US2098405A - Dynamic amplifier - Google Patents

Description

1937. c. M.-S|NNETT DYNAMIC AMPLIFIER Filed Sept. 50, 1935 HE 007F071 Patented Nov. 9, 1937 UNITED STATES PATENT OFFICE DYNAlVIIC' AMPLIFIER Chester M. Sinnett, Westmont, N. J assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1935, Serial No. 42,982 12 Claims. (01. 178-44) The present invention relates to dynamic elec- Direct current, grid biasing, screen grid and antric signal amplifiers for use in audio frequency ode operating potentials are derived from a suitcircuits and the like to expand the volume range able source (not shown) having positive and or amplitude of signals transmitted therethrough. negative output leads represented by the positive It is an object of the present invention to prosupply lead 23 and the negative ground lead 25. vide an improved system for volume range ex- The positive supply lead 23 isconnected to the pansion of electrically amplified orchestra and anode ll through a filter resistor 21 provided other music, with pleasing effect, and without with a suitable bypass condenser 29 and an anode distortion from normal radio broadcast programs output resistor 3!.

and phonograph records as normally recorded. The cathode l is operated above ground or 10 It is a further object of the present invention negative B potential by connection through a to provide a volume range expansion system lead 33 with a tap point 35 on a bleeder resistor which includes an improved electric discharge comprising sections 31, 39, 4| and 43. The bleedmixing or volume control device having a simplier resistor is connected through a lead 45 between fiecl control circuit in connection therewith. the positive supply lead 23 and ground indicated 15 It is a still further object of the present inat 41. The control grid 9 is connected through vention to provide an improved volume range exa grid resistor 49 and a lead 5! with a more pansion system for audio frequency signals negative tap point 53 on the bleeder resistor, which permits the signal level and the degree of whereby the potential drop in the section 4! of 00 expansion to be adjusted independently. It has the bleeder resistor is applied between cathode been found that volume range expansion may be and signal grid to provide a fixed negative biaseffected without appreciable signal distortion, ing potential on the latter. The sections M and through the medium of a variable gain amplifier 43 are bypassed to ground through a suitable having separate grid electrodes for signal amplibypass capacitor 55 and the section 43 is further 5 fication and gain control as a mixer device, for bypassed by a capacitor 51. The screen grid 5 the rectified and audio frequency components of l'I-I9 is connected through a lead 59 with a the controlling and amplified signal. more positive tap point 6! than the cathode The invention will, however, be better undertap 35. stood from the following description when con- As an audio frequency amplifier, signals are apsidered in connection with the accompanying plied across the grid resistor 49 and are amplified 30 drawing and its scope will be pointed out in the in connection with the signal or first control grid appended claims. 9 and the output anode H, from which the am- In the drawing, Figure 1 is a schematic circuit plified signals are derived across the output rediagram of a variable gain amplifier and variable sistor 3| and may be applied to any suitable range volume expander system embodying the output circuit, represented by the output lead 35 invention, and 85, through a suitable coupling capacitor fill.

Fig. 2 is a similar schematic diagram showing The audio frequency signals to be amplified a modification of the circuit of Fig. 1. may be derived from any suitable source such as Referring to Fig. 1, an audio frequency elecan audio frequency detector of a radio receiver or 40 tric discharge amplifier device 5 is provided with from a phonograph pickup device, (not shown) 40 an equi-potential cathode 'I, a control grid 9 adand may be applied to the amplifier device 5 jacent thereto, an output anode II, a suppressor through a suitable coupling device such as an grid l3 adjacent to the anode, and a second audio frequency coupling transformer 69. In control grid 15 provided with a screen structure the present example, the secondary 1| of the comprising two grid elements l1 and I9 intercontransformer is provided with output leads l3 and 45 nected to effectively shield the grid l5 from the 15, the latter being grounded as indicated at H. first control grid and from the anode and sup- The leads 13 and 15 may be taken to represent pressor grid. Both the control or signal grid 9 any suitable audio frequency signal channel, the and the control grid l5 are interposed in spaced signals through which are to be amplified and relation to each other in the electronic stream expanded in volume range. 50 or space path of the tube between the cathode The signal grid 9 of the amplifier is coupled and the anode. The cathode is provided with a to the audio frequency circuit 13-15 through heater l2. a coupling capacitor 19 and a volume control The grid structures l1 and 19 may be considered potentiometer 8| connected to the leads 13 and as a screen grid for the second control grid l5. 15 in shunt across the secondary II. The mov- 55 able volume control tap 83 of the potentiometer is connected 7 with the coupling capacitor 19. The audio frequency circuit return to the cathode is effected through the bypass capacitor 55 and the cathode lead 33. The latter capacitor,

