Production year 1928
The Eveready A-C Model 3 receiver was built by the National Carbon Company Inc. and, according to the metal I.D. plate found inside the receiver, this company was located in New York and in San Francisco. The plate goes on to state that the company is a "Unit of Union carbide and Carbon corporation".
The Eveready A-C Model 3 receiver employs 3 stages of neutralized Tuned Radio Frequency(TRF) amplification, along with a detector and two transformer coupled Audio Frequency(AF) stages, while ending in a push-pull power stage.
The Eveready A-C Model 3 is housed in a wooden cabinet and is only capable of receiving the AM broadcast band. The receiver has four controls on the front panel. Directly above the display or tuning dial is an on/off toggle switch. The knob on the far left is the adjustable variometer or antenna trimmer. The center knob directly under the tuning dial operates the four ganged variable capacitor and tunes the receiver to the desired frequency. The knob at the far right operates a potentiometer to control the volume. The three knobs are operated by turning them in a clockwise-counterclockwise fashion.
This receiver employs four type 26 tubes, a type 27 tube, two type 71a tubes and a type 80 tube for a total of eight tubes. Three of the type 26 tubes are used for the three RF stages of the circuit. The fourth type 26 tube is the first AF stage. The two type 27 tubes are for the push-pull audio output. The type 71a tube is the detector. The type 80 tube is the rectifier.
The power requirement of the Eveready A-C Model 3 is 105 to 125 volts A/C and it has a current draw of .5 amp at 25 cycles(Hz). Normal U.S. house current today operates at 120 volts A/C at 60 cycles(Hz). The receiver requires the use of an external speaker or headphones.
The Eveready A-C Model 3 does not couple the antenna to the input of the first tube by way of the stationary coupler as encountered in similar circuit designs, but rather incorporates an adjustable variometer which is tuned by one of the ganged sections of the variable tuning capacitor. This antenna trimmer acts as an auxiliary tuning control which can be used to bring the antenna circuitry into resonance with the desired frequency. This is extremely helpful when attempting to bring in a weak signal as there is a marked increase in audio gain when the antenna input circuitry is in resonance.
Volume control is accomplished with the aid of a high resistance potentiometer which is located in the first Radio Frequency(RF) circuit. The volume control is electrically connected between the stator of the first RF section of the ganged variable capacitor and ground. The movable arm of the potentiometer connects to the grid of the RF tube.
The two transformer coupled AF stages are used with a push-pull power stage thus producing much better tone quality than can be obtained from an equivalent single large power stage tube. There are two main reasons for this. First, the demand for plate current in the two tubes is constant. When the plate current is decreasing in one tube it is increasing to the same extent in the other. This helps to avoid a fluctuating B supply voltage which occurs when the output power stage employs a single large tube. A fluctuating B supply voltage will also affect the other tubes of the set and produce a tendency to over-emphasize the notes of certain audio frequencies. Second, the two push-pull tubes are working out of phase with each other thus canceling out the second harmonic which is chiefly responsible for distortion when a single large power stage tube is being driven to a loud audio level.
The final audio output stage is coupled to the loudspeaker terminals through an output transformer. Besides aiding in tone quality, the output transformer prevents the heavy plate currents of the push-pull power tubes from reaching and destroying the windings on the loudspeaker. Thus the only voltage leaving the receiver is the safe low voltage of the secondary of the output transformer.
The circuit design of the Model 3 incorporates eight bypass capacitors. These bypass capacitors help to stabilize the receiver and contribute greatly to the tone quality of the set. The power unit contains the A, B, and C voltage supplies as well as the last audio stage and output transformer. The power transformer has three taps in the primary to compensate for the variations in line voltages encountered at different localities. These three taps correspond to line voltages of 105, 115, or 125 volts A/C and takes the form of three holes on a terminal board. A plug that comes with the set is inserted in the hole that corresponds to the line voltage in which the receiver will be operated.