Radio Broadcast (May 1928-Apr 1929)

OCTOBER, 1928 "OUR READERS SUGGEST—" 365 The procedure is as follows: Have the condenser sections of the combination to be matched half way out. (If the condensers are to be matched with the aid of trimmers, the trimmers also should be at half maximum capacity.) Start the oscillator and snap the clip to the grid side of one of the condensers. Tune the oscillator to resonance, which will be indicated by a maximum dip on the meter. If the dip is too broad, reduce the capacity of the twisted rubbercovered wire — by cutting down the overlap — until a sharp dip is obtained. Snap the clip on the next condenser, and adjust this to resonance (without touching the oscillator adjustment) either by use of the trimmer, or by tapping the plates into place. At maximum deflection the two stages are in resonance with each other. The procedure is the same for additional circuits. John Benedict, Maspeth, L. I. An Amplifier Kink L_J AVING an occasion to revamp an old set for * *■ a friend of mine, I used the audio-frequency arrangement shown in Fig. 5. The Thordarson 3§:i transformers, originally supplied with the set, were merely rewired to conform with the diagram. The tone quality was considerably imp roved, particularly on the lower register. I effected still further improvement by the use of 1 12A tubes in the detector and first audio sockets. Edwin M. Wright, Phila., Pa. 6-voIt I S 1 -HI— To loud Speaker FIG. 4 STAFF COMMENT HpHE arrangement sug•* gested by Mr. Wright emphasizes the low notes. Reproduction of the higher notes can be enhanced by the substitution of a 30-henry choke for the 50,000-ohm resistor in the plate circuit of the first audio tube. Dynamic Speaker Field Supplied from B-Power Unit IT IS possible to excite the windings of several * of the d.c. type of dynamic loud speakers from the plate current to a receiver operated from a line power supply source. It is merely necessary to connect the excitation windings of the loud speaker between the choke coil nearest the output of the socket power unit and the maximum voltage post. The connection is broken and the circuit recompleted through the windings, care being observed to connect the positive loudspeaker post to the choke coil. The current drain to the receiver should be in the neighborhood of 100 milliamperes. This, however, is generally the case where a dynamic speaker is justified. A. V. Svendsen, New Zealand. A Simple Audio Channel Equalizer I HAVE found several sets in which the com1 bination of a good audio channel and a good speaker gives an overemphasis to the low notes. I find that the addition to the usual choke output device suggested diagrammatically in Fig. 4, is most effective in controlling the amount of low-frequency reproduction. The 0.006-mfd. condenser bypasses the high frequencies anyway, and varying the 100,000 A simple equalising arrangement that controls the reproduction of low notes. This device will eliminate the "boom" in many modem speaker-amplifier combinations ohm resistor (a Pilot Resistograd) adds the low notes to taste. H. D. Hatch, Wollaston, Mass. STAFF COMMENT UR. HATCH'S idea should prove most effective in eliminating the "boom" experienced with many cone and airplane cloth speakers. A. C. Tube to Reduce Microphonics \A ANY battery-operated receivers. the operation of which is characterized by excessive microphonic disturbances, can be improved by the substitution of a ux-227 a.c. tube in the detector socket with the proper filament resistor to permit its operation from a battery. Previous to making the suggested change, enjoyable reception from the writer's batteryoperated set was practically impossible. Footsteps in the room, or the passage of a truck in the street, was sufficient .100,000 ^ t0 set UP ringing micro phonics in the loud speaker. Appreciating the fact that the rigidity of cathode structure in a.c. detector tubes tends to reduce the vibratory motion responsible for microphonics, 1 replaced the ux socket in my receiver with a 5-prong socket, wiring a 6-ohm rheostat in series with the filament circuit. The cathode is connected to the positive filament terminal on the socket — that is, "C" and plus "F" are strapped together. A ux-227 tube can now be used as a detector. If the time lag — 30 to 40 seconds — is objectionable, an Arcturus type 127 tube can be used instead. This latter tube "comes up" in about seven seconds. STAFF COMMENT HTHE Editor of this department has done con■*■ siderable experimenting with the use of heater type tubes in airplane receivers, where they are very effective in the reduction of microphonics. If the reader does not care to change the socket in his receiver, an Arcturus type 171 tube can be plugged into the detector socket, without making any changes in the receiver at all. No additional rheostat is required for this 5-volt tube. An Antenna Booster For Loop I HAVE a loop-operated set which is badly ' shielded by surrounding walls. To correct this I am using an outside antenna inductively coupled to the loop by means of a coil. The coil is placed inside the loop and connected to the outside antenna and to the ground. I am using a 3" coil with 50 turns of No. 20 wire. I find that by tapping this coil in four places, that is, every ten turns, the reception is greatly improved over the entire range of wavelengths. That is, on short waves the 10-turn tap is just right while on long waves 40 turns or 50 turns is about right, with proportionate taps in between. William D. Esch, Cherryvale, Kans +180 v. FIG. 5 Rearranging an old amplifier in accord with this circuit will give fine quality even with poor amplifying transformers STAFF COMMENT AN ARRANGEMENT practically identical with Mr. Esch's suggestion was described in this department for December, 1927, and January, 1928. A commercial coupling coil for this purpose is made by the Jenkins Radio Company of Davenport, Iowa. An Output Filter Without a Condenser AVERY effective output device for use with the average receiver may be made with the use of a choke only (the condenser being omitted) providing the characteristics of power tube and speaker are known so that the proper choke may be employed. In a receiver constructed by the writer, a 171 type tube was used in conjunction with an R. C. A. 100 speaker, which is comparable in characteristics to most cone reproducers. A condenser of sufficient capacity not being available, the speaker was connected in parallel with a 25-henry choke as shown in Fig. 6-A. The equivalent circuit, based on a frequency of 100 cycles, is shown in Fig. 6-B. At this frequency the impedances are, approximately: tube — 2000 ohms, choke — 15,700 ohms, and speaker — 2000 ohms. The d.c. resistance of the choke is 600 ohms and that of the speaker is 1800 ohms. It will readily be seen that the speaker, being of rela ively low impedance, will take about 90 per cent, of the a.c. current, but due to its high resistance, compared to that of the choke, will allow but one third of the d.c. current to pass. This will be in the neighborhood of 6 milliamperes, which is not objectionable. The resistance of the plate circuit is considerably lower than that of the actual output device, and hence the voltage drop will be smaller. The quality of reproduction, using this combination, was found to be excellent. Glenn R. Taft, Ticonderoga, N. Y. FIG. 6 An output device using a choke coil without the conventional condenser. The wiring circuit is shown at A, and at B is the equivalent schematic circuit