Radio broadcast .. (1922-30)

No "Motor-Boating" A Quality Audio Amplifier Which Will Not "Motor-Boat" By H. O. Ward RADIO BROADCAST Photograph A HIGH-QUALITY RESISTANCE-COUPLED AMPLIFIER The two condensers and the resistance in the lower left-hand corner prevent the amplifier from "motor-boating." The loud speaker con- nects to the two Fahnestock clips alongside of the output condenser: the two similar clips in the lower right-hand corner are used to read the plate current of the last tube (EVERAL years ago the resistance-coupled amplifier was much in vogue, and justly so, for it is an excellent type of audio amplifier and is capable of giving practically equal re- sponse over the entire audio-frequency range. Its popularity at that time was comparatively short-lived—just why, it is hard to say. Since the introduction of the 240 type tube some time ago, however, there have been indications that the resistance amplifier will stage a comeback. The amplifier described in this article has a fiat frequency response curve from 60 cycles up to about 6000 cycles, it will not "motor-boat" when operated from any ordinary B power unit, and it has a voltage amplification of about 400 from the input of the amplifier to the grid of the power tube. A curve showing the frequency re- sponse curve of the amplifier is given in Fig. i. In Fig. 2 is given the circuit diagram of the ampli- fier. There is nothing unusual about this amplifier with the exception of the anti "motor-boating" circuit which we have indicated by enclosing it in dotted lines. This anti "motor boating" circuit originated in the engineering department of the F. T. Cunningham Company a short while ago, and the circuit, tested in RADIO BROADCAST Laboratory, has proved very satisfactory. It should be noted that the circuit is arranged so that the plate current of the detector and first audio tubes must pass through resistance RI. To test the circuit the amplifier was con- nected to a B power unit and with the resistance RI short-circuited the amplifier immediately began to "motor-boat." As soon as the short- circuit was removed the "motor-boating" stopped. It was found that the value of RI, necessary to produce stable operation of the amplifier, varies with different B power units. With some power units a resistance of 20,000 ohms is sufficient, while with other power units a resistance of 100,000 ohms is necessary. Since the latter size resistance seems to be effective in all cases, it is suggested that those who con- struct the amplifier use this value of resistance. As the plate current of the first tubes in the To Plate Circuit 0 006 mfd of Del. Tube FIG. I amplifier and the detector tube must flow through this resistance, it might be thought that there would be an excessive loss in voltage across it. Such is not the case, however, as the following data will show. For this test the amplifier was set up with a detector circuit connected ahead of it. The detec- tor tube used was also a 240 type tube and the same voltage was applied to the detector tube as was applied to the first and sec- ond audio tubes in the amplifier. 433 This table indicates the voltage actually at the plates of the detector and first audio tubes for two different values of applied volt- age. A difference of 10 or 20 volts at the plate of a tube will not change the characteristics of the amplifier and since the loss in voltage due to-the resistance RI is of this order, it is evident that the amplifier will not be affected. The following data were obtained: FIRST AUDIO 83 95 115 93 The reason that the detector plate voltage is lower than'that of the first audio stage for the same values of resistance is because the current drawn by the latter is less than that drawn by the detector. The recommended values of plate and grid resistances and coupling condensers are as in- dicated in Fig. 2 and the frequency response curve shows that the values specified give the desired flat characteristic. If larger values of resistances are used the gain of the amplifier will fall off at the high-frequency end. Larger values of coupling condensers tend to better the response at low frequencies but since the re- sponse is satisfactory with the values given, it is a waste of money to use any larger condensers unless you have them on hand. The correct voltages to use on the grids and plates of the three tubes of the amplifier are given below. The letters Bi, Ci and B 2 , C 2 in the table below refer to similar notation in Fig. 2. TST AND 2ND AUDIO Tubes Grid Plate Voltage Voltage Ci B, i ,o to 1.5 1.3 3.0 135 '5? POWER TUBE 112 TYPE Grid foliage Ci 6 9 10.5 Plate Voltaic 82 go 135 157 Grid Voltage Q 16.5 27 33 40.5 171 TYPE Plate Voltage 62 90 137 157 180 The following parts were used in the amplifier: i—Durham o.i-Megohm Resistor 3—Durham o.25-Megohm Resistors 3—Durham 2. o-Megohm Resistors 3—Sangamo o.oo6-Mfd. Fixed Condensets 6—X-L Binding Posts 3—Benjamin Sockets i—4-mfd. Tobe Condenser i—Amertran Choke Coil, Type 854 5—Fahnestock Clips i—6-Ohm Pacent Rheostat 4—i Mfd. Dubilier Bypass Condensers 3—Lynch Double Resistor Mounts i—Lynch Single Resistor Mount 0.006 mfd. UX-112(CX-3ir)Of 2-4 mfd. Output dipt FIG. 2