US2340904A

US2340904A - Signal amplifying system

Info

Publication number: US2340904A
Application number: US380265A
Authority: US
Inventors: Allen A Shoup
Original assignee: SOUND Inc
Current assignee: SOUND Inc
Priority date: 1941-02-24
Filing date: 1941-02-24
Publication date: 1944-02-08
Anticipated expiration: 1961-02-08

Description

Feb. 8, 1944. A. A. sHoUP SIGNAL AMPLIFYING SYSTEM Filed Feb. 24, 1941 Patented Feb. 8, 1944 l,

SIGNAL AMPIJFYINGSYSTEM Allen Shoup, Chicago. lll.,

Inc., Chicago, Ill.,

assignor to Sound,

a corporation of Illinois Application February 24, 1941, Serial No. 380,265

24 Claims.

My invention relates to signal amplifying systems, and has for its primary object to provide an improved amplifying system and method of operation with amplifying tubes having comparatively low power rating, to eect relatively large power output capacity without distortion, and with a high degree of eiiiciency and dependability.

More particularly, my invention relates to audio frequency amplifying systems for use in sound-n-iilm projectors, public address systems and the like, and has for its further object to provide an improved amplifying system having electrodal control and potential supply circuits for safely operating relatively low power output screen-grid amplifier tubes with increased power output and with a high degree of emciency.

In low power amplifiers of the character referred to, it is advantageous to utilize, in the out! put or power stage thereof, beam power or other suitable screen grid tubes of the 6L6 and 6V6 type, for example, which may be provided with electron deilecting electrodes, a screen grid and a control grid, in addition to the usual anode and cathode. In providing maximum power output with minimum cost of construction and operation, such ampliers are often subjected to overload. In sound-on-lm projectors, public address systems and the like, this condition must be provided for while maintaining a minimum size and weight. y

It is, therefore, a still further object of my invention to provide an amplifying system which may be constructed at low cost and of minimum size and weight, and which may operate the power output stage' with beam power tubes and other screen grid tubes, far beyond previously recognized and accepted commercial ratings for output wattage and power supply voltages and currents, without reducing the tube life.

It is an object of my invention further, to provide an audio frequency amplifier having relatively low wattage screen grid tubes provided with a voltage supply network between the anode, cathode and screen grid circuits, for effecting and maintaining a wide separation of screen and anode voltages, 'a substantially constant bias, with signal input amplitude variations, and a substantially constant anode output current.

It is also a further object of my invention, to provide an amplifier for audio frequency signals and the like, having a high wattage handling capacity employing relatively low wattage screen grid amplifying tubes, and a protective voltage supply network therein for operation with high screen and anode voltages to provide a relatively high' power output without reducing the operating life of the tubes.

I have found that the power handling capacity of an amplifier stage embodying beam power tubes may safely be increased beyond former accepted limits by providing a circuit or voltage supply network in connection therewith which, in the presence of high amplitude signals, will maintain the electron beam through the tube in focus, that is, within its normal path between the cathode and the anode as directed by the deflecting electrodes through the control grid and screen grid structure. In response to a'heavy or abnormally high signal amplitude, the electron beam may be driven out of the normal focused path through the control grid and screen grid to such an extent that the screen grid is subjected to excessive current sufficient to disintegrate the relatively small grid wires of which it is composed. This out of focus condition at heavy overloads often occurs when the potentials of the screen grid and the anode are permitted to be substantially equal or of the same order with respect to the cathode.

However, it has been the practice to operate beam power and other screen grid power output tubes in this manner in an attempt to obtain good voltage regulation on the screen, that is, with the screen and the anode circuits connected to substantially the same voltage supply and with this Voltage las high as the commercial ratings of the tubes would permit. With this mode of operation, however, it has been found that the tube life is greatly shortened` and in most instances is terminated by destruction of the screen grid on overload with excessive signal.

It is a further object of my invention, therefore, to provide a power amplifier for modulators and other amplifying systems, in which beam power and other screen grid tubes may be utilized with high power output ratings and without danger to the screen grid structure thereof in the presence of signal overload.

In accordance with the invention, the screen grid and the anode are initially operated with a relatively wide difference in potential, obtained preferably by raising the anode potential above the screen grid potential by a considerable amount and providing means in connection with the screen grid for maintaining a wide difference in potential between the screen grid and the anode under all conditions of operation, including .a condition of heavy signal overload.

