Radio Broadcast (May 1928-Apr 1929)

90 RADIO BROADCAST JUNE, 1928 placing the volume control in the audio amplifier but this control has the disadvantage that it will not prevent the detector overloading on strong signals. There are several satisfactory volume controls that can be applied to ax. receivers which will not affect the selectivity and which will also prevent detector overloading on strong signals. We mention two types. One fairly good control is a variable resistance connected in series with the B-plus lead to the primaries of the r. f. transformers. This resistance should have a maximum value of about 200,000 ohms and a o. i-mfd. or larger bypass condenser should be connected across it. As the amount of resistance in the circuit is increased, the effective voltage applied to the plates of the r. f. is lowered and the volume is thereby decreased. Such a control will not impair the selectivity, but, with some receivers will cause an undesirable increase in voltage applied to the other tubes in the set. A simple type of volume control that may generally be used merely consists of a variable resistance connected between the antenna and ground posts on the set. The resistance should have a maximum value of about 50,000 ohms. Centralab, Yaxley, and others make a special resistance for this purpose. Some Baffle Board Experiments STAFF COMMENT '"THE baffle board consists of a reflecting sur* face applied near a cone speaker. In many cases this takes the form of a large box, in which the cone is placed. In others it is a short horn having a relatively large bell, the cone being used as a diaphragm. In many instances, the use of baffle boards will improve reproduction of the low frequencies especially. As experimental boards are easily constructed, they are worth trying. The two following contributions consider the possibilities of the use of baffles. i. Rotor 7 0.00025 mfd. To Ant Post on Set FIG. 4 A baffle arrangement with a 540 AW cone speaker FIG. 5 An antenna tuning circuit for use with neutrodynes and other receivers having an untuned primary circuit IN EXPERIMENTING with a Western Elec* trie 540 AW cone I found it possible to attain a marked improvement in the reproduction of the lower audio frequencies, without apparent impairment of the higher notes through the use of a baffle board. The accompanying photograph, Fig. 4, illustrates the baffle arrangement employed by the writer. I made a 36 by 36-inch wood frame, 12 inches deep. The front was faced with heavy roofing paper, with a circular hole, eighteen inches in diameter, cut out in the center. The cone was placed in this cabinet with the face of the cone brought as close to the opening as the frame of the loud speaker would permit. The assembly was then mounted on top of the chest of drawers as shown in the photograph, and placed in the corner of the room so that the sides of the enclosing cabinet touched the walls. By so doing the baffle effect of the sides was greatly increased, and the supporting furniture also functioned as part of the baffle. The power tube used is a ux-171 outputting to the speaker through a 100-henry choke coil and a 9.0-mfd. condenser. This tube outputs adequate distortionless power to the baffled loud speaker with 135 volts on the plate.. A slight barrel effect in speech was counteracted by placing a piece of heavy cloth on the back of the cabinet, thus avoiding sound reflection. Paul S. Foster, New York City. A CAREFUL consideration of the most effective method of mounting a freeedge baffle type cone speaker is necessary if the best results are to be secured. Fairly good results can be expected by using a flat baffleboard of sufficient thickness and rigidity to be non resonant throughout the audio range. A deep cabinet or console, however, will tend to improve the lower frequency response of the speaker by allowing a greater effective baffling area. The greater the baffling area the better will be the reproduction of the low notes. Deep box baffles of the type to which we have reference often give rise to a resonance effect within the audio range, resulting in the exaggeration of certain frequencies, noticeable as a booming sound or barrel tone. One method of correcting this condition is to vent the baffle by cutting holes or louvres in the sides of the console. This has the disadvantage of reducing the effective baffling area and often impairs the appearance of what otherwise would be a handsome bit of furniture. At best my present suggestion is a trial and error method. In mounting a free-edge cone in a baffling arrangement of this type, the writer finds that resonance can be completely eliminated by lining the interior with felt. Felt packing of a thickness of I inch was used, although a somewhat thinner lining would probably have worked just as well. The entire interior of the cabinet, was lined with the packing and secured by glue and tacks. D. C. Redgrave, Norfolk, Virginia. Antenna Tuning Device '"THE apparatus described below is an indis*■ pensable portion of my receiver equipment for distant reception. Many broadcast fans, located like myself at some distance from broadcast centers, will find this simple device of use to them. On many evenings, when stations two hundred to three hundred miles away are practically inaudible, a variometer and two variable condensers, connected as shown in Fig. 5 boost the volume from ten To twenty times, often making enjoyable loud speaker reception possible. The device does net change the original dial settings of the receiver. Once the variable condensers are set to the proper capacity (to be determined experimentally) they need not be touched again, all tuning being effected on the variometer. However, the 0.0002 5-mfd. variable is very effective as a volume control. This apparatus is not effective on all receivers, but is designed primarily for use with sets having untuned antenna primaries, such as the average neutrodyne and tuned r.f. receiver. A. Gaudette, Lewiston, Indiana. STAFF COMMENT HpHE arrangement described above is an an* tenna tuning device. The control described by Mr. Gaudette is really a combined antenna tuning device and a wave trap. In the majority of instances it can be simplified to the circuit shown in Fig. 6. Coil L may be the secondary of any available radio-frequency amplifying transformer, or sixty turns of wire wound on a three-inch diameter form. This device will be most effective on short indoor antennas. A Spark Plug Lightning Arrester L-JERE is a simple and effective lightning ar1 1 rester. It consists of a good heavy spark plug, and a piece of pipe, three or four feet long, into which the plug can be screwed. The pipe is driven into the ground and the spark plug screwed into it. The ground connection is automatically taken care of. The wire from the antenna is led to the binding post on top of the spark plug and from there to the receiver. That is all there is to it. George Koether, Jr., Round Bay, Maryland. 0.0005 mfd. To Ant. Post on Set FIG. 6 A simplified version of Fig. 5. Both Fig. 5 and Fig. 6 are most effective when used with short antennas