Radio broadcast .. (1922-30)

331 RADIO BHOADCAST Laboratory Information Sheet Modulated Oscillators and Output Meter March, 1930 ONE OF the most important tasks in the servicing of radio receivers is the accurate alignment of the tuning condensers and the accurate adjustment of the neutralizing condensers. Although these operations may be carried out by tuning the set to some local station and making the necessary adjustments while listening to the a.f. output, this method is not very accurate. It is much better to set up a local oscillator and an output meter so that the input to the set and the a.f. output are reason- ably constant and so that slight changes in the adjustments can be detected readily. On "Laboratory Sheet" No. 332 are given the circuits of two simple modulated oscillators and two output meters that may be used in checking a receiver. Oscillator No. 1 is designed for operation on a.c. and oscillator No. 2 for operation from bat- teries. The a.c.-operated oscillator uses a 226-type tube supplied from a filament transformer and plate potential is obtained by connecting the plate lead to the primary of the power transformer. The oscillator will then have 110 volte a.c. on its plate and will be modulated by the a.c. The bat- tery-operated oscillator uses a 199-type tube and the grid leak and condenser values are such that they will function to modulate the output. It is, essential, of course, that the oscillator be modu- lated so that a note will be audible in the output of the loud speaker connected to the set under test. The output meter No. 1 uses a 226-type tube as a rectifier in series with an 0-10 milliammeter and a 1-to-l output transformer. The output meter of oscillator No. 2 simply uses a 1-to-l transformer to whose secondary an a.c. milliammeter is con- nected. If an a.c. milliammeter is available this is, of course, the simplest circuit but if a d.c. milliam- meter must be used it is necessary to rectify the output by some circuit such as is indicated by cir- cuit 1. In use the output meter terminals marked "to set" are connected either directly across the moving coil of the electrodynamic loud speaker or, if neces- sary to get sufficient reading, across the primary of the transformer supplying the loud speaker. The oscillator is set up near the set and located at such a point that a satisfactory deflection is obtained on the output meter when the set is tuned to the frequency oeing generated by the oscillator. The various condensers can then be accurately aligned until maximum deflection is obtained on the meter. 332 RADIO BROADCAST Laboratory Information Sheet Modulated Oscillator and Output Meter March, 1930 THE CIRCUITS on this sheet show arrangements that can be used to supply a constant modu- lated signal to a receiver for testing purposes and also output meter circuits that can be used to in- dicate qualitatively the output of the set. All specifications are given on the circuits and some notes on their use will be found in "Laboratory Sheet" No. 331. OSCILLATORS PLATE COIL ( ?0 TURNS ON END OF TUBE O o SO TURNS ON 3- TUBE 0.00035 MFD 1MFO -^MMAr-" 1 1000 -MW/WV FIL TRANS •—OUTPUT METERS 1-1 OUTPUT TRANS TUBE, TAPPED AT CENTER No. 333 RADIO BROADCAST Laboratory Information Sheet Calculating Power Output March, 1930 ONE OF the simplest and most effective ways of calculating the power output of an ordinary three-element power tube is by the use of "load lines" plotted across a group of characteristics showing the relation between the plate current and plate voltage for various grid biases. A group of plate current-plate voltage curves for the 17lA- type tube are given on " Laboratory Sheet" No. 334 and the following notes indicate how the load lines are determined. It should be noted that these curves show the plate current obtained for various plate voltages at grid biases corresponding to from 0 to —80 volts. The first thing to do is to pick out the normal oper- ating point of the tube, which in this case is —40, volts and 20 milh'amperes. The tube has a plate resistance of 2000 ohms and for maximum undis- torted output the load resistance would therefore be 4000 ohms. We now have to lay off the line to indicate the manner in which the plate current will change v/ith grid voltage. This is not difficult. With no signal on the grid the plate current will be 20 milliamperes. Now assume that the plate current changes from 20 milliamperes to 40 milliamperes. This means that there will be a change of 20 milli- amperes in the current flowing through the 4000- ohm resistor. By Ohm's Law the resistance, 4000 ohms, multiplied by this current, 20 milliamperes, gives the change in voltage across the 4000-ohm resistance, or in this case 80 volts. We, therefore, mark on the diagram point B at a plate current of 40 milliamperes and a plate voltage of 100 (80 volts less than the normal operating potential of 180 vojts). A line is then drawn from the operating point at 20 milliamperes and 180 volts so as to pass through the point B. This is the load line corres- ponding to a load resistance of 4000 ohms. Load lines for values other than 4000 ohms can be calculated in this same manner. For example, if the load resistance is 2000 ohms then a 20-inA. increase in plate current will produce a 40-volt change across the load resistance. This gives us point C at a current of 140 milliamperes and a plate voltage of 40 volts. Drawing a line between C and the operating point A gives the load char- acteristic corresponding to 2000 ohms. In future Sheets we will show how these load lines may be used to determine the power output of the tube and also the percentage of second-harmonic distortion. STRAYS FROM THE LABORATORY (Continued from page 273) Electric company and used in their labora- tories (and probably for sale), but takes considerable space to describe how it is used, the importance of the shape of con- denser plates, how to get good wave form at low frequencies, etc. It should be di- gested by every engineer engaged in radio- and audio-frequency laboratory work. Despite the fact that QST is edited primarily for the audience whose interest and activities lie in the frequencies higher than 1500 kc., we are continually surprised and pleased with the amount of material of interest to broadcast-frequency engi- neers. In the November (1929) issue will be found an article on the use of an inter- esting mechanical rule for determining the proper load resistance for power ampli- fiers, and to simplify power output and distortion calculations. This article is by K. S. Weaver, Westinghouse Lamp Co., Bloomfield, N. J. Another article by Technical Editor James J. Lamb describes the uv-845, a low-impedance linear power amplifier and modulator tube of the 50- watt type. A third article on "Operating Characteristics of Vacuum Tube Oscilla- tors," by H. A. Robinson, belongs with the engineer's file of vacuum tube circuit articles. THE ADAPTORON (Continued from page 253) have some effect on the wave form, but, as indicated in Fig. 8, load resistance of from about 6000 ohms up to about 40,000 ohms does not have a serious effect. The effect of changing the length of the com- mutator segments for a constant load and a specific filter system had a somewhat greater effect. Fig. 9 gives the data for this case. Second and third harmonics are (Continued on page 303) SEGMENT LENGTH IN ELECTRICAL DEGREES 1 10 130 140 150 VOLTAGES D-C- INPUT 115 115 115 115 86 PRI. (A.C.) 22.5 65 83 SEC. (A.C.) 315 455 570 600 WAVE FORMS ACROSS PRIMARY \J FUNDAMENTAL ADAPTORAN CIRCUIT COMMUTATOR BRUSHES = 90 MECHANICAL DEGREES APART. LOAD =9800 OHMS SPEED =1800 R.P.M. Fig. 4 — Effect of varying length of commutator segments. 302 • RADIO BROADCAST FOR MARCH