therefore, is preferably of relatively high capac- It will be notedthat the screen grid lead 59 is also provided with bypass capacitor to ground as indicated'at 85 thereby to provide a steady screen potential and in conjunction withthe resistor sections 31 and 39 to provide an effective filter for the screen structure. 1

By means of the volume control device 8I-83, the amplitude of the signal potential applied to the amplifier may be controlled independently of other controls preceding it and directly at the amplifier, as means for establishing a desired initial signal amplitude or volume. In any case, the volume control device for the amplifier 5 is interposed between the control grid 9 and the signal source, which, in this case, is the secondary of the transformer 69 and the circuit leads 13 and 15. a w

The device 5 is a variable gain amplifier and is of the mixer type wherein two control grids are utilized in the electronic stream and are effectively shielded from each other to reduce coupling to a minimum. The present amplifier being of the pentode type provides a relatively high gain and the gain is directly controllable by the application of a direct current. biasing potential to the second control grid I5. In the construction of the tube, the control or signal grid 9 is the inner grid, while the second control grid I5 is the outer grid; The tube is enclosed in a. metallic casing which is suitably grounded'as indicated at 81. This tube is preferably of the type now being placed on the commercial market and known as the RCA 6L7 type or pentagrimixer amplifiern While this tube is normally adapted for mixing two separate signals by electronic coupling, it has been found that it maybe utilized to advantage in a volume expander amplifier circuit by placing the signals to be amplified and controlled on the inner or signal grid 9 operated at a suitable fixed bias, for minimum distortion as a normal amplifier, and controlling the amplification wholly by varying the bias on the outer control grid between upper and lower negative values with respect to cathode to provide an amplification factor between unit and a maximum value. In the case of the tube mentioned, this may be as high as I3 in the circuit shown. For volume range expansion, the outer control grid bias is increased to a negative value such that unity amplification is obtained and negative bias is reduced in signal amplitude to provide increasing gain with increased signal amplitude.

The volume control means 8I-83 is preferably provided with tone compensating means such as the series circuits indicated at 89 and 9| in connection with spaced taps93, for attenuating signals in certain frequency ranges with reduced volume adjustment. As such tone compensating means m be employed in-any suitable manner and are well known, -further,de cription is believed to be unnecessary.

' I29, an output anode accordance with increased nected through a lead The outer control grid I5 is bypassed directly to cathode through a filter capacitor 95, and biasing potential therefor is supplied through a series filter resistor 91 and a bias supply lead indicated at 99. The lead 99 may be connected through a switch arm I OI with a contact I03 and a lead I05 connected with the tap point 53 on the bleeder resistor whereby the same fixed negative biasing potential may be'applied thereto as is applied to the signal grid 9 as derived from the resistor section 4i. tion, the amplifier operates in any normal manner to amplify signals without volume range expansion. For the tube shown, this may be approximately 10 volts negative bias, for normal operation of the device as an amplifier.

The switch IN is provided with a second contact I91 for connecting the control grid I5 further in series with a resistor I09 across which are developed direct current potentials variable with signal amplitude, and in series with a third resistor III across which a direct current potential is provided which may be selected by an adjustable potentiometer tap H3. The resistor IE9 is provided with positive and negative terminals II5 and H1 respectively, the positive terminal being connected with the switch point I91 through a connection lead H9 and the negative terminal being connected with the movable tap 5 I3 through a connection lead I2I.

