Figure 7. Quality preamplifier. On the left is a 4 position switch which is connected to 4 input jacks. The builder can change this number if desired. The switch has two poles. The swinger of the switch connects to the input point of the standard cathode follower. It will be described at the bottom of this description. The B plus point goes to B plus 2. The output goes to the center of the tape output jack. The shell is grounded. There is a 3.3 meg ohm resistor connected from the center to the shell of the jack. The swinger of the switch also goes to the top of the volume control, a dual 500 k ohm audio taper pot. The bottom is grounded. The wiper goes to the input of another standard cathode follower. The b plus point of this cathode follower also goes to B plus 2. The output point goes to the input point of the response control circuit (R C C) which is part of the tone control circuit. The input point of the RCC goes through a 56 k ohm resistor to the CW end of the bass control, 250 k ohm linear pot. The other end goes through another 56 k ohm resistor to the output point of the RCC. There is a 0.0022 u f capacitor connected from one end to the other of the bass control. Returning to the input of the RCC, it goes through a 24 k ohm resistor then a 220 p f capacitor to the CW end of the treble control, 250 k ohm linear pot. The other end of the treble control goes through another 220 p f capacitor then another 24 k ohm resistor to the output point of the RCC. The wiper of the bass control goes through a 1 meg ohm resistor to the center of the RCC. The wiper of the treble control goes through a 220 k ohm resistor to the center of the RCC. The center of the RCC goes to the grid of one of the triodes in a 12 A X 7. This grid also goes to ground through a 2.2 meg ohm resistor. The cathode goes to ground through the parallel combination of a 1.8 k ohm resistor and a 100 u f capacitor. The plate goes through a 100 k ohm resistor to B plus 3. The plate also goes to one end of a 0.47 u f capacitor. The other end of this capacitor goes to the output point of the RCC. This end of the capacitor also goes to through a 120 k ohm resistor to the CCW end of the balance control 250 k ohm linear dual pot. Assuming we are describing the left hand channel. The bottom end of the pot goes through another 120 k ohm resistor to ground. The wiper goes to the input of the gain block output amplifier.

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.

Standard Cathode Follower.

The input point goes to one end of a 0.1 u f capacitor. The other end goes to the grid of one of the triodes in a 12 A U 7. The cathode goes through a 3.9 k ohm then a 33 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 of the same triode. The plate goes to the B plus point. The cathode goes through another 0.1 u f capacitor to the output point. End verbal description.
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