Radio Broadcast (May 1928-Apr 1929)

378 RADIO BROADCAST ADVERTISER TWO PIONEER TELEVISION ACCESSORIES The Raytheon Laboratories invite correspondence from both engineers and amateurs in regard to these two accessories now in successful operation. Raytheon Kino-Lamp is the first television-reception tube developed to work on all systems. Raytheon Foto-Cell, an extra sensitive broadcasting tube, is supplied in either hard vacuum or gas-filled types. RAYTHEON MFG. CO. Kendall Square Bldg., Cambridge, Mass. ^Raytheon' \ FotPCell . SHEETS HpHE aim of the Radio Broadcast Laboratory Information Sheets is to present, in a 1 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. 225 Radio Broadcast Laboratory Information Sheet October, 1928 Calculating Grid Bias for A.C. Tubes correct resistance values TN ALL a.c. receivers, grid bias for the various *■ tubes is obtained by connecting resistances of the correct value at the correct point in the circuit. The calculation of the value of the resistance and its placement in the circuit have been the subject of quite a few letters written to the Technical Information Service and we have therefore devoted this Laboratory Sheet to the subject. The circuit diagrams of six combinations are given on Laboratory Sheet No. 226. If these diagrams are examined one important point will be noted, which is that the resistance, R, which supplies C bias to the tube, is always connected between the center of the filament, or the cathode in the case of heater type tubes, and negative B. The resistance is placed in this position in relation to the circuit no matter what tube or combination of tubes is used. With the resistor in this position the plate current of the , tube must go through it in order to reach the filament, or cathode, and therefore the voltage drop across the resistance is equal to the plate current times the resistance in ohms. To calculate the value of resistance, we must therefore know the value of grid bias that we desire to obtain and also the plate current flowing through the resistance. For example, in diagram A we have indicated a 226 type tube. By reference to any table of tube characteristics we can determine that the 226 type tube with 90 volts on the plate requires a grid bias of 6 volts and the plate current is 3.5 milliamperes. R is found by dividing the grid voltage required, 6, by the plate current in amperes, 0.0035, which gives a value of 1700 ohms as the required value of resistance. In diagram C, a 171 tube is used, forty volts of grid bias are required if the plate voltage is 180 volts. The plate current under such conditions is 20 milliamperes, and 40 divided by 0.02 amperes gives 2000 ohms as the value of resistance required for C bias. If a circuit utilizes more than one tube of the same type for which we require the same value of grid bias, the circuit is arranged as indicated at E, in which case the plate current of both of the tubes flows through resistance R. If the plate voltage on the 227 type tube is 90, the plate current is 3.7 milliamperes and the required grid bias is 6 volts. The grid bias resistance is then equal to 6 divided by 7.4 (the total current of the two tubes) which gives 800 ohms as the correct value for R. • 226 Radio Broadcast Laboratory Information ?heet October, 1928 Grid Bias Circuits for A.C. Tubes Type 226 1 ype 227 Type 171