The resistor II I is connected at one end with the cathode lead 33 and the terminal 35 while the opposite end is grounded-thereby providing a connection through it to the cathode and impressing across it the potential existing between the terminal 35 and ground in the bleeder resister.

7 With the switch IOI moved to the contact I 91 as shown, the gain or outer control grid I5 is connected in series through the filter resistor 91,

thence through the leads 99 and H9, through the resistor I09 and'the lead IN to the movable contact H3 and through the portion included between the contact II 3 and the positive termi nal thereof to the cathode through the lead 33. There are, therefore, in series between the outer control grid and the cathode, va filter resistor and. two sources of biasing potential, one of which is adjustable and'the other of which is variable in accordance with signal variations.

The latter source of. biasing potentials is provided by a rectifier device I23 connected to the signal source or signal circuit13-15 through a buffer amplifier device I25. The amplifier device I25 comprises a cathode I21, a control grid I3I and a suppressor grid I33 connected with the anode. The device is preferably of the metallic shielded type, the metallic shielding being connected to ground as indicated at I35 and the cathode is provided. with a suitable heater indicated at I31.

The rectifier device may be of any suitable type but is preferably of the diode type and may be provided by an electric discharge device having a cathode I39 provided with a heater I, a

grid electrode I43, an anode I45 and a suppressor grid I41, the latter being connected with the anode.

V In the present example, the cathode is connected with the positive terminal II5 of the resistor I99 and the control grid I43 is connected through a grid resistor I49 with the negative terminal II1 of said resistor.

The suppressor grid and anode are also con- I5I to the cathode and the terminal 5. This'device is also preferably of the metal shielded type, the metal shield being grounded as indicated at I53. Both the devices I23 and I25 may be provided by a metal tube now appearing on the commercial market and known as the RCA 6C5.

The device I23 is utilized as a rectifier with the grid I43 as the anode, and is coupled to the amplifier device I25 through a coupling capacito-r I55 and an anode output resistor I5I for the latter. The anode I3I and the suppressor grid I33 of the amplifier are utilized as a combined anode electrode as shown. The resistor I5! is the positive supply source 23 through a supply lead I59. With this arrangement, the rectifier device I23 is resistance coupled to the amplifier device I25 across the resistor I49 as the input resistor between the diode anode electrode I43 and the cathode electrode I39. a

The resistor I09 is the diode rectifier outpu impedance and the direct current component of the signal, rectified by the diode, is provided at the terminals H5 and N1. the polarity of the terminals will be asindicated, thereby introducing into the series circuit between the control grid I5 and the cathode I of the amplifier mixer tube 5, a potential opposing that established by the fixed source I I I. A signal serves therefore, to reduce the negative bias initially established on the grid I5 and to cause the amplification of the device 5 to increase with increase in signal strength thereby to provide volume range expansion, the degree of which .depends upon the amount of signal admitted to the rectifier.

The amount of signal admitted to the rectifier or the degree of expansion is controlled independently of the volume control device 8IB3 for the signal amplifier 5, by a potentiometer device comprising a resistor element I6I havinga movable contact I63. The input potentiometer is grounded at one end as indicated at I65 and connected through a coupling capacitor IB'I at the opposite end with the high side of the audio frequency circuit represented by the lead I3. It is, therefore, connected substantially in parallel with the secondary II of the transformer 69 and the movable contact I63 is connected through a lead I69 with the control grid I29 of the buffer amplifier I25.

The cathode of the amplifier is provided with a series bias resistor I'II having a bypass capacitor I13 toground I75. Since the grid I29 is connected conductively to the potentiometer device I6I, the self. biasing potential is applied to the grid I29.

Operation of the contact I63 serves to increase the signal applied to the grid I29 as the contact I63 is moved in an upward direction as viewed in the drawing. The amplified signals received on the grid I29 are amplified and applied to the rectifier I23 for controlling the amplifier mixer device 5 as hereinbefore described.

