Radio broadcast .. (1922-30)

itiinmim i: Data on a Novel Automatic Volume Control System THE PHILCO "95" SCREEN-GRID PLUS jiimimmimiiiiiiiiiiiii By WALTER E. HOLLAND* and W. A. MAC DONALDf 'Chief Engineer, Philadelphia Storage Battery Company }Chief Engineer, Haxeltine Corporation PUSH-PULL TRANSFORMER DURING THE past few months the Engineering Department of the Philadelphia Storage Battery Com- pany has been working in cooperation with the laboratories of the Hazeltine Corpora- tion to produce a moderate-priced com- mercial broadcast receiver that would utilize all the advantages and overcome all possible disadvantages of the screen- grid tube. Furthermore, it was intended that this re- ceiver embody a practical automatic volume control system of the type described by H. A. Wheeler, of the Hazeltine Corporation, in a paper he presented before the I.R.E. in November, 1927. (This paper gave data on a method using a two- element tube as a combined detector and automatic vol- ume control in a radio re- ceiver.) Such a receiver has been designed and was an- nounced by Philco about September 1, 1929. It is known as the Model 93 Screen-Grid Plus. It has been general prac- tice in the design of receivers with automatic volume con- trol to use an ordinary three- element tube as the detector and to employ an additional tube in the circuit to control the volume automatically. This latter tube is operated on the lower bend of its Eg-Ip characteristic, its grid being excited by the output of the r.f. amplifier across which the grid-filament cir- cuit is connected. The plate circuit of the tube is used to control the volume auto- matically by supplying to the control grids of the r.f. amplifier tubes a negative voltage proportional to the output of the r.f. amplifier. Only One Rectifier In such a circuit the automatic volume control tube is actually working as a" detector, the changes in average d.c. plate current with changes in input voltage being used to supply negative bias to the r.f. amplifier grids. In the plate circuit of this tube there are, of course, audio-frequency currents, but these are bypassed to ground by means of by-pass condensers. In a sense, therefore, such sets contain two detectors or rectifiers. In one rectifier, the detector, the audio-frequency output is used to supply signal voltages to the audio- frequency amplifier and the d.c. component of plate current is not used. In the other rectifier, the automatic volume control tube, the audio-frequency components are not used and the steady d.c. component of the rectified signal is used to control the volume. Why not use a single rectifier and utilize both components of its output? This is possible and in the Philco 95 a two-element tube (227 type with grid and plate tied together) is used to rectify the r.f. signal and to supply both an a.f. signal for the audio-frequency amplifier and a d.c. bias voltage to the r.f. amplifiers. The two-element rectifier, when its re- turn circuit has a high d.c. impedance, is linear over practically its entire rectifica- tion curve. In this particular receiver it is than the time of a single cycle of the lowest audio-frequency current it is desired to amplify. Actually the control system should be designed so that its time con- stant (the time it takes to effect the gain) is comparable with the period of the lowest desired audio frequency. The time constant of the volume-control circuit is determined by the values of the resistance and by-pass condensers in the r.f. grid bias circuit, and in this par- ticular receiver the time con- stant is about one fortieth of a second so that the cir- cuit nearly reaches equilib- rium in about one twentieth of a second. Such a value of time constant has a negligi- ble effect on low-frequency modulation down to twenty cycles. DETECTOR AMPLIFIER TUBE ELECTRO-DYNAMIC SPEAKER PLUG DETECTOR RECTIFIER TUBE 4ThR.F.TR«NSFO«Mtn n COMPENSATING CONDENSER nRFTRANSFORMER TUNING CONDENSER 'ON-OFF SWITCH LOCAL-DISTANCE gt SWITCH ""• B FILTER CONDENSERS a COMPENSATING CONDENSER 0 R.F.TRANSFORMER Ur R.F.TUBE 2».> COMPENSATING CONDENSER IsrR.F.TRANSFORMER TERMINAL PANEL B.FILTER CHOKE 1 COMPENSATING CONDENSER POWER TRANSFORMER RECTIFIER TUBE Tuning the Set The location of all parts on the Philco 95 chassis is clearly indicated in the above picture. linear from about 1 volt up to over 100 volts input. Such a detector gives, there- fore, the much desired linear detection characteristic (unlike the three-element rectifiers which are linear over only a small part of their detection curve, the two- element tube is linear over practically its entire curve) and it is also devoid of any overloading, even at input voltages far in excess of those at which it operates in this receiver. Actually, overloading of this type of detector can be due only to opera- tion of the tube at input voltages in excess of the maximum safe value. These proper- ties of a two-element detector contribute to the simplicity of the system and at the same time the signal modulation is recti- fied without distortion. All automatic volume control systems must be designed so that they will not seriously affect low-frequency modulation. If, for example, the control system were in- stantaneous in action, the control circuit would function to eliminate much of the modulation in the incoming r.f. signal. The control system should not be able to con- trol the gain of the system more rapidly Automatic volume con- trol receivers are generally rather difficult to tune by ear because the volume is essen- tially constant over several degrees of the dial. The more "perfect" the control sys- tem the more pronounced is this effect. This problem can be overcome either by means of a tuning meter to give a visual indication of reso- nance or by designing the control system so that a fairly definite peak is audi- ble to the ear as the set is tuned through a station. Af- ter all, absolutely complete equalization of the volume is not essential (differences in the percentage modula- tion of different broadcast- ing stations will prevent any system, no matter how per- fect, from giving absolutely equal volume from all stations) and it seemed advisable to the designers of this receiver to make a slight peak apparent in the tuning so that the user of the receiver would be able to tune the set in a normal manner and not have to learn some new method of tuning, such as tun- ing by means of a meter. Everyone who has operated this receiver has had no diffi- culty in tuning it accurately to resonance by listening to the output of the loud speaker. The automatic control is effective over a ratio of signal voltages of 300 to 1, and in combination with the local-distance switch, which gives a 40 to 1 change, complete control is obtained over a ratio of 10,000 to 1. In normal operation the r.f. input to the rectifier is about 5 volts. No better indication of the effectiveness of the volume control circuit can be had than by tuning in an average station with the "local- distance" switch in the local position, and then switching to the distance position. This changes the input voltage in a ratio of 40 to 1—and there is but a barely per- ceptible change in volume as the switch is DECEMBER 1929 1 11