Radio Broadcast (May 1928-Apr 1929)

A Resistance-Coupled Amplifier and Power Supply HERE are the constructional details for a high quality audio and power amplifier together with a power supply unit which vill supply B voltages to the r. f. amplifier with vhich it is used. If a.c. tubes are used in the r.f. ind a.f. amplifier, the filament windings on the >ower transformer may be utilized for filament urrent, thus providing complete light-socket iperation. The entire unit is one of good engileering practice as applied to both amplifier and >ower elements. The combined unit was designed to meet the xacting requirements of an audio amplifier sysem which in itself should be capable of giving ligh quality reproduction ; that is, this part of the ssembly was expected to possess a fairly flat requency characteristic over the range from apiroximately 50 cycles to about 5000 cycles. It fas realized, of course, that the radio-frequency nd of the receiver might appreciably alter this miformity of audio-frequency characteristic. Glancing at Fig. 1 and 2 it will be noted that he entire assembly of parts is such as to form everal sub-groups; that is to say, the power ransformer and filter condensers form one lineip; the rectifier tube, C-bias resistor and choke oils a second line; while a third group embodies . potential dividing resistor and bypass conlenser network together with an anti-motorloating feature incorporated in the amplifier. ~he audio amplifier system makes up a final ;roup assembly which, incidentally, is placed in a losition as far away as possible from the transormer, so as to eliminate all stray field coupling. :urther, the method of wiring largely helps in emoving noise background from the audio outiut caused by stray fields, a.c. lines, etc. Although the amplifier illustrated in this artile employs d.c. type tubes, there is no reason thy a.c. tubes cannot be used in the unit so as to nake it completely a.c. operated; the filament /indings on the power transformer may be used o supply filament current for the a.c. tubes. We lave therefore indicated in Fig. 2 in dotted lines he circuit arrangement to use with a.c. tubes. *lo changes are necessary in the values of couiling resistors and the only additional parts retired for a.c. operations are two resistors for C>ias, one with a value of 6000 ohms and another fith a value of 1000 ohms. The a.c. tubes of ourse replace the type 340 and type 112 tubes nd make it unnecessary to run C-battery or A>attery leads to the amplifier. For VTi we sug;est the use of a CeCo type G high-mu tube, rube VT2 may be any standard type 227. THE AUDIO AMPLIFIER THE audio amplifier system is made up of two stages of resistance coupling feeding into a ligh-grade push-pull output stage employing 10 type tubes. Such an arrangement generally ulfills the requirements of even the most exactng set owner. This type of amplifier was adopted only after lose study of the problem. It was felt that a iroperly designed resistance scheme of tube oupling in the detector and first audio stages fould result in an amplifier having a flat charxteristic, provided the number of high-mu ubes could be held at a minimum. For the latter By J. GEORGE UZMANN Dubilier Condenser and Radio Corp. — to ESI STANCE-COUP LED amplificaHon has not been used to any great extent in home constructed power amplifiers and its use in Mr. U^mann's unit in combination with a push-pull output stage should prove interesting to many of our readers. Resistance-coupled amplifiers — properly designed— are noted for their excellent frequency response. The amplifier utilises a circuit arrangement which prevents motorboating, a fault experienced with some resistancecoupled amplifiers and which has, in the past, perhaps made some builders hesitate to construct an amplifier of this type. Storage battery type tubes are used in this amplifier because it was constructed before a.c. high-mu tubes were available, but tubes of this type can now be obtained from CeCo, A returns and others and may be used to make the amplifier completely a.c. operated. Their use in this amplifier is covered in the text of the article. — The Editor. reason either a 30 Mu or CX-300A tube should be employed as a detector, while a cx-340 high-mu tube is used in the first audio stage. A detector plate resistor of 100,000 ohms is suggested for the CX-300A detector, and a resistor of 250,000 ohms for the cx-340 tube. The grid leaks are of 2-megohm size. Any three-stage audio amplifier system supplied from a common plate voltage source, and particularly from a socket power device, possesses characteristics which tend to throw such a cir cuit into audio-frequency oscillation — more commonly called "motorboating." In an effort to eliminate such a condition an anti-motorboating device was incorporated in the amplifier circuit. It consists of a 25,000-ohm wire wound resistor, R2, connecting between the B plus 135-volt tap of the power supply and the detector plate resistor, together with a bypass condenser, C3, of 1.0-mfd. capacity. Incidentally, the audio input terminals of the amplifier do not include an r.f. choke and bypass condenser since it was believed that this practice is almost standard and usually included in the detector output. It is important, however, that such an arrangement be used. The C-bias potential for the 340 tube, being small, is obtained by taking the voltage drop across its filament. For the second audio stage the bias voltage is obtained from a small external C battery. A high-grade push-pull input transformer is used to connect into the cx-i 12A output circuit. The complete stage is of standard design and employs a push-pull output transformer, T2. 310 type power tubes were adopted for this stage. THE POWER SUPPLY SYSTEM THE power transformer, A, has a tapped primary which readily permits of circuit adjustments suitable to meet varying line conditions without the necessity of variable resistors. Its high voltage winding outputs either 550 or 750 volts, making it suitable for a cx-381 type half-wave rectifier tube. For the audio and radiofrequency plate supply under discussion it will be found that the 550-volt tap is correct, otherwise the cx-310 plate voltage will be excessive and will result in poor tube life. Further, at this