In a preferred embodiment of my invention,

both the screen and the anode voltages are raised appreciably above former accepted commercial ratings and are maintained in operation relatively widely separated, the screen voltage being always lower than the effective anode voltage and, in the case of beam power tubes, by a sufficient amount to prevent the beam from being shifted out of focus. t

It is desirable to provide a simplified power supply source for relatively small and low cost amplifying systems for sound-on-film projectors and the like, and to this end it is further desirable that the anode current drawn from the source by the amplifier should remain substantially constant. With substantially constant current drain on the supply source, self-bias may be utilized with the same advantages as fixed bias in the amplifier stage. I have found that, by the proper control of the screen grid current in the presence of an applied signal, the current taken by the power amplifier stage may be maintained substantially constant.

Accordingly, it is a further object of my invention to provide an improved amplifying system embodying beam power or other suitable screen grid power tubes, with a circuit comprising controlled impedance elements connected to the screen grids and other electrodes of the tubes, and so proportioned '1n impedance value between the various tube elements and the voltage supply circuit that the screen voltage in the output stage may vary in accordance with signal amplitude variations at the control grid or grids of the amplifier, thereby to maintain the anode current substantially constant and to permit the use of substantially fixed self-bias potentials.

An amplifying system in accordance with my invention further permits the anode potential to be suiliciently greater than the screen potential at various signal input conditions to provide relatively higher than rated voltages or currents o'n the anode and the screen electrodes, a higher' than normal undistorted output, a greater power output before grid current starts to flow, and a higher plate impedance than is normally allowable, Ithereby improving the tone quality of the amplifier preventing the electron beam from going out of focus to an excessive degree in the presence of heavy signals.

In accordance with another feature of the invention, the driver and preamplifier stages for the power amplifier may be operated to provide a compensating harmonic distortion in opposition to that of the output stage, thereby effectively cancelling the major harmonics at the output stage. The input or preamplifier may also be operated further to compensate for other undesired harmonic distortion, and if a balanced or push-pull amplifier is utilized in the output stage, the second harmonic is substantiallyveliminated.

Amplifiers for sound-on-film projectors, public address systems and other similar` audio frequency amplifying purposes must be capable of relatively high undistorted power output with relatively low current consumption, and to withstand heavy signal load without exceeding accepted limits of distortion.

It is an object of this invention, therefore, to provide an audio-frequency amplifying system having an improved power output characteristic as herein'before referred to, and means in connection with one of the earlier stages for variably controlling or adjusting the treble and bass tone characteristics imparted to the signal through the amplifier, throughout a relatively wide control range, and without introducing distortion by reason of phase shift in the tone control network.

In accordance with the invention, I have provided an amplifying system having an improved over-al1 frequency characteristic and which is within accepted limits for distortion on signal overload. As a controlling element, the tone control network provided is of an improved and simpliiied construction having a single control element for controlling both the high or treble and and, in the case of beam power tubes,

sidered in connection with the accompanying v drawing, and its scope is pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of an audio frequency amplifier embodying the invention, and particularly adapted for use inaI soundon-i'lm projector or a public address system, and Figure 2 is a schematic representation of the structure and arrangement of the electrodes of.

one of the power output tubes shown in Fig. 1, illustrating-its operation in accordance with the invention.

Referring to Fig. 1, a three-stage audio frequency amplifier is shown, comprising a first stage amplifier tube 5, a'second stage amplifier tube 6 also containing oscillator elements 1, and a push-pull or balanced power output stage comprising a pair of amplifier tubes 8 and 8.

As this amplifier is particularly adapted for sound-on-film or public address systems, it is of compact and simplified construction and capable of handling a relatively high audio frequency output with minimum power consumption and with relatively low wattage power output tubes.

For this reason, in accordance with my invention, the output stage comprising the tubes 8 and 9 is made to operate in response to signal amplitude variations with a variable plate impedance, the average of which is appreciably higher than the normal accepted value for the particular tubes employed. For example, assuming that the tubes 8 and 9 are of the 6V6 beam power type, having cathodes' I0, control grids I I, screen grids I2, output anodes I3 and deflector electrodes I4, the output impedance, normally 8000 ohms for two tubes in push-pull or balanced relation, as shown, is made to have an average output impedance of the order of twice that value, and the screen grid is caused to vary in potential with signal input amplitude Variations to effect a variation in plate impedance.

