The Input buffer. The input terminal on the left connects to a 0.1 u f capacitor. The other end goes to the grid, pin 2, of a 12AU7. The plate, pin 1, connects to plus 250 volts. The cathode, pin 3, goes to a 3.9 k ohm resistor. The other end goes to a 33 k ohm resistor. The other end of that one goes to ground. The junction of the 3.9 k and 33k connects to one end of a 470 k ohm. The other end goes to the grid, pin 2, of the same tube. The other triode in this 12AU7 is used in the other channel of the amplifier or preamplifier. The cathode, pin 3, is the output of the buffer.

The response control circuit. A 2.2 uf 200 volt capacitor connects between pin 3, cathode of the 12AU7 and the input of the response control circuit. Do not use an electrolytic capacitor here. It must be a plastic film capacitor. A 470 k ohm resistor connects from the input of the response control circuit to ground. The input of this circuit is at the junction of two 10 k ohm resistors. The other end of the upper resistor connects to a 560 P F capacitor. The other end goes to the clockwise end of the treble pot, 100 k ohm linear. The other end of the lower 10 k ohm resistor goes through a 0.015 u f capacitor to ground. The junction of the resistor and capacitor goes to the clockwise end of the bass pot, 100 k ohm linear. The wiper of the bass pot goes through a 0.1 u f cap, then a 470 k ohm resistor then a 47 k resistor to the wiper of the treble pot. The junction of the 470 k ohm and 0.1 u f goes to ground through a 2.2 meg ohm resistor. The junction of the 470 k ohm and 47 k ohm is the center of the response control circuit and goes to the input, pin 7, grid, of the 12AX7 in the feedback amplifier. The counter clockwise end of the treble pot goes to a 560 p f cap then a 10 k ohm resistor then another 10 k ohm resistor then another 0.015 u f cap to ground. The junction of this 0.015 u f cap and the 10 k ohm res connects to the counter clockwise end of the bass pot. The junction of the two 10 k ohm resistors is the output side of the response control circuit. This point goes through a 470 k ohm resistor to ground. This point also goes through a 0.1 u f 200 volt capacitor to the output of the tone control circuit. This output point goes to ground through a 470 k ohm resistor.

The feedback amplifier. The input of the feedback amplifier is the grid, pin 7 of a 12AX7. The cathode, pin 8, goes to ground through the parallel combination of a 3.9 k ohm res and a 100 u f cap. The plate, pin 6, goes through a 750 k ohm resistor to plus 250 volts. The other triode in the 12AX7 is used in the other channel of the amplifier or preamplifier. The plate, pin 6, of the 12AX7 goes to the grid, pin 7, of a different 12AU7. The plate, pin 6, goes directly to plus 250 volts. The cathode, pin 8, goes through a 33 k ohm to ground. The other half of this 12AU7 is used in the other channel. Pin 8 goes through a 0.22 uf 630 volt capacitor to the output side of the response control circuit. Pin 8 also goes through a 0.22 u f capacitor to the signal output. End verbal description.
Go back.