Radio Broadcast (May 1928-Apr 1929)

18 RADIO BROADCAST ADVERTISER hrit use old or InferiorTubes with New ones Use new Jf^ RADIO ^TUBES> throughout and enjoy modern radio reception at its best E. T. Cunningham, inc. ;. New York Chicago .•; j:). San Francisco 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 cut out 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. This month we print an index covering the sheets published from August, 1927, to May, 1928, 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. 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. No. 185 Radio Broadcast Laboratory Information Sheet Tube Overloading May, 1928 EFFECT OF INCORRECT VOLTAGES TOURING recent years many familiar types of radio tubes have played the role of "Jack of all trades," and as a result have frequently been placed in service under conditions never intended or contemplated by the manufacturer. What constitutes "overload" on a tube, resulting in shortened life? It might be imagined that the last tube in a receiver tuned-in on a strong local station, and with the volume turned up beyond the point where the music sounds clear, would fall under this classification, but this is not the case. This is a form of overloading, but one which only results in distorted music, and in general the tube is not affected at all. A severe overload permanently affecting the tube occurs, however, when the manufacturer's specifications as regards filament, plate, and grid voltages are disregarded and higher voltages are used. One of the popular tube types affords a good illustration of this condition. The voltages recommended for type 201-A tubes are a filament voltage of 5.0 volts, and plate voltages of 90 to 135 volts, with the grid bias specified as -4.5 and -9.0 volts respectively. If the grid bias of 4.5 volts recommended at 90 volts is omitted it is equivalent to adding about 35 volts to the plate voltage, or in other words, is equivalent to operation of the tube at 125 volts with — 4.5 volts bias. The overload is, of course, correspondingly more severe if the plate voltage is raised. This is clearly shown in the table below : Plate Grid Current Extent of Volts Volts MA. Overload 90 4.5 2.0 Below maximum 135 9.0 2.5 Normal 90 0 6.0 58% 120 0 9.8 240% 135 0 12.0 380'; The 201-A type tube is capable of withstanding some overload more successfully than other types of tubes, but as a general rule it is always advisable to follow the manufacturer's ratings regarding tube voltage. No. 186 Radio Broadcast Laboratory Information Sheet The UX-250 and CX-350 , 1928 A NEW POWER AMPLIFIER THE ux-250 (cx-350) is the latest tube designed for use as a power amplifier to supply large amounts of undistorted power for the operation of loud speakers. The large output obtainable from this tube prevents any possibility of overloading of the last stage of an audio amplifier. The filament rating is 7.5 volts, 1.25 amperes. The material used in the filament is the rugged coated ribbon form, similar to that used in the UX280 (cx-380) rectifier, the filament operating at a dull red. The filament current may be supplied from the 7.5-volt winding of a power transformer. The low operating temperature and the increased size of this type of filament results in minimum ripple voltage or "hum." It should be noted that, although the filament and plate voltages are the same as those for the ux-210 (cx-310) tube, the plate current is 55 milliamperes at a plate voltage of 400 volts whereas under similar conditions, the plate current of the ux210 (CX-310) is only 18 milliamperes. The grid voltages for these two tubes, at a plate voltage of 400 volts, are respectively 70 and -31.5. the larger voltage being necessary on the ux-250 (cx-350) tube. Because of the higher plate current and grid bias required by this new tube it cannot always be used to replace the ux-210 (cx-310) tube without changing the circuit. Plate Voltage Negative Grid Bias Plate Current Plate Resistance (a.c.) Mutual Conductance Voltage Amplification Factor Max. Undistorted Power Output Filament 7.5 Volts 1 . 25 Amperes Max. Overall Height 6}" Diameter 2 \\" Base: Large Standard ux (cx) Recommended 250 300 350 400 45 54 63 70 28 35 45 55 2100 "2000 1900 1800 1800 1900 2000 2100 3.8 3.8 3.8 3.8 900 1500 2350 3250 Maximum 450 Volts 84 Volt 55 Milliamp. 1800 Ohms 2100 Micromhos 3.8 4650 Milliwatts