Figure 6. Simple preamplifier. On the left are 3 input jacks connected to a three position switch. You can change this to any number you desire. The swinger of the switch goes to the top of the volume control pot, most likely dual 500 k ohm audio taper. The bottom end of the control is grounded. The wiper goes to one end of a 270 k ohm resistor. The other end goes to the grid of one of the triodes in a 12 A X 7. There is a 47 p f capacitor in parallel with the resistor. The cathode of the triode goes to ground through the parallel combination of a 1.8 k ohm resistor and a 47 u f capacitor. The plate of the triode goes through a 270 k ohm resistor to a point we will call B plus 2. Note: This is a stereo preamplifier so there will be another triode, the other one in the 12 A X 7, with a 270 k ohm resistor in its plate for the other channel. But there is only one point called B plus 2. There is a 22 u f capacitor connected from B plus 2 to ground. There is only one such capacitor. There is a 75 k ohm resistor connected from B plus 2 to plus 300 volts. There is only one such resistor. These power supply connections power both channels at the same time. The plate of the triode also connects to one end of a 0.1 u f capacitor. The other end of this capacitor goes to one end of a 3.3 meg ohm resistor. The other end of the resistor goes to the grid of the triode. The junction of the 0.1 u f and 3.3 meg ohm goes to the input point of the response control circuit. (RC C). Now the RCC will be described. The RCC input point connects through a 270 k ohm resistor to the CW end of the bass pot, 500 k ohm audio taper. The CCW end goes through a 27 k ohm resistor to ground. There is a 0.0015 u f capacitor connected from the CW end to the wiper. There is a 0.015 u f capacitor connected from the CCW end to the wiper. Back to the RCC input point, it goes through a 270 p f capacitor to the CW end of the treble pot, 500 k ohm audio taper. The CCW end goes through a 0.0027 u f capacitor then a 4.7 k ohm resistor to ground. The wiper of the treble pot connects to the wiper of the bass pot and both connect to the RCC output point. This point goes through a 120 k ohm resistor to the CCW end of the balance control, 250 k ohm linear taper dual. That's assuming this is the left channel. The CW end goes through another 120 k ohm resistor to ground. In the right hand channel the resistor from the RCC output point goes to the CW end and the CCW end goes to ground through another 120 k ohm resistor.

Gain block output amplifier.

The wiper of the control goes through a 0.01 u f capacitor to the grid, pin 2, of another 12 A X 7. The cathode, pin 3, goes through a 1.1 k ohm then a 5.1 k ohm resistor to ground. The junction of the two resistors goes to one end of a 470 k ohm resistor. The other end goes to the grid, pin 2. The plate, pin 1, goes through another 270 k ohm resistor to plus 300 volts. A 1 u f capacitor goes from 300 volts to ground at a point which is physically close to where the 270 k ohm resistor connects to 300 volts. The right hand channel has another 1 U f capacitor near where its 270 k ohm resistor connects. The plate, pin 1, goes through a 0.022 u f capacitor to the grid, pin 7 of the other triode in the same tube. The grid also goes to ground through a 470 k ohm resistor. The cathode, pin 8, goes to ground through the parallel combination of a 680 ohm resistor and a 470 u f capacitor. The plate, pin 6, goes through a 56 k ohm resistor to plus 300 volts. The plate also goes to one end of a 220 k ohm resistor. The other end goes to the cathode, pin 3 of the first triode. The plate, pin 6, also connects to one end of a 0.1 u f capacitor. The other end goes to the center of the preamp's output jack. There is a 3.3 meg ohm resistor connected from the center of the output jack to ground. End verbal description.
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