In the output stage shown, the tubes 8 and 9 are provided with a balanced or push-pull output circuit I5-I 6 connected to the end terminals of the balanced primary winding I'I of an output transformer I8. The output circuit across the primary I1 is provided with a shunt capacitor I9.

The secondary 20 of the transformer I8 is provided with a short-circuiting jack output connection 2 I, to which any suitable load or output device for the amplifier may be connected. Normally, however, a loudspeaker or group of loudspeakers is connected to this jack. For the sake of simplifying the drawing, and for the reason that the loudspeakers do not concern the invention, the same are not shown. It will be noted that the secondary 20 is preferably connected to ground or chassis, as indicated at 22. The input circuit for the output stage 8-9 is likewise of the balanced or push-pull type arranged for class A or class'AB-I voperation, and comprises the secondary 24 of an input coupling transformer 25 connected at its terminals to the signal input grids II, throughleads 26-21. The center tap 28 on the secondary is connected to ground, as indicated at 29, and to the cathodes in parallel, through a selfbias.resistor 30 which have a value of 50,000 ohms in the present example.

The resistor 30 is a common self-bias resistor f for the amplifier. stage, since ground is a common return circuit for the various tube circuits. It is connected at the cathode with a voltage control or protective network interconnecting the anode,

screen grid and cathode circuits, compri-sing a resistor 35 connected between the cathode and the screen grids I2, a bypass or filter capacitor 36 connected directly to ground from the screen grids, and a resistor 31 connected between the screen grids and the center tap or positive supply terminal 38 for the anodes on the output primary I1. The resistors 35 and 31 may have values of the order of 100,000 ohms and 15,000 ohms, respectively, and the capacitor 36 may have a value of 10 mfd.

The positive supply terminal 38 is connected through a supply lead 39 with the positive supply lead 40 from a high voltage rectifier 4|, energized through a suitable transformer 42 from alternating current supply leads indicated at 43. The supply leads 43 are provided with a suitable power switch 44, and a plug-in terminal connection, indicated at 45. The negative terminal 46 of the power supply rectifier is connected to ground or chassis 41 through a lead 48 and a bias' supply resistor 49, through which the combined anode and screen grid current of the amplifier is taken. The high voltage rectifier or plate supply means is provided with a filter capacitor 50, which may have a value of the order of 40 mfd. to smooth the rectifier output.

The resistor 49 is utilized as a fixed source of bias potential for the driver amplifier 6, and the negative terminal, accordingly, is connected to the control grid 51 through series filter resistors provided with by-pass capacitors 52, a bias supply lead 54, a; grid lead 55 and a grid resistor 56. The cathode 58, as well as the cathode 59 of the oscillator section 1, is connected to ground, as

indicated at 60, thus completing the bias circuit to the ground connection 41 of the resistor 49.

The grid lead 55 is also connected to an intermediate tap 6| on a tone control potentiometer 62, one terminal 63 of which is connectedthrough a resistor 64 to a feedback lead 65.` The latter is coupled through a stopping capacitor 66 with a source of feedback potential or current comprising an output circuit 61 connected with the anode 68 of the amplifier tube 6. The output circuit 61 is connected through the primary 69 of the coupling transformer 25 with the positive supply lead 40, and a voltage drop producing resistor is included between the lead 40 and the primary circuit connection 12. In the present example, the resistor` 16-1l is approximately 23,000 ohms and, together with the filtered ixed bias supply for the driver stage, permits operation of the amplifier without the usual filter choke coil in circuit with the rectifier 4I.

The elimination of the choke coil, through the use of the bias supply circuit as shown and described, eliminates a major source of hum pickup in the amplifier, and in addition aids in materialthe terminal end 63 and the opposite terminal end indicated at 15, to which feedback currents 'are also conducted lthrough a frequency discriminating circuit 16 containing a capacitor 11 and a resistor 18. In the other branch, for the terminal 63, a capacitor 19 provides a frequency discriminatng connection across a section of the potentiometer 62, between the terminals 63 and 6|. 'I'hus the potentiometer 62 receives feedback potentials from the lead 65 through a branch circuit 16 containing the capacitor 11, to the terminals 15, and through the connection to the terminal 63, shunted by the capacitor 19. The terminal 6| and the lead 54 are at substantially ground or cathode potential for audio frequency signals, and the grid 51 receives no feedback through the lead 55 and the resistor 56.

