Long Tail Pair with Pentode Current Sink. The input signal goes through a 0.1 microfarad capacitor to the grid, pin 2, of a 12AX7. The grid also goes to ground through a 470 k ohm resistor. The cathode, pin 3, connects to the cathode of the other triode, pin 8, in the 12AX7. We will return to this point later. The plates of both triodes, pin 3, left, and pin 6, right, go through 100 k ohm 1 watt resistors to plus 300 volts. Each plate goes through its own 0.1 mike cap to each output terminal. These outputs are labeled quote, driver tube grids, end quote. Each output terminal goes through its own 1.0 Meg ohm resistor to a point labeled bias. The grid, pin 7, of the right hand triode, goes through the parallel combination of a 0.1 mike cap and a 470 k ohm resistor to ground. The two cathodes, pins 3 and 8, go to the plate, pin 5, of a 6BH6. The cathode, pin 7, of this tube goes through a 240 ohm resistor to minus 70 volts. The suppressor grid, pin 2, is tied to the cathode. The screen grid, pin 6, goes to ground. The control grid, pin 1, goes to minus 70 volts. A legend on the figure says, quote, At V o = 17 v, T H D = 0.065%. At V o = 25 v, T H D = 0.11%. Gain = 23.8. F2 = 280 k c. End quote. End verbal description.
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