Schematic diagram of the A-300. The primary of the power transformer is presumed connected to the on/off switch, fuse, and line plug, although none of this is shown. The 6.3 volt secondary is presumed connected to the heaters of all of the tubes except the 12AX7s in the phono preamp but none of it is shown. The high voltage secondary is not center tapped but uses a full wave voltage doubler. The top of this winding goes to the anode of D1 and the cathode of D2. The anode of D2 is grounded. The cathode of D1 goes to the positive of C1, a 100 mike 200 v cap. The negative goes to the positive of C2, another identical cap. The junction of the two caps goes to the bottom of the high voltage secondary. The negative end of C2 goes to ground. The positive end of C1 goes to a point labeled B plus 1. B plus 1 goes through a 470 ohm 1 watt res to B plus 2. B plus 2 goes through a 50 mike 400 v cap to ground. B plus 2 goes through a 1 k ohm res to B plus 3. B plus 3 goes through a 40 mike 350 v cap to ground. B plus 3 goes through a 3.3 k ohm res to B plus 4. B plus 4 goes through a 40 mike 350 v cap to ground. B plus 4 goes through a 39 k ohm res to B plus 5. B plus 5 goes through a 40 mike 350 v cap to ground. This is a stereo amplifier. Both channels are identical so only one will be described. The magnetic phono input goes to grid a of V1, 12AX7. It also goes through a 47 k ohm res to ground. Cathode A goes through a 3.3 k ohm res to ground. plate A goes through a 470 k ohm res to B plus 5. Plate A also goes through a .01 mike cap to grid B of V1. Grid B goes through a 6.8 meg ohm res to ground. Cathode B is grounded. Plate B goes through a 220 k ohm res to B plus 5. Plate B also goes through a .1 mike cap to point C. Point C goes through a 3.3 meg ohm res then a 100 k ohm res to cathode A of V1. There is a .0033 mike cap in parallel with the 3.3 meg. There is a 750 pf cap in parallel with the 100 k. Point C goes to the phono position of the selector switch. The other three positions of the selector switch go to phono jacks. The swinger goes to the top of the volume control the bottom is grounded. The loudness switch connects to the volume pot but I don't have the circuit. The wiper of the volume control goes through a 270 k ohm resistor to grid A of V2, 12AX7. There is a cap of unknown value in parallel with the 270 k. Cathode A goes through a 1.8 k ohm res to ground. Plate A goes through a 270 k ohm res to B plus 4. Plate A also goes through a .1 mike cap to point D, tone control input. Point D goes through a 120 k ohm res to the top of the bass pot, 500 k ohm. The bottom of the bass pot goes through a 27 k ohm to ground. There is a .003 mike cap from the top of the bass pot to its wiper. There is a .015 mike cap from the bottom of the bass pot to its wiper. The wiper of the bass pot connects to the wiper of the treble pot, another 500 k ohm. point D goes through a 750 pf cap to the top of the treble pot. The bottom of the treble pot goes through a .0033 mike cap to ground. The wipers of the two tone pots go to the top of one gang of the balance control 500 k ohm two ganged pot. In the other channel the other gang of the balance control works the opposite way. The wiper of the balance control goes to grid B of V2. Plate B goes through a 330 k ohm res to B plus 4. Plate B also goes through a .02 mike cap to grid A of V3, 12AU7. The other half of V3 is used in the other channel. Cathode A of V3 goes through a 2.7 k ohm, then a 20 k ohm, then a 300 ohm res to ground. The junction of the 300 ohm and 20 k ohm goes to cathode B of V2. The junction of the 20 k and 2.7 k goes through a 1 meg ohm res to grid A of V3, 12AU7. We will be returning to both of these junctions later. Plate A of V3 goes through a 22 k ohm res to B plus 3. The plate also goes through a .1 mike cap then a 1 k ohm res to the control grid of the upper 6V6. The junction of the res and cap goes through a 470 k ohm res to ground. The junction of the 2.7 k and 20 k goes through a .1 mike cap then a 1 k ohm res to the grid of the lower 6V6. The junction of the res and cap goes through a 470 k ohm res to ground. The cathodes of the two 6V6s are tied together and are also tied to the cathodes of the other two 6V6s in the other channel. The four cathodes connect through a 200 mike 50 v cap to ground. The four cathodes also go to pin 5 of V1, one of the 12Ax7s used in the magnetic phono preamp. Pin 4 of V1 goes to pin 5 of the other magnetic preamp tube and pin 4 is grounded. Each 12Ax7 heater is series connected and the two tubes are in series and serve as the cathode resistor for the four 6V6s. The suppressor grids are internally tied to the cathodes. The screen grids are tied together and go to B plus 2. Each 6V6 plate goes to one end of the output transformer. The center tap goes to B plus 1. The common end of the secondary is grounded, the 4 ohm tap goes to the 4 ohm output, the 8 ohm tap goes to the 8 ohm output, and the 16 ohm tap goes through a 47 k ohm res to the junction of the 20 k ohm and 300 ohm resistors mentioned much earlier. End verbal description.

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