n 50 watt monoblock amplifier. We will begin with the power supply. The wide prong of a polarized plug goes to the bottom end of the primary of a filament transformer. It also connects to the bottom end of a plate and filament power transformer. The narrow prong goes through a fuse then an s p s t switch to the top end of both primaries. The filament transformer is labeled Hammond 166L6. The secondary of this transformer connects to pins 4 and 5 of a 6CL6 tube and to another 6CL6 tube. An arrow indicates that the side connected to pin 4 on the tubes also connects to a point labeled B plus 2. The filament winding on the power transformer has a center tap which is grounded. The ends connect to pins 2 and 7 of four 6L6s. It connects to pins 7 and 8 of a 6SN7, to pins 3 and 4 of two 6BH6s, to two 12AX7s as described below, and to a number 47 lamp. The 12AX7s are connected as follows. One side of the winding goes to pin 9. Pins 4 and 5 are tied together and go to the other side of the winding. This power transformer is labeled Hammond 272JX. The high voltage winding has its center tap grounded. There is a .002 microfarad 6 k v capacitor connected across the entire secondary. Each side of the secondary goes to an AC input of a bridge rectifier, anode and cathode, which is made up of 4 1N4007 diodes. Positive 400 volt supply. The positive output of the bridge, cathodes, goes to the positive side of a 100 microfarad 450 volt capacitor. The negative end is grounded. The positive of the cap goes to one side of a filter choke which is labeled Hammond 193L 5 H 300 mA. The other side of the choke goes to the positive side of another 100 mike cap with the negative side grounded. The positive side of this cap goes to a connection point which is labeled plus 400 volts. There is a 220 k ohm 1 watt resistor in parallel with each capacitor. Negative 300 volt supply. The negative output of the bridge, Anodes, goes to one end of a resistor made up of 3 1 watt resistors in parallel. They are 3.3 k ohm, 3.3 k ohm and 4.7 k ohm. The other end of this parallel combination goes to the negative end of a 100 microfarad 450 volt cap. The positive end is grounded. The negative end of this cap goes through a 1 k ohm 1 watt resistor to the negative end of another 100 mike 450 volt cap. The positive end is grounded. There are 220 k ohm 1 watt resistors in parallel with each capacitor. The negative end of the second 100 mike cap goes through a 1 k ohm 1 watt resistor to pin 7, cathode, of a 0A2 tube. Pin 5, anode, connects to pin 7 of another 0A2. Pin 5 of the second 0A2 is grounded. Pin 2 of the first 0A2 goes to a connection point labeled minus 300 volts. There is a connection inside the tube tying pins 2 and 7 together. Adjustable bias supply. A 1.8 meg ohm resistor has one end connected to minus 300 volts. The other end connects to the counter clockwise end of a 100 k ohm pot. The clockwise end goes through a 100 k ohm resistor to ground. The wiper of the pot goes to a connection point labeled bias. The pot is labeled bias adjust. Adjustable screen supply, B plus 2. The cathode, pin 2, and suppressor grid, pin 7, of a 6BH6 are grounded. A 20 k ohm 5 watt resistor connects from plus 400 volts to the screen grid, pin 6. The cathode of a zener diode also connects to pin 6, the anode of the diode is grounded. The diode is labeled 1N5378A, 100 volt 5 watt. Coming off the plate of the 6BH6, pin 5, are two 1 k ohm resistors and a 100 k ohm resistor. Each of the 1 k ohms has its other end going to the grid of a 6CL6, pin 9. The plates, pins 6 and screen grids, pins 3 of both 6CL6s are connected together and go to plus 400 volts. The cathodes, pins 1 and suppressor grids, pins 7, of both 6CL6s are tied together and go to the b plus 2 point. The other end of the 100 k ohm resistor also goes to this point. The B plus 2 point also goes through a 150 k ohm resistor to the clockwise end of a 500 k ohm pot. The counter clockwise end goes to the grid, pin1, of the 6BH6. The wiper of the pot is connected to it's counter clockwise end. The pot is labeled Screen voltage adjust. Pin 1 of the 6BH6 also goes through a 470 k ohm resistor to minus 300 volts. Pin 1 also goes through a .068 microfarad 630 volt capacitor to the point labeled B plus 2. Gain block. The center of the input jack goes to the top, clockwise end, of a 500 k ohm audio taper pot. The bottom is grounded. The