In a grounded grid amplifier using a triode vacuum tube, what would be the approximate B+ voltage required for an output of 400 watts at 400 mA with approximately 50 percent efficiency?

To find the required B+ voltage, you must first calculate the total DC input power needed. Since the amplifier operates at approximately 50 percent efficiency and provides an output of 400 watts, the total input power must be double the output. Therefore, the required input power is 400 watts / 0.50, which equals 800 watts. Next, use the standard power formula (Power = Voltage x Current) to find the voltage. You know the input power is 800 watts and the plate current is 400 milliamperes (0.400 amperes). Rearranging the formula to solve for voltage gives Voltage = Power / Current. Dividing 800 watts by 0.400 amperes results in 2000 volts. Distractors like 1000 volts are incorrect because they fail to account for the 50 percent efficiency, effectively calculating for an impossible 100% efficient amplifier.