Radio Broadcast (May 1928-Apr 1929)

"Our Readers Suggest OUR Readers Suggest" is a clearing house for short radio articles. There are many interesting ideas germane to the science of radio transmission and reception that can he made clear in a concise ex-position, and it is to these abbreviated notes that this department is dedicated. While many of these contributions are from the pens of professional writers and engineers, we particularly solicit short manuscripts from the average reader describing the various "kinks," radio short cuts, and economies that he necessarily runs across from time to time. A glance over this "Our Readers Suggest" will indicate the material that is acceptable. Possible ways of improving commercial apparatus is of interest to all readers. The application of the baffle board to cone loud speakers, is a good example of this sort of article. Economy "kinks," such as the spark-plug lightning arrester, are most acceptable. And the Editor will always be glad to receive material designed to interest the experimental fan. Photographs are especially desirable and will be paid for. Material accepted will be paid for on publication at our usual rates with extra consideration for particularly meritorious ideas. — The Editor. An Emergency Detector B Supply IT HAS been my experience that the voltage distributing resistor system in the average Empower unit is the weak point of these devices. On several occasions the resistor passing the current to the detector tube in my receiver has gone bad. I found that an emergency connection could be effected in a few seconds, by wiring an outside resistor from the "detector B" post on the set to the "amplifier B" post on the unit. A ioo,ooo-ohm resistor is about the correct value. The set works quite as well operating the detector tube from the 90 volts power-unit tap, through the external resistor, as it did from the original "detector B" supply post. I rigged up the resistor mounting, shown in Fig 1, to enable me to try different values of resistors. The mounting is wired, as shown, to the "amplifier B " positive post, and the detector lead is caught under the Fahnestock clip. Perry White, New York City. HG. I This simple arrangement can be used to supply either detector or intermediate-amplifier plate potential from a high voltage tap in the case of resistor break-down. mmmmmmmmimmmmmmmmm mm 1 * n m ■ ..lainiiiiiiiyiiiiuiiiiNiiiiniiiiiiiiiiiiNiiiiiiii STAFF COMMENT IN SOME cases it may be desirable to bypass 1 this extra resistor. This can generally be accomplished by leaving the lead to the "detector B" post on the set (now wired to the special resistor) also connected to the original "detector B " supply post on the power unit, taking advantage of the bypass condenser included in that circuit. As the faulty resistor is probably " open " this will have no effect on the potential. If desired, the resistor may be bypassed by connecting any convenient condenser from 1.0 mfd. up, across the resistor. It is also possible to connect the external resistor directly across the "amplifier B" plus and the "detector B" plus posts on the receiver itself, without going back to the power supply unit. The clip-wired Clarostat and the clip-wired FIG. 2 The use of clip connectors permanently fastened to flexible leads for condensers and resistors greatly facilitates experimental and emergency work for many uses around the radio set bypass condensers suggested by Mr. Graham, elsewhere in this department, are particularly convenient in effecting temporary arrangements of this kind. Emergency and Experimental Connections IN MY experimental work, as well as in emer* gency set-ups in the perpetual endeavor to keep the family set functioning properly, I have found it very convenient to have a complete set of parts available for immediate connection by means of clips. The idea, illustrated by a few of the parts so arranged, is made clear in the accompanying photograph (Fig. 2.). The clips used are of the small battery type, obtainable at most electrical and radio supply houses. Any type of ordinary hook-up wire may be used for the leads — I used flexible Celatsite. In the cases of certain parts, such as the condensers which already have long and flexible leads, these may be soldered directly to the clips. The following parts which I happened to have on hand, were arranged with two-foot leads and clips: 1 Volume control Clarostat (variable resistor) 89 1 Universal range Clarostat (variable resistor) 1 Low range Clarostat (variable resistor) 1 Amsco 2000-ohm Duostat 1 400-ohm potentiometer 2 Fixed condensers, 0.001 mfd. 1 Gridleak mounting 2 Bypass condensers with leads, o. 1 mfd. 3 Filter condensers with leads, 4.0 mfd. 1 1000-ohm fixed resistor 1 2500-ohm fixed resistor 1 5000-ohm fixed resistor 1 10,000-ohm fixed resistor 3 60-ohm center-tapped resistors It is also a good idea to have on hand a half dozen or so three-foot lengths of flexible wire with clips on the ends. The above parts arranged for instant clip connections, will be more than handy in all experimental work. Herbert Graham, Chicago, Illinois Volume Control for ResistanceCoupled Amplifiers COME receivers, particularly sets using a.c. ^ tubes, employ a type of volume control such that the signal cannot be reduced without impairing selectivity to a serious extent. This consideration justifies the control of volume at a point in the circuit following the detector tube where it will have no effect on selectivity. Some circuits employ a high resistance potentiometer across the secondary of the first audio transformer in a transformer-coupled amplifier. When using a resistance-coupled amplifier a somewhat similar arrangement can be used, and is suggested in Fig. 3. The coupling resistor in the detector plate circuit is a high range potentiometer (.1 to .25 megohms maximum) such as the Electrad Royalty, with the movable arm connected to the coupling condenser. This arrangement provides adequate volume control without changing the frequency characteristic of the amplifier. H. F. Kucks, New York City. STAFF COMMENT AS MR. KUCKS points out there are certain types of volume controls which impair the selectivity, as for example a variable resistance across the primary or secondary of an r. f. transformer. Such difficulties can be prevented by FIG. 3 A volume control circuit for use with resistance-coupled amplifiers