Transformer Coupled L s

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This amplifier was designed for the reproduction of musical programs with a high degree of realism. The amplifier had to be more than just ordinarily good, as its ultimate purpose was to please the discriminating music lover.

In the tube line-up, triodes are used, with the exception of the output stage which uses 6L6-G's in push-pull with inverse feedback.

High-fidelity audio transformers are used in input and output of the

Amplifier Builders Guide
Fig. 45—Appearance of the finished transformer-coupled amplifier.

6L6 stage and are largely responsible for the excellent results of which this amplifier is capable. These transformers have a frequency response of ±y2 db from 30 to 15,000 cycles and have hum-bucking construction, with heavy iron cases. The overall gain of the amplifier is approximately 75 db. Power output is 15 watts with less than 2-percent distortion, although higher output is possible with an increase in distortion content. High power output was not considered an essential factor during design; medium power output, with excellent fidelity, was the keynote. However, an output up to 15 watts provides an adequate reserve for the handling of loud signals without noticeable distortion.

The first audio stage is a 6J5, self-biased. To take advantage of the small amount of degeneration occurring in the 2,500-ohm bias resistor, there is no bypass condenser from cathode to ground. The grid of this stage receives its signal from the volume control which is connected to a phono-radio switch. A high-frequency tone control, consisting of a .02-/if condenser and a 150,000-ohm potentiometer, is connected from plate to ground of the 6J5.

Direct coupling between the first audio stage and the phase inverter was made practical by adjusting the first audio stage's operating plate voltage to the proper voltage for the grid of the phase-inverter tube (100 volts). A study of the voltages in the phase-inverter diagram reveals that the plate supply for the first audio stage is 250 volts. This tube draws 1 ma plate current, causing a 150-volt drop in the combination of R2 and R3, developing an operating potential of 100 volts for its plate, and, by virtue of the direct coupling, the same potential for the grid of the phase-inverter tube. This voltage automatically biases the 6J5 phase-inverter tube to where its plate current is approximately 2 ma. This current causes a voltage drop of 102 volts across R4 and a similar drop across R5, leaving 158 volts as measured from plate to cathode of the phase-inverter tube. R2 is the first-stage plate resistor and R3 performs the dual function of filtering and dropping the voltage to the proper level at the point of the direct coupling, i.e., 100 volts.

The plate voltage supply for the phase-inverter tube was made high purposely, in order to avoid too low a potential on the plate of the first audio stage. The plate voltage of the first audio stage and the cathode voltage of the phase-inverter tube are practically the same, with the exception of a few volts of bias that are developed in the phase-inverter cathode resistor. This high voltage is taken from the output stage plate supply where 375 volts are available. Additional filtering was required at this point to keep all traces of hum out of the phase-inverter stage. This is adequately accomplished by CHI and the 8-/if condenser C2.

Care was exercised in selecting the phase-inverter cathode and plate resistors R4 and R5 to be certain of obtaining as near a match as possible between the two.

Following the phase inverter is a stage of push-pull 6J5's. Push-pull at this point was considered desirable for the elimination of second-harmonic distortion at high signal levels. Also, while the grids of the output stage are never driven positive, the use of feedback doubles the signal voltage required of the driver stage. A high-fidelity transformer couples this stage to the output stage.

6l6 Amp Layout Diagram
Fig. 46—Schematic diagram of the transformer-coupled amplifier.

Two 6L6-G's, in push-pull, are utilized in the output stage, with 16-percent inverse feedback. This stage is coupled to the speaker voice coil by means of a high-fidelity output transformer. The primary of this transformer is rated at 6,600 ohms with appropriate taps for matching the speaker voice coil.

The first audio stage and power supply are on the tuner chassis which is used with this amplifier for the reception of radio broadcasts. The tuner is not shown. The power supply is conventional, as can be seen in the schematic diagram. The phase-inverter, push-pull driver stage and push-pull output stage are on the chassis pictured in the

Fig. 47—Under-chassis view of the amplifier.

photo and are within the dotted line portion of the schematic diagram. Heavy stranded wire is used to connect the 6.3-volt heater winding to the amplifier. Each wire is connected to two pins on the plug to which it connects, and, correspondingly, two lugs on the plug socket are joined, to avoid the possibility of a voltage drop and consequent heating at this juncture.

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