Radio Broadcast (May 1929-Apr 1930)

.RADIO BROADCAST ADVERTISER. Tone OualitySensitivity Voltime^Clarity a 7 this andmore by usm t\adioTubes F you're tone conscious you'll sense the finer, purer tone quality that CeCo's provide... the greater sensitivity they give the set ...the greater volume without distortion they afford. What you will discover also (after you've enjoyed them clear to the end of their long, useful life) is the many extra months of perfect service CeCo's provide. Hear a set of CeCo Tubes today — most good dealers have them. Listen to the sparkling program of music and comedy by the CeCo Couriers Monday Evenings — 8:30 Eastern Daylight Saving time over WOR and the Columbia Broadcasting System. CECO MANUFACTURING CO., INC. PROVIDENCE, R.I. Used By Millions The Radio Broadcast LABORATORY INFORMATION SHEETS By HOWARD E. RHODES T^HE aim of the Radio Broadcast Laboratory Information Sheets is to present, in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, engineer, or serviceman. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, Radio Broadcast's Data Sheets, may now be bought on the newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New \ork, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Broadcast accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00 instead of the usual subscription price of $4.00. — The Editor. No. 283 Radio Broadcast Laboratory Information Sheet Julie. 1929 Hum-Voltage Characteristics (226-and 227-type tubes) TT IS becoming increasingly common to find ■*■ that most recent models of various wellknown receivers use 227-type tubes in all the sockets rather than only in the detector socket with 226-type tubes in the r.f. stages and in the first a.f. stage. To explain this trend in receiver design the statement is generally made that 227-type tubes produce less hum than 226-type tubes. This is true — but it isn't an explanation. Why the 227 is a better tube is indicated by the curves on "Laboratory Sheet" No. 284 taken from the Cunningham Tube Data Book. Curve A is for a 226-type tube and shows the relation between the hum voltage in the plate circuit of one of these tubes as a function of the plate current. The minimum hum voltage indicated by this curve is about that obtained from a 226-type tube under normal conditions. If, however, the plate voltage increases or decreases somewhat there is a rapid increase in the amount of hum. If the plate potential were 90 volts and the bias about 6 volts minimum hum would be obtained, but a 10 or 15-volt decrease in plate voltage would double the hum output. Curve B shows a comparison between the 227 and 226-type tubes with reference to hum. This curve shows hum output as a function of the accuracy of the center-tapped resistor connected across the tube's filament. It should be noted that the 227-type tube is affected only slightly by an unbalance of the centertapped resistor whereas the 266-type tube necessitates the use of a very accurate centertapped resistor. Specifically, the curve shows that if the resistor is unbalanced ten per cent, the hum voltage from a 227-type tube is increased very slightly. On the other hand, a ten per cent, unbalance in the resistor across the 226-type tube causes the hum voltage to increase from a minimum of 10 millivolts to about 600 millivolts! The rather recent improvements in radio receivers— in loud speakers particularly — has made it especially important that everything possible be done to keep the hum output at the lowest possible level. The hum is not only annoying, as of itself, but it also has an apparent effect on the fidelity. When stimulated by a tone such as a hum, it is difficult for the ear to hear other tones of the same or nearly the same frequency and so we get an apparent reduction in low-frequency response. This technically is known as the "masking effect" of one tone on another. No. 284 Radio Broadcast Laboratory Information Sheet Hum-Voltage Characteristics (226-AND 227-TYPE tubes) June, 1929 TYP E226 3 4 5 6 PLATE CURRENT (A) 600 § 400 i 300 s 200 X 100 0 226J 227, "15 10 5 0 5 10 15 20 PERCENT UNBALANCE (B) • june, 1929 1>U£4' 116 •