Feedback, however, is applied to the grid 51 through a grid resistor and a supply lead 8| connected to the potentiometer ann or contact 82. When the latter is moved to the terminal 6|, no feedback through the grid is provided. As the contact is moved toward the terminal 15, the series feedback through the capacitor 11 is effective to reduce the high frequency component of the signal, thereby effectively increasing the low frequency or bass component of the amplified signal.

When the contact 82 is moved from the terminal 6| toward the terminal 63, the shunting effect of the capacitor 19 on the feedback of high frequency signals causes an increase in the high frequency or treble component of the amplifled signal. Various intermediate positions provide corresponding controlling effects on the signal whereby treble or bass compensation may be added to the signal as desired. It will be noted that the tone control network and inverse feedback circuit does not include any inductive element. Also the neutral tap 6| is substantially -at grid and ground potential. Therefore, operation of this tone control circuit produces minimum phase shift in the feedback and, accordingly, does not introduce appreciable distortion.a.t any setting of the tone control contact 82. In addition to providing overall tone compensation for the amplifier, it has the advantage that it does not appreciably add to the cost or size of the apparatus.

Signals are introduced into the driver stage 1 through a coupling capacitor 85 for the grid 51 from the output anode circuit 86 of the pentode input yamplifier stage 5, which receives anode current through a filter resistor 81 and a bypass capacitor 88 from the terminal 12. is of the self-bias type having a suitably bypassed, self-bias resistor 89 connected between the cathode 90 and ground 9| and having a screen i grid 92 suppliedfrom the resistor 81 in the positive supply lead for the system. The screen grid is provided vwith a filter resistor 93 of the order of l megohm and a bypass capacitor 94 of the order of .05 mfd.

The signal grid 95 is connected to an input circuit 96 provided with a branch input connection 91 and a second branch input connection 98. The circuit 96 is coupled to the branch 98 through a coupling capacitor 99 across an input coupling impedance or resistor |00 which may be of the order of several megohms, while the capacitor 99 may have a value of .005 mfd.

The circuit 98 is also yconnected through the;

Biasing potential supply for the cell |06 is taken from the positive supply lead for the stage 5 through a bleeder circuit |06 which includes a resistor ||0, the potentiometer resistor or variable source |05, and a series resistor connected to ground as indicated at ||2.

The oscillator section 1 of the amplifier tube 6 is provided with an oscillator circuit.||5 which energizes a projector lamp ||6 with substantially constant potential. 'Ihe brightness of the lamp is controlled by a variable resistor ||1 connected in the anode supply circuit ||8 for the oscillator which is provided with a suitable filter capacitor ||9 and a ilxed series resistor |20. The anode circuit is connected with the positive supply lead 40 through the lead 39.

As the projector lamp and oscillator circuits do not concern this invention, further description is believedto be unnecessary. However, by utilizing a single tube for -both the oscillator and amplifier elements, the amplier system is simplified and rendered more compact, and by so doing, both the oscillator cathode and the amplifier cathode are connected to ground by a common connection, which is permitted because of the fact that a separate fixed bias supply is utilized for the driver stage, as hereinbefore described, as an aid in simplifying the filtering system for the ampliilen An additional or auxiliary signal source may be connected with the input circuit of the tube 5, through the branch 91. The latter includes a volume control potentiometer |25, the movable contact or arm |26 of which is connected through a resistor |21 to one terminal |28 of a microphone or pickup jack |20. 'I'he opposite terminal |30 of the jack is grounded.

Signals introduced through the pickup jack |29 may, therefore, be mixed with the signal from the photoelectric cell |06 to any desired degree, by operation of the contact |26, moving the contact upwardly as viewed in the drawing, causing an increase in the amplitude of the signals introduced into the input circuit of the amplifier 5 along with the photocell pickup. When the contact |26 is moved to its lowermost position, the input potential from the extern-al pickup is zero.

As hereinbefore pointed out, itis desirable that the harmonic distortion in the amplifier be neutralized or minimized as far as possible, and to this end the amplifier stages are operated with the anode and grid potentials and circuit impedances so adjusted that harmonic distortion in the successive stages is minimized or balanced out, and thus the overall output of the amplier is substantially free of the major undesired harmonies. To this end, in the present example the resistor 89 may have a value of 3,000 ohms, the resistor 93 may have a value of l` megohm, resistors 53 and 56 may have values of 200,000 ohms and resistor 01 may have a value of 2,000

ohms. The tubes 5 and 6 may be of the type 6J1y and 6N?, respectively. However, these and other values of circuit components are given only by way of example, as illustrating a present preferred embodiment of the invention.

