Radio Broadcast (May 1928-Apr 1929)

"Our Readers Su iii!:iU!i!i!i.,i;r,, ■:;i!,::i.»!,.ii:ii:;i:ii:iir,;i;i;:i:ii.ii:iinn Increasing the Output Voltage of B -power Units IN PAST numbers of this department suggestions have been offered by readers for the modernization of power supply devices designed before the advent of high-voltage rectifying tubes and power tubes. These suggestions have generally considered the possibilities of improved voltage divider systems, the addition of C-supply resistors and the slight gain effected by the use of new rectifying tubes. Two readers of this department, William C. Millar of Long Island City, N. Y., and Wallace Allen of Denver, Colorado, have simultaneously contributed details of a simple device that, connected exterior to the power supply unit, increases the output voltage of the transformer, making it possible to take full advantage of the new tubes and voltage distribution system. Both contributors have suggested the same arrangement, but, as they individually cover slightly different phases of its possibilities, the editor has combined their two contributions into a single article, rather than accepting only one of them. The device recommended has two principal advantages. It permits the use of C bias resistors in the negative plate current return without reducing the applied plate voltage below the potentials secured without the bias resistors and the booster. Also, it raises the output of powerpacks designed for the original Raytheon and similar tubes to voltages suitable for the efficient operation of modern power tubes. The arrangement, fundamentally, is a transformer booster, raising the line voltage from approximately 115 volts to 130 volts before applying it to the input of the power supply transformer. The output of the power supply source is increased in proportion. The filter systems of present-day line power devices built by reputable manufacturers have a factor of safety permitting this overload. This is not generally true of units designed several years ago and there is a possibility that filter condensers will break down if this method of increasing the output voltage is applied to some older B-power units. The only additional part required is a small toy or bell ringing transformer having a secondary output of about fifteen volts. This is connected between the line and the power supply unit in accordance with the diagram, Fig. 1. The secondary of the step down transformer is connected in series with the primary or line voltage, giving an additive effect when connected properly. (If the output voltage of the power unit is less when the booster is used, the connections to the secondary should be reversed.) So connected, the device is, in effect and actually, an auto-transformer. A new BH Raytheon tube (or similarly perfected rectifying tube) should be substituted for the type employed with the lower voltage in order to take full advantage of the increase. Mr. Millar suggests that this arrangement might well be employed even with modern power fl)SV 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 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 hoard 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. supply units to compensate poor line regulation in suburban districts. In this case a tapped boosting transformer such as the Ives type 204 and the Thordarson TY121 will provide the desired regulation. Cleaning Corroded Battery Terminals ASIDE from being unsightly and causing general corrosion of A-battery leads, dirty terminals on a storage battery are responsible for much of the so-called "static" experienced in many receivers. Dirty terminals are easily cleaned by washing with a saturated solution of baking (bicarbonate) or washing soda. The terminals should be swabbed with this solution until there is no more effervescence. The terminals and battery top may then be wiped perfectly clean with a rag. In addition to cleaning the battery, the soda wash effectively puts a stop to the corrosive action by neutralizing the acid — an effect that cannot be secured by ordinary cleaning. A. J. Peters, San Francisco, Cal. New Use for a Block Filter Condenser THE more popular models of dynamic cone loud speakers, such as the Jensen or Magnavox, require a six volt source of magnetization current. I find that this can be adequately supplied by many of the popular trickle chargers with a Tobe A Block connected across its output to eliminate hum. This combination is altogether satisfactory, not a trace of a.c. being perceptible in the speaker, which functions at full efficiency. Albert E. Chase, Quebec, Canada. Simplified Plate Rectification THE advantage of plate rectification — the lessened possibility of introducing distortion through overload of the detector tube — has been described on various occasions in "Our Readers Suggest" columns. In some cases this advantage fails to justify the loss in detecting efficiency. It would be well to determine, before permanently changing the wiring in the receiver, whether or not satisfactory signal strength will be obtained on the desired stations when using plate rectification. This can be found out in a half minute by substituting a C battery for the grid leak in the average receiver. The C battery is connected across the grid leak mounting. It is not necessary to remove the grid leak. The negative side of the C battery is connected to the grid • side of the leak. The potential should be varied between 1.5 and 9.0 volts, the plate voltage being changed from 22.5 to 60.0 volts for each C battery adjustment, in an endeavor to determine a combination giving the greatest signal response. Henry Whitehall, Camden, N.J. Parallel Plate Feed THE use of a parallel feed, i.e., a separation of the direct current and alternating current paths in the plate circuit of a vacuum tube, certainly has no claim to novelty. This system of connection has not, however, achieved the popularity to which its merit would seem to entitle it. So far the set builder is familiar with the circuit only as associated with the loud speaker, where the use of the so-called "speaker filter" has become general. The parallel plate circuit applies the principle of the speaker filter to the audio amplifier. The circuit is illustrated in the diagram in Fig. 2. Inductance L should have a high value of about 100 henries. It must be of such construction as to maintain its inductance at currents of several milliamperes. Capacitor C is a condenser of sufficiently large capacity to offer a low impedance at low frequencies — from 2 to 4 >I15 / V. Ill" \ £< Toy £■ <S RJ Trans -O C E former O al 111' CO <lll[li 15V. 130 V to Power Supply FIG. I A simple and inexpensive booster arrangement which will increase the output of a power supply device from ten to fifteen per cent. 215