The contact I53 may be connected with the switch I in such a manner that, as the contact I63 is moved to the minimum input position adjacent to the ground end of the resistor IIiI, the switch arm is moved tothe contact I03 thereby placing the amplifier device under normal operation without volume range expansion. This connection for operating the switch is indicated by the dotted connection I".

From the foregoing description, it will be seen that a variable gain amplifier tube 5 is provided with electrode means whereby it may operate It will be seen thatstage may be increased with decrease in bias potential, which is initially adjusted by means of the contact H3 for substantially unit amplification.

A portion of. the input signal from the audio frequency circuit 'I3'I5 is taken therefrom preceding the audio frequency volume control means and is supplied to a volume amplifier I25 which servesto isolate the load of the rectifier device I 23 from the audio frequency transmission circuit, thereby minimizing distortion of the signal.

by operation of the rectifier. The rectified voltage derived from the rectifier device across the rectifier or diode output resistor I09 is applied, through a filter system comprising the resistor 91 and the bypass capacitor 95, to the outer or second control grid I5.

As the input signal is increased, this rectified potential increases in opposition to the potential initially established on the grid I5 and increases the gain on the amplifier device 5 proportionately. Likewise, as the signal strength decreases, the rectified voltage decreases, thereby increasing the bias potential on the control grid I5 and permitting the gain to decrease. In this manner additional dynamic volume control is obtained over that normally present when the amplifier device operates independently with the switch arm IOI in the position I03.

For producing a pleasing effect in musical reproduction With volume range expansion partic-- ularly in the production of orchestra music both from phonograph records and radio signals, it is desirable that the gain of the amplifier mixer device 5 having a more rapid build-up or increase of amplification and a slower decay or reduction of amplification, and this is provided by the circuit in connection with the control grid" I5 as follows: with increasing signal strength,-' the volume provided by the rectifier output resistor tends to change the bias upon the grid I5 through the resistor 9! causing the capacitor 95 to be charged at a predetermined rate. With the capacitor 95 of .5 microfarad and a resistor 9'! of approximately 500,000 ohms, the build-up time is substantially .2 second and has been found to be a preferred time lag in response for most pleasing reproduction of music, such 'as orchestra music.

When the signals are decreasing in volume and the voltage across the output resistor I09 is decreasing, the capacitor 95 discharges not only through the resistor 9'! but also through the resistor I09, the two being in series for this condition of operation and the decay time is, therefore, increased and in the present example to substantially .5 second, with a resistor I09 of substantially 200,000 ohms shunted by a stabilizing .5 microfarad. By adjusting the values of the resistors 91 and I99, and the capacitor 95, the time of build-up and decay may be adjusted for any preferred volume expansion effect."

For normal operation of the system described,

with a fixed potential of substantially volts on the inner of signal control grid 9, and the contact H3 adjusted to apply a negative potential to the other or gain control grid I5 of approximately 18 volts to provide substantially,

unity amplification, the contact 163 is then adjusted 'to'pro'vide a degree of volume expansion such th'at with the maximum signal potential tOJYbG sreceived through the circuit l3 '|5, the resulting bias' potential on the gain control grid may "be that necessary for'maximum' amplification or'gain. For example, zero bias potentialonthe grid |5 may produce maximumgain, in 'which case the rectified signal potential across the-resistor |'09.is substantially equal to and'the adjusted potential provided by the source I. Lesser'degreesof expansion may be obtained by adjustment of the contact 163 of the expansion control device |6| to correspond to a lower signal'ipotential. V r

A system,- in accordance with the invention, has the advantage that the signal channel is not loaded-or otherwise affected by the signal rectifier and'g'ain control means to cause signal distortion, "since the rectifier device is isolated from the signal channel by a buffer amplifier of the electric discharge type, and the gain control means comprises a control grid inserted in the electronic stream of the signal amplifier independently of the signal control grid and effectively'shielded or screened therefrom and from the 'output anode. Furthermore, both the signal volume and the degree of expansion are independently adjustable whereby the .degree of expansion may be maintained for various signal levels-established by the volume control means and the same range of expansion may be obt'ainedfor low volume adjustments as for high volume adjustments for the signal volume control device.