Referring now to Fig. 2, along with Fig. l, the electronic stream and operation of the output amplifier tubes 8 and 9 will briey be considered.

In Fig. 2 the same reference numerals are appliedl to the tube electrodes as in Fig. 1.

In known amplifier systems utilizing screen grid tubes of the character shown, it is desirable, in the interest of higher power output, to provide as low resistance as possible in the screen grid circuit, and to this end the screen grid is usually connected to the source of anode potential for the tube. In the present example, this. would provide a direct connection for the screen grid |2 to the terminal 38 or the lead 39, without the use of the resistors 35 and 31, thereby placing vthe screen and anode electrodes at substantially the same positive potential value with respect to the cathode. For that mode of operation, recommended values of voltages for the screen and' anode of beam power tubes, for example, were greatly reduced and tentatively established as 2'10 for the anode and 250 for the screen, following serious diillculties in the ileld with tube breakdown, mainly from failure of the screen grid when voltages of the same value were applied to both electrodes. In accordance with the invention, however, these values may be raised with safety to 315 volts for the anode and 250 volts for the screen. Such ratings are now accepted.

The cause of failure in the field in amplifiers of the type described is due mainly to the tendency to overload the output ampliiler stage continuously with excessive signals to produce wide sound coverage, and to the fact that the ampliiler may operate oftentimes when' just set up, without the speakers being connected and with the volume control increased beyond a desirable limit before itis discovered that sound output cir- .,cuit is not complete. Under such conditions, the output transformer is protected by the output jack 2| which is of the short-circuiting type.

The screen grid failure on excessively heavy signal is prevented, in accordance with the present invention, by the grid-cathode-anode network eifectively floating the screen grid connection between the cathode and the anode, while holding the anode current and control grid bias substantially constant. The initial potentials established between the screen and the anode are such that the anode potential is relatively high to insure high power output, and the screen potential is substantially lower initially, that is, in the absence oi' signals. However, this screen potential is higher than the accepted ratings and may safely be used.

In the absence of signal, the electron steam indicated at |35 ilows between the cathode I0 and the anode I3, being focused by the deilector or focusing electrodes I4 between the grid wires of the control grid and the grid wires of the screen grid |2, two only of which are indicated in the schematic showing for the purpose of illustrating the mode of operation and control. Upon striking the anode, some of the electrons are driven off to form a cloud or virtual electrode |36, between the anode and the screen grid. Because of the wide diilerence in potential between the anode and the screen grid, the electrons in the virtual electrode move into a more negative region as they approach the screen grid region and hence are held near the anode. Therefore, the screen current is not affected by the cloud of elecrons.

However, as a signal is applied to the control grid, and when such signal reaches excessive amplitudes, the electron stream increases and then becomes extended to such a degree that it goes out of focus and causes the virtual electrode |36 to permit electrons to flow to the screen grid. since the region it occupies is no longer increasingly highly negative in the direction of the screen grid. Thus the current may become so strong that the grid structure of the screen is disintegrated by failure to carry the current.

I have found, however, that if the virtual electrode may be maintained in a negative condition, as by decreasing the positive potential on the screen grid in the presence of heavy or excessively high amplitude signals on the control grid, then the screen current may be maintained below a value sufllcient to disintegrate it, andto this end, in accordance with the present invention, the control network 30, 35 and 31 comprises resistor or impedance elements so proportioned that the screen voltage is prevented from rising excessively and eventually is caused to decrease to a relatively low value in the presence of excessive signals on the ,control grid or grids, thereby always maintaining the virtual electrode |36 in a negative region between the anode and the screen grid. This at the same time maintains the electron stream in focus through the screen grid.

