US2473265A - Audio-frequency amplifier - Google Patents

Description

June 1949- 'E. WATKINSON EI'AL 2,473,265

AUDIO-FREQfiENCY AMPLIFIER Filed Dec. 4, 1946 300 VOLT 500 \vOLT ANDRE WILLEM .STORM CHARLES ORDON SMITH &

ERIC WATKINSQN INVENTOR6 Patented June 14, 1949 AUDIO-FREQUENCY AMPLIFIER Eric Watkinson, Hinders Park, South Australia, Australia, Andre Willem Storm, Eindhoven, Netherlands, and Charles Gordon Smith, Sydney, New South Wales, Australia, assignors to Hartford National Bank & Trust Company, Hartford, Conn., as trustee Application December 4, 1946, Serial No. 714,074

In Australia August 28, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires August 28, 1965 6 Claims.

The present invention relates to audio frequency amplifiers.

It is well known that, with class B and class AB audio frequency amplifiers, the current re- 2 nected to ground via the two resistors RI and R2, which form part of the resistive network including te resistors RI, R2, R3 and R4. These resistors are each by-passed by a condenser of quirements of the output tubes vary over a con- 5 suitable capacity.

siderable range. To enable an amplifier of this The voltage for the anodes of the 807 tubes type to function at maximum efiiciency, it has is obtained from the positive termin l of th hitherto been necessary to operate the amplifier 500 volt supply while the screen electrodes of from a direct current source having very good the 807 tubes and the anode of the 6V6 tube are regulation and/or to make use of a stabilizing 10 fed from the positive terminal of the 300 volt tube or tubes, the current consumption of which varies inversely with respect to the current consumption of the amplifier.

It is the primary object of the present invention to provide a class B or class AB amplifier which requires a power supply having only moderately good regulation. This object may be accomplished in a preferred form of the invention by utilizing one or more of the tubes in the initial stages of the amplifier as stabilizing tubes in addition to their normal use as voltage amplifying or driving tubes.

The invention will now be described with the aid of the accompanying drawings which gives the circuit diagram of an amplifier comprising two tetrode output tubes, for example, of the 807 type driven by a driver tube, which may be any grid-controlled tube but which is preferably of the 6V6 type.

In this amplifier the plate and the screen grid of the 6V6 tube are connected together so that it operates as a triode. The tube is transformer coupled to, and drives the grids of the 807 tubes which operate in a push pull circuit in a well known manner.

The voltages for the electrodes of the tubes are preferably obtained from two power supplies, one of the order of 500 volts and the other of the order of 300 volts. These power supplies may be derived from the alternating current mains by means of rectifiers, which, by reason of the improvement resulting from the use of the in vention, do not require a high degree of regu lation, nor does the current supplied thereby need to be filtered to any great extent.

The high voltage windings connected to the rectifiers may be wound on the same core and may be energized by a single primary winding. A filter consisting only of condensers or of condensers and series resistors will be found to be satisfactory for smoothing purposes.

In the embodiment of the invention shown in the drawing, the negative terminal of the 500 volt supply is connected to ground while the negative terminal of the 300 volt supply is con supply. The cathodes of all the tubes are connected to ground.

The negative bias voltage for the 807 tubes comprises the voltage drop across the resistor RI, while the negative bias voltage for the 6V6 tube is obtained from a voltage divider formed by the two resistors R3 and R4 connected in series between the negative terminal of the 300 volt supply and the positive terminal of the 500 volt supply. The 6V6 tube is adapted to function as a control tube as hereinafter described and the total negative bias voltage applied to this tube is the resultant of the voltage across the resistors RI, R2 and R3; that produced across the resistor R3 being opposite in direction to the voltages across the resistors RI and R2.

Referring to the drawing, it is seen that there are two direct voltage circuits having resistors R1 and R2 in common. The first of these circuits can be traced from the positive terminal of the 300-volt power supply through the internal resistance of the 6V6 tube in parallel with the internal screen-to-cathode resistances of the 807 tubes, through R1, through R2, and back to the negative terminal of the 300-volt power supply. The point between resistor R1 and the internal resistance of the tube is at ground potential. The second of these circuits can be traced from the positive terminal of the 500- volt power supply, through R4, through R3, through R2, through R1, and back to the negative terminal of the 500-volt power supply, which is at ground potential. Hence the currents flowing through R1 and R2 can be seen to oppose each other and the resultant voltages developed across these resistors depends on the difference in the current flowing. In the preferred embodiment of the invention resistors R1 and R2 are traversed by the full load current of the 300-volt supply, and by only a current of low value derived from the 500-volt power supply. In this fashion, a negative voltage is derived across resistor R1 and is applied to the grids of" the 807 tubes as bias. Similarly, a

negative voltage is derived across resistors R1 and R2 and; a positive voltage, which; is some: what less than the voltage derived across re-, sistors R1 and R2, is derived across resistor R3, the resultant voltage being negative and being applied to the grid of the 6V6 tube When a signal is applied to the amplifier the plate current of each of the 89;?- tubes,"increases thus causing a decrease in thevoltage-of the 50. volt supply which will cause a decrease in that portion of the bias voltage applied to the-6N6 tube derived from the resistor R3. As this portion of the bias voltage is positive, the resultant negative bias voltage applied-to the 6V6 tube, will increase thereby causing a decrease/in theplate current of this tube. This. current flows from the cathode of the tube to the negative ter. minal of the 300 volt supply, via resistors RI and R2, and therefore the bias voltage applied;to the.

grids of the 807 tubes is decreased. At the same time, thevoltage applied to the screen electrodes.

in the screengrid voltage of the. 807 tubes thus tending to maintain theoptim'um power output of the tubes.

