Radio Broadcast (May 1928-Apr 1929)

202 RADIO BROADCAST ADVERTISER SHEETS "THE 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, set builder or service man. 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 York, 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. 249 Radio Broadcast Laboratory Information Sheet January, 1929 A Resistance-Coupled Amplifier Clear Silver ToiieS -/rom the microphone to you EXPERT workmanship, correct design and the careful selection and testing of all materials are responsible for the great popularity of CeCo tubes. You'll find a CeCo tube will last longer, perform better and give you more genuine enjoyment from your set. There is a Ce Co tube for every meed and they cost no more. They are the best engineered tube in the industry. Sold everywhere. CeCo Mfg. Co., Inc. * Providence, R. I. Radio Tubes PARTS required f\N LABORATORY Sheet No. 250 is published ^ the circuit diagram of a resistance-coupled amplifier illustrating the use of filter circuits in the plate and grid leads. As explained in Sheet No. 243, lack of proper filter circuits will cause distortion due to common coupling in the plate supply. It will frequently be worth while to incorporate such filter circuits in existing resistance-coupled amplifiers, especially if the amplifier exhibits a tendency to " motorboat " or distort. In operating a resistance-coupled amplifier it is especially important that overloading be prevented by keeping the volume down to the point where none of the tubes draw grid current, and it is up to the user of the amplifier to operate it so that grid current does not flow. In constructing the amplifier illustrated on the next sheet the following parts will be required: Ri — Three plate-coupling resistors. 250,000-ohm; R2 — Three grid resistors, 2-megohm; Ri — Three plate-circuit filtering resistors, 25,000 ohm; R4 — Three grid-circuit filtering resistors, 50,000ohm; Rs — Filament rheostat, 6-ohm; Ci — Three coupling condensers, 0.01-mfd.; C2 — Six by-pass condensers, 1-mfd.; C3 — One by-pass condenser, 0.0002-mfd.; C4 — Output condenser, 4-mfd.; Li — R.F. choke coil; L» — Output choke coil, 30-henries; Sw — Filament switch. The detector and the first two of the audio amplifiers may be 240-type tubes and the power tube may be any type, depending upon the personal preference of the builder. The voltages applied to the B-plus power terminal and the C-minus powerterminal will, of course, depend upon the type of power amplifier; it is recommended that a 171atype power tube be used. The simplest and most satisfactory construction to follow in building a resistance-coupled amplifier is to mount the tube sockets and the resistor mounts for the grid and plate-coupling resistors all in a line. With this arrangement the grid and plate leads between the tubes and the coupling resistors are very short.