The following tabulated data are the result of the operation of the circuit of Fig. 1 under various conditions of signal input with the type 6V6 beam power tubes as shown at 8 and 9:

Maximum Typical Zero signal o e possi e signal signal Heater voltage... 6. 3 6. 3 6. 3 Plate voltage -do. 400 400 376 Screen voltage.... .do 300 228 147 Grid voltage (se1f-blas) ..do. 22. 5 23. l 29. 5 A. F. signal grid to grid (R. M. S.)

voi o 34 4s Load resistance, plate to plate ohms-. 1e, 00o 16, 000 16, ooo Plate current .milllamperes.. 60 56. 5 71 Plate dissipation watts.. 24 4. 8 2. 7 Screen current -milliamperes 2. 5 8. 8 13. 6 Screen dissipation watts.. 75 2. 05 2. 02 Power output ..do... l5 20. 0 Distortion percent.. 4. 3 41. 0

It will be noted that the power output is substantially twice that normally considered obtainable from a pair of 6V6 beam power tubes within the limits of allowable distortion, and after extensive use in the field this operation has been found in no way to reduce the operating life of the tubes.

It will be noted that, in the tabulated operating data above, this circuit arrangement causes the screen -potential to fall from 300 to below 150 vcts on maximum signal, thereby effectively maintaining a negative virtual electrode region between the anode and the screen and maintaining the screen current below an excessively high value suicient to disintegrate the screen wires.

It will also be noted that between zero signal and maximum undistorted signal, the total plate and screen current remains substantially unchanged, between 62.5 m. a. and 65.3 m. a. This feature is desirable for the reason that, as hereinbefore pointed out, self-bias may be applied to the control grid with substantially the same eect as fixed bias, and the load on the power supply does not vary, thereby permitting a simplied and low cost power supply unit of small size.

It will further be noted that the plate impedance of the tubes varies with the screen grid voltage and is caused in the present example all times maintained at a relatively safe and low value. The plate or anode voltage is highwith respect to the screen voltage. \Both 'the screen and anode voltages, with respect to the cathode, are relatively high and are set and, held widely separated, preferably by causing the screen voltage to be reduced in the presence of signals of increasing amplitude. Thus the output stage is permitted to deliver substantially twice its maximum power rating with safety and without drawing excessive current from the power supply.

In the case of beam power tubes, the beam is maintained in focus in the presence of heavy signals, and while high voltages are employed on the screen and plate, the tube life is not shortened thereby.

n While the invention has been described in connection with a specific amplifier for sound-onlm or public address systems, and with speciic tube types in the output stage, it should be understood that it is applicable to any amplifier subjected to excessively heavy signal input and wherein the screen grid or grids are controlled in accordance with signal potential variation and with the anode potential at all times suilciently greater than the screen grid potential to permit (1) relatively high voltages or currents to be applied to the plates and screen grids to provide higher power output without drawing the grid current (class A or AB operation), (2) less distortion in the presence of heavy applied signals, (3) higher output plate impedance and hence less distortion, and (4) control of the screen voltage to permit higher plate impedance without excessive screen current in the signals.

I claim as my invention: 1. The method of operating a beam power amplifier, which comprises applying a relatively high potential to the screen grid thereof, applying a higher operating potential to the anode presence of heavy thereof, providing a self-bias potential for thev control grid thereof, and causing the control grid bias to remain substantially constant and the screen grid potential to decrease in accordance with an increase in the amplitude of an applied signal potential with respect to the anode potential in such proportion that the direct current component of the anode current remains substantially constant.

2. In an audio frequency amplifier, the method l signal strength variation, and maintaining the electron stream of said tube substantially in focus in response to relatively heavy signal strength variations thereby to protect the screen grid from breakdown by automatically reducing the screen grid voltage in response to high levels of signal strength.

3. An audio frequency amplifier comprising,

infcombination, a first stage agriplier tube having a, signal input circuit and a signal output circuit, means for applying signals to said input circuit, a driver amplifier tube having an input circuit coupled to said output circuit, an oscillator having a common cathode connection with said amplifier, an inverse feed- `back circuit connected with the input circuit of said driver stage, variable control means in said inverse feedback circuit for selectively accentuating the treble and bass tone characteristic 'of signals transmitted through said amplifier, an output power amplifier stage coupled to said driver stage including a'power amplifier tube, a screen grid circuit for said tube, a voltage supply network therein for establishing a relatively wide anode and screen grid potential difference and responsive to signal amplitude variations whereby the screen grid potential is reduced to increase said difference in accordance with an increase in signal strength, and means providing a high impedance output circuit for said output stage.