Thus a decrease in the 50.0,volt supply, due to increased plate current with v signal, is I compensated by a decrease in the negative grid. bias voltage and an increase in the screen. grid voltage.

of the 807 tubes.

The. degree of compensation can bevaried if so desired by varying the value of theresistorB3.

In the operationof theamplifier, the flow. of, grid current in the grid circuits of the. 807 tubes causesan increase in the, voltage. drop across the resistor .RI, and an increasein. thernegative bias. voltage applied to the.,80'l tubes. Theincreas ed. voltagedrop across the resistor RI also increases the negative bias voltage applied to the .6 V6..tube

thus reducing the plate current flowin through;

this .tu. be. As the. plate current of .the; 6V6 .tube. fiows through theresistor RI -the,r,educed plate.

current of the (W6 tends to maintain. the voltage drop across the resistor RI atits original value; The negative biasvoltage applied tot he-80l tubes is thus, substantially unaffected by the flow of grid. current.

Likewise, if thescreengrid current of the 80T tubes increases, theincreased current. flows.- through the resistorsRI and-R2, thereby in-v creasing. the negative, grid. bias voltage applied to the 6V6 tube and decreasing .thesplate current flowing, through this I tube. Since the plate cu rrent .of the 6V6 tube also flows through the resistors RI andRZ the reduction-in thiscurrent offsets the increased screen grid current, thereby-- maintaining a substantially constantvoltagedrop acrossthe resistors RI and R2; Since thescreengrid voltage for the-807 tubescomprises the. dif-. ference between the 300 volt supplyavoltageand. the voltage developedacross. the resistors Rtand- R2, it will be maintained substantiallyconstant. Consequently, thescreen grid voltage of the out-. put tubes is substantially unaffected by variations in the screen grid current.

The decrease in th positivebomponent of the negative grid bias voltage of theGVfi tube when.

the .500; volt supply decreases. is partly '4 the decrease in voltage across the resistors R2 andRI whentha plate current of the 6V6 tube decreases. The decrease in the positive com ponent is also partially offset by the increase in the-screen grid current of the 807 tubes when a signal is applied to the control grids of these tubes. The effect-is degenerative but it cannot completelyoffset the increase in the negative bias voltage app1ied=to the 6V6 tube following upon the decrease in the positive component of the bias voltage resulting from the decrease in the voltage of the-500 vo lt supply.

In a similar manner, any changes in the voltagelo ftheBflO volt supply from which the potential fonthescreen grid electrodes of the 807 tubes is takenwill produce a counter change in the bias voltageapplied to the 807 tubes.

In practice, changes of voltage in the power sppplie s.-. are .mainly caused by variations in the current consumption of the tubes following upon changes in the level of the signal being amplified. The invention can substantial} overcome these variations in the. power supply voltages. which would otherwise limit the optimum output of the tubes.

It is. evident,that the invention may alsobe applied to modulating systems in which class B modulators are utilized, thus reducing the degree of regulation required in the power supplies for the modulator, and such applications are. intended tobe Within the scope of the invention. Having, now described our invention what we claim as new and desire to secure by Letters Patent is:

1. An audio-frequency amplifier comprisinga driverstage having a first electron discharge tube.

having an anode, a control grid and a cathode, an output stage coupled to said. driver stage and havinga.se c-ond electron discharge tube having. an anode, acontrol grid and a cathode, a first direct. pQtfintial supply, a second direct poten- 75. electron discharge tube in series with a portion tial supply, a resistance network connected across saidsecondpotential supply, means to apply the output, potential of said first. potential supply across thegridand cathode of said first electron discharge tube, in series with a portion of said resistance network, means to apply the output potentialof said second potential supply across the anode and cathode of said-second electron discharge, tube, means .to connect said resistance network tothe grid of said first electron dischargetube, and means to connect said resistance network to the grid of said second electron discharge tube thereby to apply negative bias voltages to the grids of said first and said second electron dischargetubes. at values and in series, at which achange in the potential of either of said potential supplies will vary the bias voltage.,applied-to the grid of said first electron discharge tube-to vary-the anode current thereof and vary. the grid bias of said second electron discharge tube to maintain the power output of said amplifier substantially constant.

