Radio Broadcast (May 1928-Apr 1929)

"Our Readers Suggest 9 9 Checking Power Unit Foliages MANY readers of Radio Broadcast have constructed socket power devices providing B and C voltages. The adjustment of the resistors in such apparatus is generally a matter of guesswork, the values being varied until the reproduction sounds about right. An accurate determination of voltage values is generally a matter of purchasing an expensive high-resistance voltmeter. The former method is generally unsatisfactory because modern receivers are designed to operate at maximum efficiency with certain definite plate voltages. The following paragraphs describe a simple manner of checking the voltage outputs of a power unit using parts generally on hand: The average experimenter possesses one or two 45-volt B batteries, an ordinary voltmeter, and a pair of earphones. The power unit and the receiver are both set in operation and rough adjustments are made by ear. Ninety volts of B battery is connected with its negative terminal to the minus post of the unit. Let us assume that we wish to adjust the 45-volt tap, supplying the detector tube, to the correct voltage. One lead from the phone should be connected to the 45-volt tap on the battery and the other should be placed on the tap of the power unit. Each time the connection is made or broken, a click will be heard in the receiver, provided the two voltages are not identical. It is therefore simply a matter of slowly adjusting the tap on the power unit until no sound is heard in the phone receiver when the circuit is made or broken, or until the click is at a minimum. Under these conditions, the voltage at that tap is exactly equal to the voltage at the battery tap. The ordinary voltmeter can be used to check the battery voltage. The same steps are taken for the 67.5 volt and 90-volt taps. Should only one 45-volt battery be available, we simply "step up" the voltage by connecting B-minus of the battery to plus 45 of the power unit as previously determined, and then use plus 45 of the battery to adjust the 90-volt tap of the power unit. Where small C bias voltages are obtained B+ Det FIG. I The preferred jack arrangement in the plate circuit of an a.c. detector tube from the same unit through the drop in a resistance, the same method of adjustment can be used by making use of a C battery with a 3-volt and 4.5-volt tap. W. A. Grobli, New York City. STAFF COMMENT HpHE arrangement suggested by Mr. Grobli A is more useful as a means of checking plate voltages than for setting them at some predetermined value. It often happens that receivers c/Ste 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 exposition, 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 overfJois "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. ?I)<3V operate most efficiently from socket-power plate voltages which are different from the optimum battery voltages. In the majority of instances the problem will be to determine the plate voltages applied to different portions of the receiver rather than to adjust these values to a given potential. When this is the case, a 2000-ohm potentiometer should be shunted across the B block with the lead from the telephone receiver wired to the movable arm. This will make it possible to obtain practically any voltage within the range of the battery. A Convenient Tele-phone jack Arrangement IT IS often desirable to plug the telephone receivers into the plate circuit of the detector tube without upsetting the audio-frequency amplifier. However, most jack arrangements in the detector plate circuit open the primary of the first amplifying transformer resulting in a loud howl from the loud speaker unless the amplifying tubes are turned off. The sketch in Fig. 1 shows a simple cir '51 cuit arrangement employing a double contact jack such as the Yaxley No. 5, which shortcircuits the primary of the transformer when the phones are plugged-in, preventing amplifier instability and howling. The current drain of the receiver remains practically constant regardless of whether phones or speaker are employed. P. H. Greely, Washington, District of Columbia. Neutralising the Short-Wave Amplifier \ A 7H1LE experimenting with radio-frequency ^ ' amplifiers on waves between 1 5 and 200 meters, I found it next to impossible to neutralize the grid-plate capacity of the r.f. tube. I tried all of the popular circuits for neutralizing with discouraging results. 1 experimented with dozens of different plate coils having from one to ten turns and with different values of coupling to the detector grid coil. I finally decided to stick to one circuit and fight it to a finish. The Rice circuit was chosen. With this circuit, if the filament tap is in the exact center of the coil and the capacity of the neutralizing condenser has the same value as the grid-plate capacity of the tube, it remains neutralized regardless of the setting of the tuning condenser. Even with this circuit I could not completely neutralize the r.f. stage although the detector and its associated parts was enclosed in a copper shielded box. I came to the conclusion that the plate coil offered too much capacity coupling to the detector grid coil. In this case an electrostatic shield would be needed to eliminate the capacity coupling. I took the primary of three turns of No. 20 d.c.c. wire as a form and wound it toroid fashion, full of No. 26 d.c.c. wire as in Fig. 2. When connected in the circuit one end of the shield winding is left open and the other was connected to the ground. It is now possible to neutralize the grid-plate capacity of the r.f. tube properly. The same shielded plate coil was used for all FIG. 2 The method of winding the shield coil about the short wave r.f. primary. One side of the toroidal coil is grounded