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Use ohms law You have two voltage dividers, one in the microphone the two 470Ks and one in the power supply the two 100k pattern controlHere is one waybuilt stockMeasure the B+ under load microphone plugged into the power supply Note If your B+ is 120VDC you don't have to change the circuit http://www.gyraf.dk/gy_pd/g7/gic_s.gifChange the top voltage divider resistor value to give 60VDC to the backplate inside the microphone. The two 470K that set the backplate voltage part of the circuit You know you want 60VDC so start with 60VDC divided by 470K this gives the series current. B+ - 60VDC give the "top" resistor voltage drop and you know the current so calculate the "top" resistor valueNext add a resistor between the B+ in the power supply and the top 100K of the pattern control switch so you have 120VDC at the top of the 100K move the top connection of the pattern switch to the node of the top 100K and the new resistor. You are looking for a series current of 60VDC / 100K =.6ma. You have 200K already so the total value will be power supply VDC / .6ma. So if you have a 160VDC power supply under load, 160VDC/.6ma=267K total, add a 68Know go back and check the voltages adjust the values, because if your power supply is weak the B+ will move up a little because of less loading via the voltage dividers that said I use another way