Radio Broadcast (May 1929-Apr 1930)

"Z_ .RADIO BROADCAST ADVERTISER. Technical data and fine descriptions may help, — but in the final analysis, nothing counts like— PERFORMANCE!— And you can rely on Electrad for that. ADJUSTABLE SLIDING CLIP U. S. Pat. No 1676869 i Pats. Pern) TRUVOLT All-Wire Resistances Nothing better for Eliminator or Power-Pack construction. Distinctive design that insures accurate values, cool, safe operation and ENDURANCE that cuts down service and replacement expense. The sliding clip, an exclusive Electrad feature, provides easy, convenient variability. 22 stock sizes. Also Variable Tru volts with same features. U. S. Pats. No. 1034103-1034104 and Pats. Pending. SuperTONATROL 5Watt Volume Control You no longer need compromise on a volume control. You can now plan to use any reasonably high voltage and depend on Electrad's new 5watt volume control to handle it perfectly without strain. A new principle throughout — smooth as velvet — and with longer life than you'll ever demand of it. Can be made with a variable or tapered curve with practically any desired rating. Sample and laboratory graphs to any established manufacturer. Test it in your own way. ELECTRAD, INC., Dept. R B 6 175 Varick St., New York, N. Y. Please send me data and sample of — Truvolt Resistance — Super-TONATROL. Manufacturer Address City State No. 285 Radio Broadcast Laboratory Information Sheet June, 1929 Frequency Vs. Capacity and Inductance In "Laboratory Information Sheet" No. 286 are given a group of curves indicating what capacity is necessary to tune a circuit to a given frequency when using a coil of known inductance. The curves are applicable to broadcast frequencies and the capacities cover the range of sizes ordinarily used in such receivers. The curves were calculated by substituting in the formula. f = 159,200 where f = frequency in cycles per second C = capacity in microfarads L = inductance in microhenries The curves were calculated for various frequencies between 500 and 1500 kc. A few examples will indicate quite clearly how the curves are used. Example. What size condenser is necessary to tune to 600-kilocycles with a 200-microhenry coil? To determine this we locate the vertical line corresponding to 200 microhenries and follow it to the 600-kilocycle line and it is found that this point corresponds to 0.00035 mfd. which is the capacity necessary to tune the coils to 500 meters. Example. To what frequency will a circuit tune if it consists of a 250-microhenry coil and a 0.0004-mfd. condenser? The unknown frequency in this case is determined by finding the intersection of the vertical line corresponding to 250 microhenries and the horizontal line corresponding to 0.0004 mfd. They intersect at the line corresponding to 500 kilocycles which is the frequency to which they would tune. Example. How much inductance is required in parallel with a 0.0003-mfd. condenser to tune to 1500 kilocycles? Determine the intersection of the horizontal line corresponding to 0.0003 mfd. with the transverse line corresponding to 1500 kilocycles. The intersection is found to fall on the vertical line corresponding to 38 microhenries which is the required value of the coil's inductance. If it is desired to make calculations of inductance, capacity, or frequency for values above or below the broadcast bands, the formula given at the beginning of the sheet may be used. It is simply necessary to substitute the known quantities and solve for the unknown. No. 286 0.002 0.0015 d 0.0007 ^ 0.0006 z 0.0005 £ 0.0004 < 0.0003 Badio Broadcast Laboratory Information Sheet June, 1929 Frequency Vs. Capacity rnd Inductance 0.0002 0.00015 0.0001 • 1 — |— r— Rimn RRDAnrAQT I jABOR ATC RY -< q *< 10 100 INDUCTANCE IN MICROHENRIES 1000 No. 287 Radio Broadcast Laboratory Information Sheet Protecting Meters June, 1929 Several Readers have written us and re^ quested suggestions on how to protect a milliammeter in a set-tester or tube-tester from damage in case there is a defect in the circuit of the device being tested which would permit sufficient current to flow through the meter to damage it. The simplest way of protecting the meter is by the use of the arrangement indicated in sketch A on this sheet. M is the meter to be protected and it is protected by the shunt circuit consisting of R and the switch S. The switch, S, is the type which is usually used as a voltmeter switch; it normally remains in a closed position and must be held in the open position by hand. The resistance, R, should nave a value such that, with the switch closed and maximum rated current of the meter flowing through the circuit, the meter gives a very small deflection. For example, suppose that the meter had a range of 10 inilliamperes. Thej procedure would be to pass 10 mA. through the meter so that the meter read a maximum and then to place across the meter a resistance such that the meter deflection decreased to, suy, 0.5 mA. Now when we use the instrument in which the meter is located we first note the reading of the meter with the switch closed (its normal position) and if the meter reads more than 0.5 mA. we will know that excessive current is flowing through the circuit and the meter will be overloaded if the switch, S, is opened. If the meter reads less than 0.5 mA. it will be safe to open the switch. Another good method of protecting a meter is by the circuit arrangement in B. In this sketch R2 is a rheostat with a resistance of about 10 ohms. The procedure here is to start with the arm at the right and then to move gradually the arm to the left end. If as this end is approached the meter needle goes off scale it is an indication that current in excess of what the meter can read is flowing through the circuit. R (A) (B) • june, 1929 page 118 •