Radio Broadcast (Nov 1926-Apr 1927)

MARCH, 1927 B POWER-SUPPLY DEVICES 479 amperes. This device, being equipped with a regulator tube has a good output. It would therefore, be suitable for use with a receiver using an ux-171 tube in the last stage. Device No. 10 would also power such a receiver satisfactorily. The voltage output from each of the other voltage terminals also falls proportionally as these curves indicate. If accurate data as to the output at various current drains is not available, it is impossible to obtain it with an ordinary voltmeter. The current drain of an ordinary voltmeter is sufficient to modify greatly the output of the B device so that the reading is valueless. A special high-resistance instrument, such as a Jewell No. 116 or a Dongan, Type A, is necessary to measure the voltage output correctly. Some power-supply devices are equipped with C battery output terminals, eliminating the necessity for a C battery as well as B battery. C batteries are inexpensive and usually give at least a full year's service, hence their elimination, although a convenience, is not of paramount importance. CAUSES AND ELIMINATION OF THE HUM THE silence of output is an important factor in the service ability of B power-supply device. This is universally claimed for every power device which we have seen, although attained by comparatively few. The fact that a hum is drowned out by a loud program does not alter the fact that it introduces distortion and imperfection in the musical output. We quote the following sentence from the advertising literature of one manufacturer to show how freely the quality of silence is claimed: The B-supply device is totally free from hum, even on earphones. Then, turning to the operating instructions which the customer receives after he has purchased this brand of B device, we read : Turn on the current by means of the switch on the supply cord. A very slight hum should be discernible when you listen with each ear about a foot from the unit. This hum shows it is functioning properly. Under the circumstances, the only criterion in judging the quietness of B-supply device is the listener's own ears. The hum may be due to inherent faults in design or, with better devices, to the manner in Receiving Tubes Grid Potential + 1 1 (Det.) 0 —11 -3 — 4§ —9 Plate Potential — > 22| 45 90 22 J 45 67* 90 45 67| 90 45 90 90 135 Tube: 201-a .65 2.3 7.7 .25 1.3 3.4 6.0 .6 2.1 4.7 .25 3.2 2.0 2.5 200-a .75 3.0 5.5 1.1 2.0 2.9 4.0 1.0 1.5 2.5 .4 1.1 .6 199 .61 2.0 5.0 .36 1.4 2.8 4.2 1.0 2.1 3.6 .5 3.0 2.5 Power Tubes Grid Potent al —9 —16* —18 — 22i 27 —35 -40i Plate Potential—^ 135 90 90 135 250 135 135 350 425 180 Tube: 112. 9.0 120 3.1 7.4 171 10.0 16.0 20.0 210 12.0 18.0 22.0 which they are installed. Cheap devices have small smoothing condensers and chokes. This accounts not only for their objectionable hum but also for the manufacturer's ability to produce the device at low cost. More expensive A TYPICAL B-SUPPLY DEVICE Similar to many tested in the Radio Broadcast Laboratory. It is a product of the Cornell Manufacturing Company, Long Island City devices sometimes hum, but this is generally due to improper adjustment. The most common cause of hum is magnetic induction from the transformers and chokes into the receiving set. By carefully changing the angle and distance of the B device from the set while it is in use, a position can usually be found which eliminates magnetic induction. Another cause of hum is excessive detector voltage or excessive amplifier voltage, or open or improper C battery connections. A slight hum is not at all objectionable, but, if it can be heard several feet from the loud speaker, your dealer should be consulted immediately, while the device is still new. He may be able to quiet the device by better placing or improved voltage adjustment; otherwise, he should replace it. There is no more satisfactory method of testing a B power-supply device than by giving it several days' trial in your own home. A cut price dealer cannot afford to do this for you. The insurance of satisfaction, resulting from buying from a reputable dealer, makes it wise to pay the extra cost involved. Another brand of blue sky claim, made by some B device manufacturers, are those dealing with the cost of operation. Apparently, B-power units cost next to nothing to operate; a few cents a month at the most. The chemical types draw a heavy current for the first few minutes of operation, when they are first installed, but the power soon falls to a reasonable figure. The average tube or electrolytic device draws from 15 to 3 5 watts, according to load; those with one stage of power amplification, between 45 and 50 watts; and regulator tube equipped devices, from 30 to 35 watts. Using a set an average of 1 5 hours a week, the total watt hours per month for a 30-watt unit is about 2000 watt-hours, or from 14 to 50 cents a month, according to the power rate. But current is not the only cost. To this must be added tube upkeep and depreciation. Figuring five years' life for a $50 device, depreciation averages 80 cents a month. Tubes at $4.00 a year add another 35 cents a month. So that instead of "ten cents a month," as one maker once advertised, costs may be as high as $1.00 to $2.00 a month. This should not, however, be compared directly with B battery costs because good B devices also give much greater convenience, there being no heavy batteries to carry and install. They also give a steady, instead of progressively falling, voltage output. From the foregoing dissertation upon the ills and weaknesses of B power-supply devices, one might conclude that they are very unsatisfactory and uncertain devices. As a matter of fact, their most serious imperfections have already been conquered and, if care is used in selection and a few simple precautions observed in care and operation, they give highly satisfactory and lasting service. NEW UNITS FOR SUPPLYING B POWER The Valley B Power, using Raytheon tube; the Balkite Combination Radio Power Unit, which, when connected to the A battery furnishes power both to A and B circuits. It is controlled by the filament switch on the set and is automatic. Price $59.50. Next is the General Radio Power Amplifier and B supply. Finally, the Ail-American "Constant B" using a Raytheon tube, which sells for $37.50