4. An audio frequency amplifier comprising, in combination, a first stage amplifier tube having a signal input circuit and a signal output circuit, means for applying signals to said inpt circuit, a driver amplifier tube having an input circuit coupled to said output circuit, an inverse feedback circuit connected with the input circuit of said driver stage, variable control means in said inverse feedback circut for selectively accentuating the treble and bass tone characteristic of signals transmitted through said ampliiler, an output power amplifier stage coupled to said driver stage including a power amplifier tube, a screen grid circuit for said tube, a voltage supply network therein for establishing a relatively wide anode and screen grid potential difference and responsive to signal amplitude variations whereby thescreen grid potential is reduced to increase said diiference in accordance with an increase in signal strength, and means providing a high impedance output circuit for said output stage.

5. In an audio frequency ampliner, an amplier tube comprising a cathode, an anode and a control electrode and having an input circuit and an output circuit connected therebetween, an inverse feedback circuit connected between said output and input circuits including a potentiometer, a connection from one end of said potentiometer to said output circuit, a second connection from the opposite end of said potentiometer to saldoutput circuit containing reslstance and capacitance to operate as a frequency discriminating circuit, a xed connection from an intermediate point of said potentiometer to said grid maintained substantially at cathode potential, a condenser in parallel to the proportion of said potentiometer between said first end and intermediate point, and an adjustable connection from said grid to said potentiometer on either side of said fixed connection.

6. In an audio frequency amplifier, an amplifier tube comprising a cathode, an anode and a control electrode and having an input circuit and an output circuit connected therebetween, an inverse feedback circuit connected between said output and input circuits including a potentiometer, a connection from one end of said potentiometer to said output circuit cantaining a resistance, a second connection from the opposite end of said potentiometer to said output circuit containing resistance and capacitance to the exclusion of inductance to operate as a frequency discriminiating circuit, a fixed connection from substantially the midpoint of said potentiometer to said grid maintained substantially at cathode potential, a condenser in parallel to the portion of said potentiometer between said first end and midpoint, and an adjustable connection from said grid to said potentiometer on either side of said xed connection. l

7. In a signal amplifying system, the combination of a signal amplifying stage including a tube having a cathode, a control grid, an anode, and a screen grid operating at higher than otherwise allowable potentials subject to damage when the control grid swings positive, means for applying signals to said stage, and a voltage supply network interconnecting the anode, cathode, and screen grid and including an impedance between the anode supply and screen grid for applying higher than otherwise allowable static potentials to said screen grid for correspondignly reducing the potential on the screen grid in response to signal swings on the control grid to prevent damage to said screen grid.

8. The combination set forth in claim 7 wherein the circuit constants of said voltage supply network are such as to maintain the cathode direct current substantially constant in response to the input signals.

9. In a signal amplifying system for obtaining relatively high power output with low distortion, the combination of a signal amplifying stage including a tube having a cathode, a control grid, an anode, and a screen grid operating at higher than otherwise allowable potentials subject to damage when the control grid swings positive, means for applying signals to said stage, a voltage supply network interconnecting said anode, cathode and screen grid for applying higher than otherwise allowable static potentials to said anode and screen grid and including resistance between the anode supply and screen grid for correspondingly reducing the potential on the screen grid in response to signal swings on the control grid to prevent damage to said screen grid.'

10. The combination set forth in claim 9 wherein said network includes a self-biasing resistance controlled by the substantially constant cathode current passing therethrough.

1l. In a signal amplifying system, the combination of a signal amplifying stage including a beam power tube having a cathode, a control grid, an anode, and a screen grid subject to damage when the control grid swings positive, means for applying signals to said stage, and a voltage supply network interconnecting said `anode, cathode and screen grid for applying higher than otherwise allowable static potentials to said anode and screen grid and including a high impedance circuit between the anode supply and screen grid for reducing the potential on the screen grid in response to signal swings on the control grid to prevent damage to said screen grid and to maintain the electron beam in focus.

12. In a signal amplifying system, the combination of a signal amplifying stage including a beam power tube having a cathode, a control grid, an anode, and a screen grid subject to damage when the control grid swings positive, means for applying signals to said stage, a voltage supply network interconnecting said anode, cathode and screen grid for applying higher than otherwise allowable static potentials to said anode and screen grid and including a high resistance between the anode supply and screen grid for reducing the potential on the screen. grid in re- 13. The combination set forth in ciaim 12,-

wherein a self-biasing resistor is provided trav-l ersed by substantially constant cathode current in consequence of the values of the circuit constants of said voltage supply network.