2. An audio-frequency amplifier comprising a driver stage having a first electron discharge tubehaving'an anode, acontrol grid and a cathode an output stage coupled to said driver stage and havinga second-electron discharge tube having an anode, a control gridand a cathode, a firstdir ect.potentialsupply, a second direct potential supply, a resistance network connected across said second potential supply, means to applythe-output potential of said first potential supply-across. theanode and cathode of said first tential of said second potential supply across the anode and cathode of said second electron discharge tube, means to connect said resistance network to the grid of said first electron discharge tube, and means to connect said resistance network to the grid of said second electron discharge tube to apply negative bias voltagesto I the grids of said first and said second electron discharge tubes at values and in senses at which a change in the potential of said potential supplies due to flow of grid current in said second electron discharge tube will vary the bias voltage applied to the grid of said second electron discharge tube to maintain the optimum power output of said amplifier substantially constant.

3. An audio-frequency amplifier comprising a driver stage having a first electron discharge tube having an anode, a control grid and a cathode, an output stage coupled to said driver stage and having a second electron discharge tube having an anode, a screen grid, a control grid and a cathode, a first direct potential supply, a second direct potential supply, a resistance network connected across said second potential supply, means to apply the output potential of said first potential supply across the anode and cathode of said first electron discharge tube and across the screen and cathode of said second electron discharge tube in series with a portion of said resistance network, means to apply the output potential of said second potential supply across the anode and cathode of said second electron discharge tube, means to connect said resistance network to the grid of said first electron discharge tube, and means to connect said resistance network to the grid of said second electron discharge tube to apply negative bias voltages to the grids of said first and said second electron discharge tubes at values and in senses at which a change in the output potential of either of said potential supplies will vary the bias voltage applied to the grid of said first electron discharge tube to vary the anode current thereof and vary the grid bias and the screen grid voltage of said second electron discharge tube to maintain the power output of said amplifier substantially constant.

4. An audio-frequency amplifier comprising a driver stage having a first electron discharge tube having an anode, a control grid and a cathode, an output stage coupled to said driver stage and having a second electron discharge tube having an anode, a screen grid, a control grid and a cathode, a first direct potential supply, a second direct potential supply, a resistance network connected across said second potential supply, means to apply the output potential of said first potential supply across the anode and cathode of said first electron discharge tube and across the screen and cathode of said second electron discharge tube in series with a portion of said resistance network, means to apply the output potential of said second potential supply across the anode and cathode of said second electron discharge tube, means to connect said resistance network to the grid of said first electron discharge tube, and means to connect said resistance net-- work to the grid of said second electron discharge tube to apply negative bias voltages to the grids of said first and said second electron discharge tubes at values and in senses at which a change in the potential from said potential supplies due to changes in screen grid current of said second electron discharge tube will vary the bias voltage applied to the grid of said first electron discharge tube to vary the anode current thereof and vary the grid bias and the screen grid-voltage of said second electron discharge tube to maintain the optimum power output of said amplifier substantially constant.

5. An audio-frequency amplifier comprising a driver stage having a first electron discharge tube having an anode, a control grid and a cathode,- an output stage coupled to said driver stage 'a'ndhaving a second electron discharge tube havin an anode, a screen grid, a control grid and a cathode, a'first direct'potential supply, a second direct potential supply, a resistor network comprising a first, a second, a third and a fourth resistor connected in series across said second potential supply said first resistor being connected to'the negative terminal of said second potential supply, means to couple the positive terminal of said first potential supply to the anode of said first electron discharge tube and to the screen grid of said second electron discharge tube, means to connect the negative terminal of said first potential supply to the junction of said second and said third resistors, means to apply the output potential of said second potential supply across the anode and cathode of said second electron discharge tube, means to connect the junction of said third and said fourth resistors to the grid of said first electron discharge tube, and means to connect the junction of said first and said second resistors to the grid of said second electron discharge tube, and means connecting the cathodes of said electron discharge tubes to the negative terminal of said second potential supply, the resistors of said network having resistance values at which a change in the potential from said potential supplies will vary the bias voltage applied to the grid of said first electron discharge tube to vary the anode current thereof and vary the grid bias and the screen grid voltage of said second electron discharge tube to maintain the power output of said amplifier substantially constant.

6. An audio-frequency amplifier comprising a driver stage having a first electron discharge tube having an anode, a control grid and a cathode, a push-pull output stage having a second and a third electron discharge tube, said second and said third electron discharge tubes each having an anode, a screen grid, a control grid and a cathode, a first direct potential supply, a second direct potential supply, a resistor network comprising a first, a second, a third and a fourth resistor connected in series across said second potential supply said first resistor being connected to the negative terminal of said second potential supply, a transformer having a primary and a center-tapped secondary winding, means connecting one end of the primary winding of said transformer to the anode of said first elec tron discharge tube, means connecting the positive terminal of said first potential supply to the other end of the primary winding of said transformer and to the screen grids of said second and said third electron discharge tubes, means to connect the negative terminal of said first potential supply to the junction of said second and said third resistors,. means to couple the junction of said third and said fourth resistors to the grid of said first electron discharge tube, and means to connect the junction of said first and said second resistors to the center tap of the secondary winding of said transformer, means to connect the ends of said secondary winding to the grids of said second and said third electron

Images