Radio Broadcast (May 1928-Apr 1929)

RADIO BROADCAST ADVERTISER omze Helium THE SECRET OF THE STANDARD REPLACEMENT [UBEA B EUMINAK YUc use ot Ionized Helium, gives to the Raytheon "BH" Tube a superior ruggedness, a far longer lite .md .i sustained voltage. Be sure you get a Raytheon BH tor your eliminator when the tube needs replacing which is generally every nine to twelve months. /Raytheon \ *%. 10NG LIFE RECTIFY! N t! TUBE ^ HERE I U BE CHECKER Which Requires NO BATTERIES Operates direct from the A. C. LIGHT SOCKET Or any other A. C. — 60 cycle — 90 to 130 volt source of supply Proper Voltage Regulation is quickly obtained by means of the Voltage Adjusting Dial and the Voltage Indicator. Will test ever) type of rube A. C. or D. C. — having filament voltages of 1.5, 2.5. 3.3. 5 or 7.5 volts, including filament type rectifying tubes. Check Tubes at Time of Sale It prevents comeback? and makes satisfied customers. Your jobber will supply you, or write direct to: Weston Electrical Instrument Corp. 604 Frelinghuvsen Ave., Newark, N. J. WESTON RADIO BNSTRDMENTi The Radio Broadcast LABORATORY INFORMATION SHEETS THE Radio Broadcast Laboratory Information Sheets are a regular feature of this magazine and have appeared since our June, 1926, issue. They cover a wide range of information of value to the experimenter and to the technical radio man. It is not our purpose always to include new information but to present concise and accurate facts in the most convenient form. The sheets are arranged so that they may be cut from the magazine and preserved for constant reference, and we suggest that each sheet be cutout with a razor blade and pasted on 4" x 6" filing cards, or in a notebook. The cards should be arranged in numerical order. In July, 1927, an index to all sheets appearing up to that time was printed. Last month we printed an index covering the sheets published from August, 1927, to May. 1028, inclusive. All of the 1926 issues of Radio Broadcast are out of print. A complete set of sheets, Nos. 1 to 88, can be secured from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Orders for the next set following can also be sent. Some readers have asked what provision is made to rectify possible errors in these sheets. In the unfortunate event that any serious errors do occur, a new Laboratory Sheet with the old number will appear. — The Editor. Radio Broadcast Laboratory Information Sheet August, 192c Selectivity AS EFFECTED BY NUMBER OF R.F. STAGES THE selectivity of a radio circuit depends upon many things, including the number of tuned stages, amount of coupling in the r.f. transformers, the characteristics of the tubes, the amount of regeneration in the circuit, the accuracy with which the individual circuits are tuned, etc. In this Sheet we will consider the effect on selectivity of increasing the number of tuned stages in a receiver. Future Laboratory Sheets will discuss the influence, on selectivity, of some of the other factors mentioned above. Curve 1 represents the selectivity curve of a single r.f. stage. At a point 5.000 cycles off resonance the circuit gives S3 per cent, of the amplification at resonance: at 10.000 cycles off resonance the amplification has dropped to 60 per cent. Now suppose we" add more r.f. stages, with characteristics the same as that of the first stage. We then get the selective action " 10.000 5000 0 5000 indicated in Curve 2. cycles off resonance If, at a certain point off resonance, the f.rst stage reduced the amplification to 83 per cent., then the second stage would reduce the amplification to 83 per cent, of what came through the first stage. Refering to the curves, at a point 5,000 cycles off resonance, the various stages introduce a selective action as indicated below. First stage = 83 per cent. Second stage = 83 x S3 = 69 per cent. Third stage = S3xS3x83 = 57 per cent. Fourth stage = S3 x 83 x S3 x S3 = 47 per cent. This means that if we had a four-stage r.f. amplifier with these characteristics, that a signal 5,000 cycles off the resonance frequency to which the stages were tuned, would be amplified only -17 per cent, as much as a signal at the resonant frequency. Since a radio wave includes modulation frequencies up to 5,000 cycles off resonance, it is evident that such an r.f. amplifier would cause considerable side band suppression with consequent signal distortion. 10 BROADCAST AB0RAT0RY RAt k I V, // ! A \ ^ / i \\\ — si SI s ii , A/ 1 r ! 1 1 1 No. 210 Radio Broadcast Laboratory Information Siheet Protecting the Rectifier Tube August, 1928 A PILOT LAMP TO INDICATE OVERLOAD TV/TOST of the rectifier tubes available at the present time will be Beverly injured if they are subjected to accidental short circuits or to excessive overload for any considerable period of time. In constructing power units it is therefore wise to place in the circuit some device which will serve to indicate any overload. Such an indicator is described in this Sheet and is applicable to power units of all types whether they use gaseous or filament type rectifiers. The main precaution to be observed in operating rectifier tubes is that of avoiding an overload with respect to plate current. The shorting of the rectifier output, such as may occasionally occur due to the failure of some part of the apparatus (as by the breakdown of a filter condenser 1 will overload the filament and result in filament failure (in the case of filament type rectifiers) unless the current is turned off promptly. To indicate an overload it is a good idea to connect a small lamp in series with the rectifier output as indicated at B in the circuit diagram on this Sheet. A small lamp such as is used as a dial light may be used for this purpose. Excessive brilliancy of this lamp will immediately indicate an overload, which can then be remedied before damage results. In constructing power amplifiers and B supplies it is also a good idea to place a fuse in the primary side of the power transformer. This fuse will protect the transformer from Filter System damage in case its secondary is accidentally short circuited. The fuse should preferably be of the ordinary plug type with a rating of about three amperes. Only one fuse need be used, connected in series with one side of the line and the transformer.