Figure 18 Initial Version of Tube Amplifier for Parametric EQ. The input goes to one end of a 10 k ohm resistor. The other end goes to the inverting input of the tube amplifier. There is another 10 k ohm resistor connected between the inverting input and the output of the tube amplifier. The input also goes to the counter clockwise end of the presents control, 4.7 k ohm. The clockwise end of this control goes to the output of the tube amp. The wiper goes through a 0.015 u f capacitor then a 1.8 k ohm resistor to one end of a 180 k ohm pot connected as a rheostat. This is one of a dual ganged pot which is the frequency control. The other end of the first rheostat connects to the second rheostat which is part of the frequency control. The other end of the second rheostat goes through another 1.8 k ohm resistor to ground. The junction of the two rheostats connects through a 0.01 u f capacitor to ground. The junction also connects to the noninverting input of the tube amp.

Now we will describe the internal circuit of the tube amplifier, tube amp. This circuit will be considerably changed in the final version to be shown below. The input stage of the tube amp will be recognized by many as a long tail pair with a current source. Others may remember it as a differential amplifier. Both groups are correct. The tube is a 12 A X 7. The grid of the left hand triode is the inverting input. It connects to ground through a 10 meg ohm resistor. The grid of the right hand triode is the noninverting input. It also goes to ground through a 10 meg ohm resistor. The two cathodes connect together and to one end of a 5.6 k ohm resistor. The other end connects to the drain of a depletion mode M O S F E T, IXCP10M45S. The gate goes through a 1 k ohm resistor to minus 105 volts. The source goes through a 100 ohm resistor, then a 500 ohm pot connected as a rheostat to minus 105 volts. The pot will later be changed to a 5 k ohm. The plate of the left hand triode goes through a 270 k ohm resistor to a point we will call B plus two. The right hand plate goes through another 270 k ohm resistor to B plus two. B plus two connects through a 22 u f 450 volt capacitor to ground. This is an electrolytic cap and the plus side goes to B plus two. A 47 k ohm resistor connects between B plus two and plus 255 volts. The left hand plate connects through an 820 p f capacitor then a 2 k ohm resistor to the right hand plate. The right hand plate also connects directly to the grid of the lower triode in the cascode amplifier, a 12 A V 7. The cathode of this triode goes to ground through the parallel combination of a 120 k ohm resistor and a 10 u f 450 volt capacitor. The plate of the lower triode connects directly to the cathode of the upper triode in the same tube. The grid of the upper triode connects through a 100 k ohm resistor to plus 255 volts and another 100 k ohm resistor to ground. The grid also connects through a 0.1 u f capacitor to ground. The plate of the upper triode goes through the parallel combination of a 15 k ohm resistor and a 47 p f capacitor to plus 255 volts. The plate also goes through a 0.1 u f capacitor to the grid of the cathode follower, 1/2 of a 12 A U 7 or a 6 C 4. The cathode of this triode goes through a 1 k ohm resistor then a 10 k ohm resistor to ground. There is a 4.7 k ohm resistor connected between the cathode and ground. The grid of this triode connects to one end of a 100 k ohm resistor. The other end of this resistor connects to the junction of the 1 k and 10 k ohm resistors. The cathode also goes through a 1 u f capacitor to the output of the tube amp. The output also goes through a 3.3 meg ohm resistor to ground.
End of verbal description.
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