Radio Broadcast (Nov 1926-Apr 1927)

MARCH, 1927 A. C. AS A FILAMENT-SUPPLY SOURCE 497 POWER TRANSRANELl IIP VOLT TAP 3 VOLT TAP 25 VOLT TAP powan TRANSFORMS, leg of the B supply to the filament circuit. The normal biasing voltage for the first and second radio stages and first audio stage is obtained from the voltage drop across the 200-ohm variable resistor in the receiving set on the right-hand side of the first radio tube socket. Since the total plate current of all the tubes of the receiver passes through this resistance, and since this resistance is variable, the biasing on these tubes may be controlled by a variation of this resistance. Its chief value is in the control over the total plate current which energizes the detector filament and which control makes it possible to change the natural vibration period of the detector filament. It is found that a small percentage of the 199 detector tubes, even with the use of the best obtainable cushion sockets, will pick up energy from the loud speaker and develop an audio frequency or acoustic howl between the detector and loud speaker. If the loud speaker is resting in its normal position on the top of the receiver cabinet it is possible to eliminate this howl by a small change in the 200-ohm biasing resistance. This will change the filament current in the detector tube, and so alter its natural vibration period that the sound energy fed back will be out of phase with the vibration in it, and thus stop the howl. The detector filament, as before explained, is inserted in the negative B lead and is energized by the total plate current (approximately 60 milliamperes), of all the amplifier tubes in the receiver. Across this detector tube filament is connected a fixed resistor of 1000 ohms which will maintain the B circuit should the detector tube be pulled out while the set is in operation. A sudden interruption of the circuit by the removal of the detector tube will produce a high inductive voltage across the choke coil which would be impressed upon the filter condensers, and might possibly cause their breakdown were the resistance not to be included. The first and second radio-frequency tube filaments are connected in parallel and have resistances connected in them. The first audio tube also has, in each leg of its filament supply, a variable resistor. These resistors are provided for the purpose of regulating the voltage of the tubes at the desired value below the ~]\ volts supplied by the A winding of the transformer for use directly upon the filament of the 210 power tube. A one-half megohm potentiometer regulates the signal voltage to the first audio tube for volume control, and a one-quarter megohm fixed resistor across the second audio grid circuit adds a slight load, which improves quality. signed to provide as satisfactory a degree of selectivity as is possible without materially, or seriously, cutting off the higher frequencies by side-band elimination. You will find receivers which are more selective than the EA receiver, but a careful examination of reproduction quality will show that side bands are probably being cut by the more selective receiver, and that the l-AMP FUSES SOOOOO-OHM RESIST t SMEOJ 3 CHOKE POWER BOX BASE SAFETY SWITCH SAFETY SW. BRACKET \ OO.OQO-OHM RESIST (.IMEd) T SELECTIVITY AND DISTORTION ESTS have proved that there is a definite point in selectivity in the radio-frequency circuit of a receiver beyond which a designer cannot go without materially increasing the distortion introduced. If the selectivity is too high, the side bands which carry the higher audio frequencies of the broadcast signals will be cut down materially, and the reproduction will suffer because of this. The EA receiver has been de TERMINAL & SOCKET PANEL . RECTIFIER TUBE SOC/TTT \ LOUD SPEAKER THE POWER CONVERSION UNIT reproduced quality is sacrificed because of it. Overall resonance curves for the whole receiver show a band width at one-half the resonant response of about 7 kilocycles at 500 meters and 1 5 kilocycles at 300 meters. In addition to the inherent design of the radiofrequency circuits which reduces side-band cutting in this receiver, there is provided an audio-frequency amplifying and reproducing system which has been designed with utmost care for the preservation of all the frequencies present in the broadcast music. The transformers are of special construction, with a large amount of iron to prevent saturation, and coils of large dimensions to prevent the loss of high frequen cies by capacitive bypass, and with windings of extremely high inductance for the preservation of extremely low tones, which form the foundation of the musical structure. In addition to these high-quality transformers, another extremely important feature is the use of power tubes for the prevention of overloading and the reproduction of low tones at high volumes. Even with the best of transformers it is not possible to reproduce the whole audio spectrum as transmitted by our better broadcasting stations with tubes of ordinary size and operating at ordinary plate voltages. The 210 power tube provides an undistorted output energy of approximately 100 times that obtainable with the 201-A tube and for this reason provides an amplifier of correspondingly lower distortion. The 201-A tube will not reprof duce the lower tones, necessary for satisfactory musical reproduction, at the volume levels ordinarily used in the home. A 210 tube will accomplish this result very easily and will, in addition, extend the volume many times without distortion so that practically full tone reproduction is available without distortion. Another factor of importance in connection with the high tone quality in this receiver is the use of an output coupling device for the loud speaker. The use of a large size choke coil and a condenser as an output coupling is the simplest and best way to eliminate distortion resulting from high plate current in the loud speaker windings. In addition, the use of this coupling removes the high B voltage of the power-tube plate circuit from the loud speaker terminals and entirely prevents any danger of accidental shock from such a cause. UX210 THE CIRCUIT DIAGRAM OF THE RECEIVER DESCRIBED Adequate protection of the power unit from the mains is provided by the fuses Radio Broadcast is the official publication of the Radio Club of America, through whose courtesy, the foregoing paper has been printed here. Radio Broadcast does not, of course, assume responsibility for controversial statements made by authors of these papers. Other Radio Club papers will appear in subsequent numbers of the magazine