Peter is right on. Since the grid takes very little current, a poteniometer
is used to adjust the bias voltage to the exact voltage. Usually you will
have a higher negative voltage that is tapped by the poteniometer and you
will have fixed resistors on each side of the pot to limit the voltage
range.
Every time you change tubes, you will need to reset the pot. Although tube
manufacturers specify the grid voltage, most manufacturers of grid driven
amps specify idle current and have you adjust the bias until you reach a
specified idle current. They usually do not care or specify the grid
voltage.
Whatever it takes is the correct answer.
Colin K7FM
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