14. In a signal amplifying system, the combinationV of' a signal amplifying stage including a pair of balancedbeam power tubes eachI having a cathode, a control grid, an anode, and a screen grid subject to damage when the control grid swings positive, means for applying signals to said stage, a balanced output circuit for said tubes having a mid-tap, a voltage supply network interconnecting said mid-tap, cathodes and screen grids for applying higher than otherwise allowable static potentials to said anodes and screen grids to obtain relatively high power'output capacities with low distortion, said network comprising a high impedance circuit between the anode supply and screen grids for reducing the potential on the screen grids in response to signal swings on the control grids to prevent damage to said screen grids and to maintain the electron beamsk in focus.

15. The combination set forth in claim 14 wherein a self-biasing resistor is provided traversed by substantially constant cathode current in consequence of the values of the circuit constants of said voltage supply network.

16. 'I'he combination set forth in claim 14 wherein said voltage supply network includes a controlling capacitor connected with said screen grids and effective ground.

17. In an amplifier employing a multiple electrode tube having a. cathode, control grid, an anode, and a screen grid operating at higher than otherwise allowable potentials subject to damage when the control grid swings positive in response to the application thereto of signals, the method of preventing damage to the screen grid which comprises decreasing the potential on the screen grigi in response to signal swings on the control gr 18. In an amplifier employing a. multiple electrode tube having a cathode, control grid, an anode, and a screen grid operating at higher than otherwise allowable potentials subject to damage when the control grid swings positive in response to the application thereto of signals of high energy level, the method of preventing damage to the screen grid which comprises floating the potential of the screen grid as the signal energy varies to always maintain the screen wattage dissipation at a safe operating value when high e1ec' trode potentials are employed to deliver relatively high signal outputs.

19. In an amplifier employing a beam power tube having a cathode, control grid, an anode, and a screen grid operating at higher than otherwise allowable potentials subject to damage when the control grid swings positive in response to the application thereto of signals of high energy level, the method of preventing damage to the screen grid which comprises decreasing the potential on the screen grid in response to signal swings on the control grid to maintain the beam in focus and to maintain the screen wattage dissipation at a safe operating value.

20. In an audio frequency amplifier, an ampli-- fier tube comprising a cathode, an anode and a control electrode' and having an input circuit and an output circuit connected therebetween, an inverse feedback circuit connected between said output and input circuits including a potentiometer, a frequency discriminating path for the treble frequencies connected to one end of the potentiometer and another frequency discriminating-path forxthe bass frequencies connected -to the opposite end of said potentiometer, and an versefeedback circuit connected between said output and input circuits including a potentiometer, a frequency discriminating path for the treble frequencies connected to one end of the potentiometer and another frequency discriminating path for the bass frequencies connected to the opposite end of said potentiometer, a connection from an intermediate point of said poteutiometer at substantially ground potential to said control electrode, an adjustable connection from said control electrode to said potentiometer to selectively transmit the bass or treble frequencies fed to the opposite ends of said potentiometer to the input circuit of said said amplier tube in dependence upon the relative location of said adjustable connection with respect to said intermediate point whereby a tone control with minimum change in volume at voice frequencies is attained.

23. The method of controlling the output tone of an audio frequency amplifier having an input circuit and output circuit and an inverse feedback arrangement therebetween including a pOtentiometer and a filter network connected thereto, which comprises feeding the treble frequency waves into one end of the potentiometer and the bass frequency waves into the other end of the potentiometer, and selectively tapping a. portion of the potentiometer for connection to said input circuit to raise and lower the bass on one side of the potentiometer and to raise and lower the treble on the other side of the potentiometer with the same control instrumentality.

24. The method of controlling the output tone of an audio frequency amplifier having an input circuit and output circuit and an inverse feedback arrangement therebetween including a potentiometer and a filter network connected thereto, which comprises feeding the treble frequency waves into one end of the potentiometer and the bass frequency waves into the other end of the potentiometer, and selectively tapping portion of the potentiometer for connection to said input circuit to raise and lower the base on one side of the potentiometer and to raise and lower the treble on the other side of the potentiometer with the same control instrumentality, and to maintain the intermediate range of the voice frequencies substantially constant throughout the range without substantially affecting the output volume at voice frequencies.

* ALLEN A. SHOUP.