers are operated successively to engage different ones of their associated contacts,

3. In a vacuum tube network, a plurality of vacuum tubes each comprising a cathode heater, a source of heating current, means connecting said heaters in series across said source, a manually operable switch comprising two sets of bank open-circuited one of said contacts and two uni-controlled wipers each associated with one of said contact sets, an indicating lamp connected. between said wipers, and circuit connections extending between the terminals of said heaters and the contacts of said switch, said circuit connections including a conductor connected to two contacts of one of said contact sets and to a junction connection between two heater terminals, and being so arranged that said indicating lamp is successively connected in parallel with difierent ones of said heaters as said wipers are operated successively to engage different ones of their associated contacts, and said lamp having a relatively high resistance such that it is substantially illuminated only at the voltage of said source, whereby an indication is given by said lamp only when it is connected in parallel with an open-circuited one of said heaters,

4. In a vacuum tube network, a plurality of vacuum tubes each comprising a cathode heater a source of heater current, means connecting said heaters in series across said source, a manually operable switch comprising two sets of contacts and two uni-controlled wipers each associated with one of said contact sets, an indicating lamp connected between said wipers, circuit connections extending between the terminals of said heaters and the contacts of said switch, said circuit connections being so arranged that said indicating lamp is successively connected in parallel with different ones of said heaters as said wipers are operated successively to engage different ones of their associated contacts, and means including said wipers, the contact of one of said sets and the corresponding contact of the other of said sets for connecting said lamp across said source to test the condition thereof, said lamp having a relatively high resistance such that it is substantially illuminated only at the voltage of said source, whereby an indication is given by said lamp only when it is connected across said source as in parallel with an open-circuited one of said heaters.

5. In an electrical network, a plurality of devices to be tested, said devices being susceptible of becoming open-circuited, a source of current, means connecting said devices in series across said source, a manually operable switch comprising two sets of contacts and two uni-controlled wipers each associated with one of said contact sets, an indicating device connected between said wipers and adapted to be energized from said source when connected in parallel with an open circuited one of said first-mentioned devices, and circuit connections extending between said first-mentioned devices and the contacts of said switch, said circuit connectionsincluding a conductor connected to two contacts of one of said contact sets and to a junction connection between the terminals of two of said first-mew tioned devices, and being so arranged that said indicating device is successively connected in parallel with different ones of said first-mentioned devices as said wipers are operated successively to engage different ones of their associated contacts.

6. In an electrical network, three devices to be tested, said devices being susceptible of becoming open-circuited, a source of current, means connecting said devices in series across said source, a manually operable switch comprising two sets of contacts and two uni-controlled wipers each associated with one of said contact sets, an indicating device connected between said wipers and adapted to be energized from said source when connected in parallel with an open circuited one of said first-mentioned devices, a first con ductor extending from the junction connection between the terminals of the first and second of said first-mentioned devices to two adjacent contacts of one of said contact sets, a second conductor extending from the junction connection between the terminals of the second and third of said devices to two adjacent contacts of the other of said contact sets, the second contact of said last-mentioned two contacts being arranged to be engaged by its associated wiper simultaneously with the engagement of the second contact of said first-mentioned two contacts by its associated wiper during operation of said switch, and circuit connections including said conductors for successively connecting said indicating device in parallel with difierent ones of said firstmentioned devices as said wipers are operated successively to engage different ones of their associated contacts.

7. In an electrical network, three devices to be tested, said devices being susceptible of becoming open-circuited, a source of current, means connecting said devices in series across said source, a manually operable switch comprising two sets of contacts and two uni-controlled wipers each associated with one of said contact sets, an

indicating device connected between said wipers andadapted to be energized from said source when connected in parallel with an open circuited one of said first-mentioned devices, a first conductor extending from the junction connection between the terminals of the first and second of said first-mentioned devices to two adjacent contacts of one of said contact sets, a second conductor extending from the junction connection between the terminals of the second and third of said devices to two adjacent contacts of the other of said contact sets, the second contact of said last-mentioned two contacts being arranged to be engaged by its associated wiper simultaneously with the engagement of the second contact of said first-mentioned two contacts by its associated wiper during operation of said switch, circuit connections including said conductors for successive 1y connecting said indicating device in parallel with difierent ones of said first-mentioned devices as said wipers are operated successively to engage different ones of their associated contacts, and means including said wipers, the contact of one of said sets and the corresponding contact of the other of said sets for connecting said indicating device across said source to test the condition thereof,

8. In a vacuum tube network, a plurality of vacuum tubes each including a cathode heater, a source of current, a, circuit serially including said source said heaters and a resistor, a manually operable switch comprising two sets of contacts and wipers individually associated with. said contact sets, an indicating lamp connected between said wipers, circuit connections extending between the terminals of said heaters and the contacts of said switch, said circuit conductors including a conductor connected to two contacts of one of said contact sets and to a junction connection between two heater terminals and a second conductor extending from the next succeeding junction connection between two heater terminals to two contacts of the other of said contact sets, one of said last-mentioned contacts corresponding to one of the two contacts of said one contact set, said circuit connections being so arranged that said indicating lamp is successively connected in parallel with difierent ones of said heaters as said wipers are operated successively to engage diiferent ones of their assohich is relatively much greater than the resistance of one of said heaters, whereby it is substantially illuminated only at the voltage or" said source and an indication is given by said lamp only when it is connected in parallel with an open-circuited one of said heaters; and circuit means serially including said resistor, an additional resistor having a resistance approximating the combined series resistance of said heaters, said wipers and a pair of corresponding contacts of said contact sets for connecting said lamp across said source to test the condition thereof. ROSWELL H. HERRICK.

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