Explanation: For Class B amplifier operation, the vacuum tube's grid is biased to the approximate plate current cutoff point. This ensures that plate current flows for only one-half of the input AC cycle. The cutoff bias voltage (Ec) can be calculated using the formula: Ec = -Ep / μ Where: * Ec is the cutoff bias voltage * Ep is the plate voltage * μ (mu) is the amplification factor Given a plate voltage (Ep) of 1000 V and an amplification factor (μ) of 25: Ec = -1000 V / 25 = -40 V Thus, a negative grid bias of 40 V is required for Class B operation. The other provided parameters (plate current, filament current, and mutual conductance) are not used in determining the cutoff bias voltage. Other bias values would place the tube in a different operating class.