However, incertain systems it may be desirable to operate the control system from the same volume control device as the signal amplifier. For'this'purposea switch 64 is provided to'break the connection for the grid I29 with the contact 1-63 of the independent expansion control device and toprovi'de a connection through a contact I66 and a connection lead I68 with the volume control contact 83. Variationof the volume control means, for a lower volume, for example,

provides a correspondingly lesser degree of 'expansion. Likewise the expansion control is increasingly more eifective as the volume is increased.

I'he seriescircuit for the gain control grid of the signal amplifier is particularly advantageous since the filter means for time delay operation is placed adjacent to the control grid and serially in circuit with the output resistorof the controlling rectifier andthe fixed source of biasing potential for adjusting the initial amplification. This permits a more rapid increase in gain with a slower-decrease in gain in response toichanges in signal strength or average amplitude of the applied audio frequency signals, as is desirable for sound reproduction with volume range expansion from normally recorded and normally broadcast signals without the addition of complicated circuit means.

The control channel, including the buifer amplifier, :is uniformly responsive throughout'the audio frequency range and provides substantially linear amplification for the rectified signals.

The circuit has the further advantage that compensated volume control means may be provided in connection with the signal amplifier independently of the volume expansion means and the latter is controlled entirely by the voltage amplitude of the signals applied to the buffer amplifier, no additional controlling signal being required, "and substantially no load being-placed upon the "audio frequency. circuit thereby, since the audio frequency signal circuit 'actuates only the signal and bufier voltage amplifiers.

. The. bufler. amplifier and signal rectifier for the control circuit may be combined if desired in one'device as shown in thecircuit modification of Fig. 2 to which 'attention'is now directed and in whichthe same reference numerals are used as in Fig. 1 for the same circuit elements. The volume expander amplifier 5 is coupled as in'the circuit'of Fig. 1 to a signal input circuit and Jvolume control through the coupling capacitor 19 and is provided with bias supply means through the griddeak resistor 49 while the output circuit includes the output coupling im pedance 3| and the coupling capacitor 61.

The gain control grid I5 is provided with the filter capacitor 95 andseries filter resistor 91 and the cathode 1 is provided with a variable self-bias resistor 2|! also provided with a bypassicapacitor 2| 9. The circuitfrom the cathode 1 through thevariable self-bias resistor 2|! is completed through av ground connection 2|8, asecondgroundiconnection 2|5, through a diode 'outputresistor 208 and thence to the resistor 91 through a lead 205. The initial biason the gain control grid |5is thereby adjusted by the resistor 2 l1 and the resistor 209 is also variable to provide a desired opposing potential in response to signals to cause the gain of the amplifier'5 to increase with increasing signal strength. The'resistor-209 is suitably bypassed by a capacitorZ l3 and it will be seen that for increasing signal strength, the resistor 91 causes the capacitor 95 to charge'at apredetermined rate and that, upon decrease of signal strength, the resistors 91 and 209 are in seriesto cause a longer timedelay for response of the amplifier to decreasing signal strength. a In the present example, the diode rectifier for supplying signals to the output resistor 209 is providedin an-electric discharge amplifier device I80 having a control grid |'83, an output anode I85, a cathode |8'|, a heater. therefor I89, and a pair of diode anodes '|9| associated with the cathode l8l;

Signals are applied to'the control grid I83 from the signal supply lead I69 and are amplified in the amplifier I80. The amplified signals are app'li'edto'thediodecomprising the anodes |9| and the cathode |8'| through an output transformer |93 connected with the amplifier output circuit I95 in which is included a coupling capacitor I91 and coupling resistor I99. The output circuit includes the primary winding 20| of the transformer I93 and the secondary 201 is connected between the diode anodes |9| and the cathode I81 through the'output resistor 209. The diode circuit comprises the leads 2 and 205. The control of the variable gain amplifier through the means shown is otherwise the same as described in connection with Fig. '1 and, accordingly, further description is beliveed to be unnecessary.

