Conventional Pentode connected 6L6 amplifier. The hot terminal of the input goes to the top of a 500 k ohm pot and the bottom is grounded as is the other side of the input. The wiper goes through a 0.1 mike cap to the grid, pin 7, of a 12AX7. A 470 k ohm resistor goes from the grid to ground. The cathode, pin 8, goes to one end of the parallel combination of a 2.0 k ohm res and a 220 microfarad cap. The other end of this parallel combination goes through a 510 ohm res to ground. We will return to this point later. The plate, pin 6, goes through a 270 k ohm res to a point we will call B plus 3. The visual schematic is not so labeled. The plate also goes through a 68 pf cap then a 56 k ohm res in series to ground. The plate also goes through a .47 mike cap to the grid, pin 2, of the other half of the 12AX7. The cathode, pin 3, goes through a 4.7 k ohm res to point A. Point A goes through an 82 k ohm res to one end of a 50 k ohm pot, AC balance control. The wiper of this control goes to ground. Point A also goes through a 470 k ohm res to the grid. point A also goes through a .1 mike cap to the grid, pin 7, of a 12AV7. The plate, pin 1, of the 12AX7 goes through a 100 k ohm res to B plus 3. The plate also goes through a .1 mike cap to the other grid, pin 2, of the 12AV7. B plus 3 goes through a 50 mike cap to ground. B plus 3 also goes through a 12 k ohm res to B plus 2. Each grid of the 12AV7 goes through its own 470 k ohm res to each end of a 100 k ohm pot, the DC balance control. Each end of the DC balance pot goes through a 1 meg ohm res to ground. The wiper of this pot goes to the wiper of another 100 k ohm pot labeled bias adjust. One end of this pot is grounded and the other end goes through a 470 k ohm res to minus 300 volts. Each cathode, pins 8 and 3, goes through its own 180 k ohm 1 watt res to minus 300 volts. The two plates, pins 1 and 6, are tied together and go through a 10 mike cap to ground. The two plates also go through a 47 k ohm 1 watt res to B plus 2. The cathode, pin 3, of the upper half of the 12AV7 goes through a 1 k ohm res to the grid, pin 5, of the upper 6L6GC. The other cathode of the 12AV7 goes through another 1 k ohm res to the grid, pin 5, of the lower 6L6GC. There are two 100 k ohm resistors in series connected between the two cathodes, Pins 3 and 8, of the 12AV7. The junction of the two 100 k ohm resistors goes to a test point labeled TP1. These two resistors are selected for equal value. The cathodes, pin 8, of both 6L6GCs goes through its own 1.0 ohm res to ground. These two resistors are selected for equal value. The screen grids, pin 4, of both 6L6GCs goes to B plus 2. The plate, pin 3, of the upper 6L6GC goes to the blue lead of the output transformer primary. The plate of the lower 6L6GC goes to the brown lead of the output transformer primary. The center tap of the primary, red, goes to B plus 1. The 8 ohm secondary of the output transformer goes to an 8 ohm load resistor. The bottom of the secondary is grounded and the upper end goes through a 1200 ohm resistor to the ungrounded end of the 510 ohm res in the cathode circuit of the first 12AX7 amplifier stage. The 1200 ohm resistor is shown connected by dashed lines. A note on the diagram says, quote, B plus 1 equals 400 volts. B plus 2 equals 290 volts. End quote. End verbal description.
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