Figure 8. Super Preamplifier. Because this preamp provides for 2 tape decks the switching is fairly complicated. It will be described first.

The circuit begins with S1A. This is the source selector switch. Each position is wired to an input jack. S1B selects for the other channel. The swinger of S1A goes to position 2 of S2A, tape monitor selector. The swinger of S2A goes to the top of the volume control. S2B is in the other channel. Position 1 of S2A is connected to the input jack where the signal from tape deck B is fed to the preamp. It is labeled tape input deck B. Position 3 of S2A goes to tape input deck A. The swinger of the source selector switch, S1A goes to position 2 of S3A and position 2 of S3C. S3B and S3D are in the other channel. The swinger of S3A goes to cathode follower A which feeds signal to tape output deck A. The swinger of S3C goes to cathode follower B which feeds signal to tape output deck B. Cathode followers A and B are powered from B plus 3. There are 0.1 u f capacitors connecting between the cathodes of follower A and B to their respective tape output jacks. There are 3.3 meg ohm resistors connected across each tape output jack. S3 is the tape copy switch. Position 1 of S3 is the copy A to B position. Position 1 of S3A is grounded. Position 1 of S3C goes to the tape input deck A jack. Position 3 of S3 is copy B to A. Position 3 of S3A goes to tape input deck B. Position 3 of S3C is grounded.

How they function.

S1 is the source selector. It selects which of the signal sources that are plugged into the preamp will be fed to the amplifier and tape decks. S2 is the tape monitor switch which comes after the source selector. When this switch is in position 1 you will hear the signal from tape deck B. In position 2 you will hear the signal that is selected by the source selector. In position 3 you will hear the signal from tape deck A. This switch has no effect on what is fed to the two tape decks. S3 is the tape copy switch. In position 2 the signal that has been selected by the source selector is fed to both tape decks. In position 1, A to B, no signal is fed to deck A. The output signal from deck A is fed to the input of deck B. In position 3 no signal is fed to deck B. The output of deck B is fed to the input of deck A. The tape monitor switch may be used to listen to the output of either deck A or deck B. Or if you are running a big boring dubbing job you can set the monitor switch to position 2 and listen to the radio while the tape is being copied.

The signal from the swinger of the tape monitor switch, S2 goes to the top of the volume control pot, 500 k ohm dual audio taper. The lower end is grounded. The wiper goes to the input of cathode follower C. the B plus point of this cathode follower is connected to b plus 2. The output goes to the input of the response control circuit (RCC). That's right, no DC blocking capacitor. The input point goes through a 10 k ohm resistor to the CW end of the bass control, 100 k ohm linear. The other end goes through another 10 k ohm resistor to the output point. There is a 0.056 u f capacitor connected across the bass control. The input point also goes through another 10 k ohm resistor to the CW end of the midrange pot, 100 k ohm linear. The other end goes through another 10 k ohm resistor to the output of the RCC. There is a 1600 p f capacitor connected across the midrange pot. The input point also goes through a 1000 p f capacitor to the CW end of the treble control 100 k ohm linear. The other end of the treble control goes through another 1000 p f capacitor to the output of the RCC. The wiper of the bass pot goes through a 100 k ohm resistor to the center of the RCC. The wiper of the midrange pot goes through 3300 p f capacitor then a 100 k ohm resistor to the center of the RCC. The wiper of the treble pot goes through 47 k ohm resistor to the center of the RCC. The center of the RCC goes through a 0.1 u f capacitor to the grid, pin 7, of a 12 A X 7. The grid goes to ground through a 1 meg ohm resistor. The cathode goes to ground through the parallel combination of a 3.9 k ohm resistor and a 100 u f capacitor. The plate, pin 7, goes through a 750 k ohm resistor to B plus 2. The plate goes directly to the grid, pin 7 of a 12 A U 7. The plate goes to B plus 2. The cathode goes through a 33 k ohm resistor to ground. The cathode connects directly to the output side of the RCC. The cathode goes through a 0.22 u f capacitor to one end of a 120 k ohm resistor. The other end goes to the top, CCW, end of the balance control, 250 k ohm linear. The other end goes through another 120 k ohm resistor to ground. The wiper goes to the input of the output gain block.

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.

The b plus connections are as follows. Plus 300 volts from the external power supply goes to the plus 300 volt points of both gain block output amplifiers. There are two 1 u f capacitors connected to the 300 volt line each one physically near one of the output amplifier blocks. The other ends of these capacitors are grounded. A 5.1 k ohm resistor connects from plus 300 volts to B plus 2. A 100 u f capacitor connects from b plus 2 to ground. A 5.6 k ohm resistor connects from plus 300 volts to B plus 3. A 100 u f capacitor connects from B plus 3 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 to the output point.

End verbal description.
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