I claim as my invention:

1. In a volume expander system for audio frequency signals, an electric discharge amplifier device having two independent control grids in the space path thereof, means for applying audio frequency signals to be amplified to one of said control grids, volume control means for said signals interposed in circuit between said last named means and said'one of said control grids,

a signal rectifier device having an output resistor, 7

means providing a series bias supply connection for the second control grid through said output resistor, means including a series resistor and a by-pass capacitor between the second control grid and the cathode providing a filter in said connection, a buffer amplifier including an electric discharge amplifier device coupled to said rectifier, means for applying audio frequency signals to said rectifier through said amplifier, and means for controlling the amplitude of said last named signals independently of saidfirst named volume control means.

:2. In a volume range expander system for audio frequency signals, an electric discharge arn-, plifier device having a cathode, and two independent control grids in the space path thereof, means for 'applyingaudio frequency signals to be amplified to one of said control grids, volume. control means for said signals interposed in circuit between said last named means and said one of said control grids, a signal rectifier device having an output resistor, means providing a series bias supply connection for the second control grid through said output resistor, a by-pass capacitor for said second control grid directly connected therewith and a series filter impedance in said bias supply connection between the second control grid and the cathode providing a filter in said connection, said resistors being in series for discharge of said capacitor and said output resistor providing a potential source for charging said capacitor, whereby a predetermined difiering delay in response of the gain control through said amplifier device is provided for increasing and decreasing signal volume level.

3. A'dynamic audio frequency amplifier comprising an electric discharge signal amplifier having a signal grid and a separate gain control grid in the space path thereof, said last named grid being provided with a screen, a signal rectifier for applying a signal variable biasing potential to said gain control grid, means for connecting said rectifier to the signal channel of said amplifier including a buffer amplifier stage, independent volume control means for said buffer amplifier and said first named amplifier, and time delay filter means providing a connection between said rectifier and said gain control grid.

4. A volume expander system comprising in combination, an electric discharge amplifier device having a pair of control grids, means for applying signals to be amplified to one of said grids more adjacent to the cathode, means for applying a predetermined fixed direct current biasing potential to the other of said control grids, means providing a time delay network in connection with said other control grid, a second signal amplifier device comprising a cathode, a control grid, an anode, and a diode anode adjacent to the cathode, means for applying signals to said last named control grid, means for applying signals derived from said anode between the diode anode and the cathode, and a diode output resistor in circuit between the time delay network and said bias supply means.

5. In a volume expander system, the combination with a signal supply circuit, of a combined amplifier and rectifier device having a. control grid connected with said signal supply circuit and having an output resistor, a volume expander amplifier device having a cathode, a signal grid connected with said signal circuit and a gain control grid connected with said output resistor, a resistor capacitor filter in series between said gain control grid and said output resistor, a selfbias resistor in circuit with said last named resistors serially, and independent volume control means for said last named amplifier.

6. In an audio frequency amplifier, an electric discharge amplifier device having acathode, a

control grid adjacent'to the cathode, a second control grid and an output anode electrode, means for applying audio frequency signals to said control grid, and means for applying a variable biasing potential to the second control grid thereby to control thegain'of said amplifier, said means comprising .a second amplifier device, means for variably applying a portion of said si nals to said second amplifier device, a rectifier device for receiving amplified signals through said second amplifier device, an output resistor for said rectifier, means providing a fixed direct current biasing potential for said control grids including a resistor across which a potential is established for the second control grid, a circuit connecting said lastrnamed resistor and output resistor serially between the second control grid and the cathode,and a resistor capacitor filter connected between the second control grid and the cathode.

