EL34 ultra linear amplifier circuit, one channel. The input terminal connects to one fixed contact of a double pole double throw switch. This contact is labeled normal. One pole will be described, the other pole is used in the other channel. The swinger connects to the top of the level control, 500 k ohm pot. The input also connects to one end of the parallel combination of a 470 k ohm resistor and a 10 pf cap. The other end of this parallel combination connects through a 24 k ohm resistor then another 24 k ohm resistor to ground. The end of the parallel combination away from the input also connects through a 100 pf cap to ground. There is a .012 mike cap in parallel with the 24 k ohm res that is connected to ground. The junction where the 470 k ohm, the 24 k ohm, the 10 pf and the 100 pf come together connects to the other fixed contact of the switch. This contact is labeled sleep. Now back to the level control pot. The bottom end is grounded. The wiper connects to one end of a 0.033 mike cap, the other end goes to the grid, pin 7, of the first 12 A X 7. The grid also connects to one end of a 470 k ohm resistor. The other end is grounded. The cathode, pin 8, connects to the feedback compensation network. It will be described later. The plate, pin 6, goes through a 100 k ohm resistor to B plus 3. B plus 3 connects to one end of a 0.1 mike 630 volt cap. The other end is grounded. The plate also goes through a 0.1 mike cap to the other grid, pin 2, of the same 12 A X 7. The grid also goes through an 82 pf cap then a 15 k ohm res to ground. This grid also connects to one end of another 470 k ohm resistor. The cathode of this triode, pin 3, connects through a 10 k ohm then a 100 k ohm res to ground. The 470 k ohm from the grid connects to the junction of the 10 k and 100 k. This junction also goes through a 0.1 mike cap to the grid, pin 7, of the second 12 A X 7. The plate, pin 1 of the first 12 A X 7, goes through another 100 k ohm res to B plus 3. The plate also goes through another 0.1 mike cap to the grid, pin 2 of the second 12 A X 7. We are now finished with the first 12 A X 7 and will be describing only the second 12 A X 7. The cathode, pin 3, goes through a 430 ohm then a 6.2 k ohm res to the top of a 1 k ohm pot labeled AC balance. The junction of the 430 ohm and 6.2 k goes through a 470 k ohm to the grid, pin 2. The cathode of the other triode, pin 8, goes through another 430 ohm res then another 6.2 k ohm res to the bottom of the AC balance pot. The wiper is grounded. The junction of the 430 and 6.2 k goes through a 470 k ohm to the grid, pin 7. The plate, pin 6, goes through a 100 k ohm res to B plus 2, 400 volts. The other plate goes through another 100 k ohm res to B plus 2. The plate, pin 1, connects directly to the grid, pin 2, of a 6CG7. The other 12 A X 7 plate, pin 6, connects directly to the grid, pin 7, of the 6CG7. Both plates of the 6CG7, pins 1 and 6, connect to B plus 2. There is a 0.1 mike 630 volt cap from B plus 2 to ground. Each cathode, pins 3 and 8, connects through its own pair of 100 k ohm 1 watt resistors connected in parallel to ground. Note, the cathodes are not connected together. The cathode of the upper triode in the 6CG7, pin 3 also connects through a 0.15 mike cap to the top end of a 47 k ohm res. The cathode of the lower triode, pin 8, also connects through a 0.15 mike cap to the bottom end of a 47 k ohm res. The other ends of the two 47 k ohm resistors connect together and to the wiper of the bias pot, 5 k ohm. The wiper of this pot is also connected to the negative end of a 47 mike electrolytic cap. The positive end is grounded. The CW end of this pot connects to the CW end of the bias pot for the other channel, also a 5 k ohm. The CCW end connects to the CCW end of the other pot. The wiper of the bias pot for the other channel is connected in a manner identical to the one described. The CCW end of both pots connects to a point labeled C minus. The CW end of both pots connects through a 3.6 k ohm resistor to a point labeled C plus. The C plus point is also connected to ground. The upper junction of the .15 cap and the 47 k res connects through a 1 k ohm res to the grid, pin 5, of the upper left EL34. The junction also connects through another 1 k res to the grid of the upper right EL34. The lower junction of the .15 cap and the 47 k res goes through a 1 k ohm res to the grid, pin 5, of the lower left EL34. The junction also goes through another 1 k ohm res to the lower right EL34. On all four EL34s the cathode, pin 8, connects to the suppressor grid, pin 1. The cathodes and suppressors of all four EL34s are tied together and go to one end of a 1 ohm 1 percent resistor. The other end of this resistor is grounded. There is a terminal connected to each end of the 1 ohm res. This pair of terminals is labeled, quote, cathode current. End quote. The plates, pin 3, of both upper EL34s are connected together and go to the blue lead of the output transformer primary. There is also a blue lead on the secondary. Don't confuse the two. The plates, pin 3, of both lower EL34s connect together and to the brown lead of the transformer primary. The screen grid, pin 4, of the upper left EL34 goes through a 100 ohm resistor to the upper screen tap, blue and white lead, of the transformer. The screen grid of the upper right EL34 goes through another 100 ohm res to the blue and white lead. Similarly, the lower left EL34 screen, pin 4, goes through a 100 ohm res to the lower screen tap, brown and white. The lower right EL34 screen goes through another 100 ohm res to the brown and white wire. The red lead, center tap, goes to B plus 1. B plus 1 goes through a 0.1 mike 630 volt cap to ground. On the transformer the 5 primary leads come out of one side of the transformer and the two secondary leads come out of the other side. The black secondary lead is grounded. The blue lead goes through an 82 pf cap to the cathode, pin 8, of the first 12 A X 7 triode. The blue lead also goes through a 4.7 k ohm then a 1.8 k ohm to pin 8. The junction of the 4.7 k and 1.8 k goes through a 220 ohm res to ground. The junction of the 4.7 k and 1.8 k also goes through a .0068 uf cap to ground. There is an 8 pin, octal, plug mounted on the chassis. This is not a socket but a plug with its pins sticking out. It will mate with an octal socket on the end of a cable coming from the power supply. Pin 1 connects to one side of the 6.3 volt heater supply. Pin 2 connects to C minus. Pin 3 connects to C plus. pin 4 connects to B plus 3. Pin 5 connects to B plus 2. Pin 6 connects to B plus 1. Pin 7 connects to B minus and ground. Pin 8 connects to the other side of the 6.3 volt heater supply. B plus 1 connects through a 330 k ohm 1 watt resistor to point A. point A connects through a 47 k ohm resistor to ground. Point A connects to the positive end of a 4.7 microfarad cap. The negative end is grounded. Point A connects through a 150 ohm resistor to one line of the heater supply. Point A connects through another 150 ohm res to the other heater supply line. End verbal description.
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