Radio Broadcast (May 1928-Apr 1929)

Record Details:

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112 RADIO BROADCAST ADVERTISER RADIO #TUBES Guarantee of hidden value and unseen protection in a radio tube is in the name Cunningham —a reputation for quality zealously guarded for thirteen years. Manufactured and sold under rights, patents and inventiona owned and/or controlled by Radio Corporation of America E. T. CUNNINGHAM, Inc. New York Chicago San Francisco No. 195 Radio Broadcast Laboratory Information Sheet June, 1928 A Resistance-Coupled Amplifier With Screen-Grid Tubes CONSTRUCTIONAL DATA THE February, 1927, Radio Broadcast reported some experiments made in the Laboratory on the use of the screen-grid tube in audiofrequency and radio-frequency amplifiers, and in the article there appeared a circuit diagram of a resistance-coupled, audio-frequency amplifier using two screen-grid tubes. Many letters have been received requesting constructional data on this amplifier and we have therefore reprinted the circuit diagram on Laboratory Sheet No. 196 and the list of parts necessary to construct the amplifier appears at the end of this Sheet. The publication of this circuit diagram and list of parts should not be taken to indicate unqualified endorsement of the amplifier for its high voltage gain of 2200 (the voltage gain of an average twostage transformer coupled amplifier is 250) in some cases will prove more of a disadvantage rather than an advantage. The disadvantage of a high gain audio-frequency amplifier will become evident when an attempt is made to operate it from a Bpower unit. When an ordinary amplifier is used with a plate-supply unit which provides hum-free operation no difficulty may ensue; but when this same supply is connected to a high-gain, screen grid amplifier, the hum is greatly magnified and may be of entirely too, high a value. If the screengrid tubes are operated from batteries, however, this amplifier will give very satisfactory results. To construct this amplifier the following parts are necessary: Ri, 0.25-Megohm Resistors i?2, 2.0-Megohm Resistors Rs, 20-Ohm Filament Resistors Ri, 4-Ohm Resistor Rh, 0.1-Megohm Resistor Ci, 0.01 -Mfd. Fixed Condensers C2, 4.0-Mfd. Fixed Condensers C3, 2.0-Mfd. Bypass Condensers Three Sockets Binding Posts No special care is required in the construction of this amplifier although it is wise to arrange the layout so that the various grid and plate leads are short. The condensers O and the resistor R-, are incorporated in the circuit to prevent the amplifier from motorboating. This circuit will also help to keep the hum low if the device is operated from a Bpower unit. A frequency characteristic curve of this amplifier made in this Laboratory showed it to be flat from 100 to 10,000 cycles. . 196 Radio Broadcast Laboratory Information Sheet Julie, 1928 Circuit of a Resistance-Coupled Screen-Grid Amplifier C Supply OOO B B+ B+ B+ B+ t Pet. 45 135 Power, B Supply No. 197 Radio Broadcast Laboratory Information Sheet Amplification Constant June, 1928 HOW IT MAY EASILY BE MEASURED TT IS not difficult with simple apparatus to ■*• measure the amplification constant of any tube The important apparatus required to make such a test are two accurate resistances, one variable, the other fixed, and a milliammeter capable of carrying the normal plate current of the tube under test. The circuit diagram to be followed in making this test is given here. The following parts are used in the circuit: Ec — C-battery with' a value correct for the tube under test. Eb— B-battery with a value correct for the tube under test. Ea — -Source of filament voltage. E — 45 volt B-battery. Ri — Filament rheostat. R» — Accurate 10-ohm resistor. Rj — Accurate variable resistor, having a maxmum value of 300 or 400 ohms. I — -Milliammeter having a maximum range of about 20 milliamperes. K — -Key to open and close the circuit. The important resistor in this circuit is Ri which must be calibrated. A good potentiometer may be used, provided it is supplied with a dial so that the amount of resistance included in the circuit can be calculated. For example, if the potentiometer has a resistance of 400 ohms and the dial reads from 0 to 100 then each degree would include 4 ohms. The test is conducted as follows. With K open, adjust Ec and Eb so that the tube is being operated under the correct conditions of grid and plate voltEge. Note the plate current reading. Now depress K and note the change in the reading of the milliammeter. Adjust Ri so that as the key is opened and closed no change takes place in the reading of the milliammeter. When resistor, Rlr has been adjusted so that the plate current remains constant, calculate the amount of resistance at Ri, included in the circuit. Divide this resistance by 10, the value of R2, and the quotient will be the amplification constant of the tube. Example: A 201-A type tube is being tested and a balance is obtained when there are 83 ohms included in the circuit at Ri. Dividing 83 by 10 we get 8.3, the amplification constant of the tube.