7. A volume range expander system for audio frequency signals, comprising anraudio frequency signal transmission circuit, an audio frequency amplifier having a signal control grid connected with said circuit, volume control means in said connection, said amplifier device further having a cathode, a second control grid and an output anode, a time delay filter connected adjacent to the second control grid and comprising a capacitor connected between said control grid and cathode and a series resistor, means providing biasing potential for said second controlgrid included serially in circuit with said filter resistor between said grid and the cathode, and means for establishing a variable potential in opposition to said biasing potential in said circuit in response to signal strength variations, said means comprising a diode rectifier device, a diode output resistor therefor in said circuit, an electric discharge amplifier stage coupled to said diode rectifier and variably connected with said signal circuit, independently of said first named amplifier device.

8. In an audio frequency amplifier, the combination of an electric discharge amplifier and control device having a cathode, a signal control grid adjacent to the cathode, an output anode and a second control grid between said first named control grid and the anode, a signal output circuit connected with said anode, a signal input circuit connected with said signal control grid, volume control means in said last named connection, a second electric discharge amplifier device connected with said signal input circuit independently of said volume control means, independent volume control means for said second amplifier device, a diode rectifier device coupled to said second amplifier device for rectifying signals therefrom, said rectifier device having an output resistor, a capacitor connected between the second control grid and the cathode, a resistor connecting said second control grid with the rectifier output resistor whereby said resistors are in series relation to each other between the second control grid and the cathode, and means providing a fixed biasing potential for said grid in said connection.

9. In a volume range expansion system for audio frequency signals, an electric discharge amplifier having a cathode, a signal control grid, a

second control grid, means for establishing a fixed biasing potential on said first control grid, means for applying audio frequency signalsto said signal control grid, means for deriving a fixed biasing potential, for said second control grid from said first named means, means for applying a rectified signal potential to said second control grid in opposition to the potential applied thereto by said last named means, said means for applying arectified signal potential including filter and bias supply resistors in series with said second control grid, and a shunt capacitorfor said series circuit connected between the second control grid and the cathode, the value of said capacitor and thevalues of said series resistors being so related that variation in biasing potential in response to variation in rectified signal potentials are applied to said second control grid with a predetermined time delay with increasing rectified signal potential and a predetermined increased time delay with decreasing rectified signal potential.

10. In a volume range expander system for audio frequency signals, a combined electric dis-- charge amplifier and mixer tube having a cath-' ode, a control grid adjacent thereto, an output anode, a suppressor grid adjacent thereto and connected to the'cathode, a second control grid interposed in the space path of the tube between the first control grid and the outputanode, and a screen grid structure'for'said second control grid whereby it is electrically shielded from the other electrodes of the tube, means providing a series bias supply andcontrol circuit between the second control grid and the cathode, a filter comprising a resistor and capacitor in combination in said circuit adjacent to the second control grid, the filter capacitor being connected between the cathode and said second control grid, a signal rectifier'having anoutput resistor included as a time delay and bias supply impedance in said series circuit, means for applying a biasing potential' to said second control grid whereby a predetermined initialjovndegreei ofgamplification .is ,ob;v tained, and means for applying signals to be am: plified to the firstnamed: control grid and-to3said.

rectifier. l 7' 11. In a volume range expander system for;

' audio frequency signals, a'combined electric 1118-;

' be amplified to the first named. control grid, and

variable means for applying signals to saidcrectifier.

12. In a volume range expander. system for audio frequency signals, a combined electricdiss charge amplifier and control tube havingla oathode, a control grid adjacent theret an output anode, a gain control grid interposed in the space path of the tube between the'first control grid and the output anode, means providing a seriesbias supply and control circuit between the sec-- ond control grid and the cathode, means .provid-' ing a common source of biasing potential'forsaid control grids connected with said lastnamd circuit, a signal rectifier having, an output resistor 9 included in said series bias-supply and-control circuit, and means responsive to an increase in'. the average amplitude of audio frequency. signals applied to said first named grid for causing'the gain control grid potential to be changed in a positive direction.

CHESTER M. SINNE'IT.

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