Adding a cathode resistor in series. The cathode of one half of a 12AU7 goes to ground through the series combination of a 1.8 k ohm, and a 4.3 k ohm, resistor. The grid goes through a 470 k ohm resistor to the junction of the 1.8 k and 4.3 k ohm resistors. The plate goes through a 100 k ohm resistor to plus 250 volts. The plate also goes through a 0.1 mike cap then a 470 k ohm res to ground. A legend at the bottom says quote gain = 6.7. AC SERIES. End quote. End verbal description.
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