Radio Broadcast (May 1928-Apr 1929)

138 article. Those who buy the Tobe A-filter need only connect it between the battery-charger and the A-terminals of the receiver and the job is done; those that want to make up their own filter circuit need to get a choke coil and two A-condensers. The completed unit will be as foolproof as it is possible for any power unit to be. [Those receivers which make use of a rheostat in the r.f. filament circuit to control the volume may find the means of superseding the storage battery suggested in this article of some disadvantage. The regulation of the system is quite poor (a characteristic of all A-power units, as far as we know) and when the volume is lowered by decreasing the filament current to the r.f. tubes, the other tubes in the receiver will be supplied with excessive filament voltage. In such cases it will be better to change over to some other type of volume control which does not function by varying the filament voltage. Several methods of alternative volume control were discussed in "New Apparatus" in our June number — Editor.] These condensers require no attention — they are dry and may be operated in any position. Don't try to use these condensers in B-power circuits, in circuits where there is a.c, or in circuits in which the voltage exceeds about 10 volts d.c. The condensers have been designed for use only in low voltage d.c. circuits, such as are found in A-power circuits. THE BEST FILTER SYSTEM THE arrangement of the filter system which will prove best is shown in the illustration Fig. 3. The condensers, Q, Q, and C3, are A type condensers and the chokes. Li and L2, are rated at about 2 amperes and 0.25 henries. The condenser, C3, is not always necessary, but if the audio amplifier in the receiver is a very good one, giving good gain at low frequencies, it may be necessary to include the third condenser, C3, in RADIO BROADCAST JULY, i9'.£ Choke Coil Red + Where to Get the Parts A-CONDENSERS CHOKE COILS COMPLETE A-FILTERS 10VOLT VOLTMETERS RHEOSTATS Aerovox Wireless American Trans Tobe Deutschmann Corp. Dubilier Condenser & Radio Corp. Elkon Works, Inc. Tobe Deutschmann Company former Company Knapp Electric Company Company Knapp Electric Company Burton Carter Rogers Centralab Jewell Clarostat Sterling Frost Ward Leonard Weston Yaxley 10 ohms C ,, C2.C3-A Type Condenser FIG. 3 The circuit of an A-power unit. The filter circuit for an A-power unit made from a battery-charger consists of two choke coils and three A-type condensers arranged as indicated in this circuit diagram. Note that one of the chokes is connected in the positive lead and the other in the negative lead. These A-condensers, unlike ordinary condensers such as are used in B-power units, have a definite polarity and the red lead or the terminal marked plus must always be connected to the positive side of the circuit. The choke coils should have an inductance of about one quarter of a henry and should be able to carry about 2 amperes of current. The 10-ohm rheostat regulates the output voltage. The entire filter circuit may be easily laid out on a wooden baseboard or arranged in more compact form to suit local conditions. Manufacturers making the various parts that can be used are listed in the table accompanying the article Choke Coil Fig. 3. On most commercial receivers with audio amplifiers of indifferent quality the third condenser is not required; good home-constructed receivers generally require the use of the third condenser. In arranging the apparatus, the two choke coils should be placed with their cores at right angles to each other. To control the output of the unit it is necessary to use a heavy duty rheostat with a A Condense; A Condenser A Condenser value of about 10 ohms. [A 10-ohm rheostat may not prove satisfactory if the receiver contains only two or three tubes. A 15-ohm rheostat is more satisfactory in such cases. — Editor.] Manufacturers of satisfactory rheostats will be found listed in the table accompanying this article. It is also wise to include a voltmeter (range 0-10 volts) in the installation so that the rheostat may be accurately adjusted to the point where the correct voltage is applied to the receiver. This voltmeter will also serve to indicate when the line voltage varies and the rheostat can then be adjusted to compensate the variation in line voltage. How the rheostat and the voltmeter are connected in the circuit is shown in the diagram Fig. 3. FIG. 5. A BASEBOARD LAYOUT OF THE FILTER SYSTEM Two choke coils, three A-type condensers, a rheostat, a voltmeter, and four binding posts are indicated in this baseboard layout of a filter system for an A-power unit. Connect a battery-charger to the two left hand terminals and you have a source of filament current for your radio receiver. Operation of your radio receiver direct from the light socket is then possible using standard storage battery type tubes, such as the 201-A. Note in this baseboard layout of apparatus that the two choke coils have been placed at right angles to each other so as to eliminate any possibility of coupling between them. Be sure to note the markings (or color of the wire) on the Acondenser and connect them as indicated in this drawing FIG. 4. PARTS FOR USE IN AN A-POWER UNIT