wiper goes through a .022 mike cap to the grid, pin 2, of a 12AX7. The cathode, pin 3, goes through a 1.1 k ohm then a 36 k ohm to ground. The junction of the 1.1 k and 36 k 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 a 270 k ohm resistor to a point we will call B plus 3. The plate also goes through a .0047 mike cap to the grid, pin 7, of the other triode. Pin 7 also goes to ground through a 470 k ohm resistor. The cathode, pin 8, goes through the parallel combination of a 680 ohm resistor and a 470 mike 25 volt cap to ground. The positive end of the cap goes to pin 8. The plate, pin 6, goes through a 56 k ohm resistor to B plus 3. The plate also goes through a 220 k ohm resistor to the cathode, pin 3, of the first triode. The point labeled B plus 3 goes to the positive end of a 100 microfarad 450 volt capacitor. The negative end is grounded. B plus 3 goes through a 33 k ohm resistor to B plus 2. Now for the Main amplifier with global feedback. First is the long tail pair phase inverter with current sink in the cathodes. The plate of the second triode in the first 12AX7 also goes through a .1 mike cap to the grid, pin 2, of a second 12AX7. The grid goes to ground through a 470 k ohm resistor. The plate, pin 1, goes through a 100 k ohm resistor to B plus 2. The grid, pin 7, of the second triode goes through a 390 ohm resistor to ground. We will return to this grid near the end of the description. The plate, pin 6, goes through a 100 k ohm resistor to B plus 2. The two cathodes, pins 3 and 8, are tied together and connect to the plate, pin 5, of a 6BH6, not the same one mentioned earlier. The screen, pin 6, goes to ground. The cathode, pin 2, and suppressor, pin 7, are tied together and go through a 270 ohm resistor to the anode of a zener diode. The grid, pin 1, also goes to the diode's anode. The anode goes through a 20 k ohm 5 watt resistor to minus 300 volts. The diode's cathode is grounded. The diode is labeled 1N5375A 82 volt 5 watt. And now, back to the triodes. The plate, pin 1, goes through a 15 k ohm res then an 82 pf cap to the other plate, pin 6. The plate, pin 1, goes through a .22 mike cap to the grid, pin 1, of the upper triode in a 6SN7. The other plate of the 12AX7, pin 6, goes through a .22 mike cap to the grid, pin 4, of the lower triode in the 6SN7. Each grid goes through its own 470 k ohm resistor to the point labeled bias. The plates, pins 2 and 5, are tied together and go to B plus 2. The cathode of the upper triode, pin 3, goes to two 1 k ohm resistors and a 100 k ohm Ľ watt resistor. The cathode of the lower triode, pin 6, goes to two more 1 k ohm resistors and another 100 k ohm Ľ watt resistor. Each cathode goes through its own 100 k ohm 1 watt resistor to minus 300 volts. The other ends of the two 100 k ohm Ľ watt resistors are tied together and go to a banana jack. A companion jack is grounded. The jacks are labeled monitor bias. One of the 1 k ohm resistors from the cathode of the upper triode goes to the grid, pin 5, of the upper left 6L6. The other 1 k ohm resistor from the upper triode goes to pin 5 of the upper right 6L6. Similarly, one of the 1 k ohm resistors from the lower triode goes to pin 5 of the lower left 6L6 and the other 1 k ohm resistor goes to pin 5, grid, of the lower right 6L6. The cathodes, pin 8, of all four 6L6s are grounded. The screen grids, pin 4 of all four are connected to B plus 2. The suppressor grids are internally connected to the cathodes. The plates of the upper two 6L6s, pin 3, connect to the brown lead of the output transformer primary. The plates of the lower two 6L6s connect to the blue lead of the transformer primary. The red lead of the transformer, center tap, connects through a 1 ohm 1 percent resistor to plus 400 volts. There are two banana jacks, one connects to each end of the resistor. These jacks are labeled current. The output transformer is a Hammond 1650K. The secondary is wired for 8 ohms. The black wire is grounded. The yellow wire connects to one end of a parallel R and C. The resistor is a 910 ohm and the capacitor is a .0022 microfarad. The other end of this parallel RC goes to the grid, pin 7, of the 12AX7 where the 390 ohm resistor goes to ground. The 8 ohm load is connected between the black and yellow leads of the transformer secondary. This completes the verbal description.
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