 By the early 1930s most car radios, no longer experimental, were superheterodynes and used a vibrator power supply to step up the low voltage to high voltage ("B+" voltage of anywhere from 90 to 250 volts) for the vacuum tubes.Vibrators are relatively unreliable as electromechanical components of limited life, buzz audibly, and produce radio interference.A few radios used a bulkier and more expensive motor-generator or motor-alternator set called a "dynamotor" that spun a high-voltage generator or alternator using a 6 or 12V DC motor.