I have to admit I am a bit of a vintage electronics technologist. One of many pass times includes bringing vintage vacuum tube automobile radios back to life. In working with modern DC sources I’ve seen innovations come about in the past decade for efficient power conversion, including soft switching and synchronous rectification. A funny thing however, for those who have been around long enough, or into vintage technologies like me, is that these issues and somewhat comparable solutions existed up to 70 years ago for automobile radios and other related electronic equipment. What is old is new again!
As we know, vacuum tubes (or valves to many) were to electronics back then as what semiconductors are to electronics today. The problem for portable and mobile equipment was that the vacuum tubes needed typically 100 or more volts DC to operate. They did have high voltage batteries for portable equipment but for automobiles the radio really needed to run off the 6 or 12 volts DC available from the electrical system. The solution: A DC/DC boost converter!
Up until the mid 1950’s most all automobile radios used vacuum tubes biased with high voltage generated from a rather primitive but clever DC/DC boost converter design. The inherent technological challenge was semiconductors did not yet exist to chop up the low-voltage, high-current DC to convert it to high-voltage, low-current DC. Of course if the semiconductors did exist this would all be a moot point! Making use of what was available the DC/DC boost converters employed what were called vibrators, which are a form of a continuously buzzing relay, to chop up the low-voltage DC for conversion. Maybe some of you are familiar with the soft humming sound heard when an original vintage automobile radio is turned on, prior to the vacuum tubes finally warming up and the audio taking over? That humming is the vibrator, the “heart” of the DC/DC boost converter in the radio.
Figure 1 below is an example circuit of vibrator-based DC/DC boost converter in a vintage automobile radio. This is just one of quite variety of different implementations created back then. Two pairs of contacts in the vibrator act in a push-pull fashion to convert the low-voltage DC into a low-voltage AC square wave. This in turn is converted to a high-voltage square wave by the transformer. Because the vibrator is an electro-mechanical device, it is limited in how fast it can switch. Switching frequencies are typically about 100 to 120 Hz. The transformers used are naturally the steel-laminated affairs similar in nature to the transformers used to convert household line voltage in home appliances. Very possibly some radio manufacturers used off- the-shelf appliance transformers in reverse to step up the voltage! Often a small rectifier vacuum tube, such as a 6X4 (relatively modern, by vacuum tube standards) would be used to convert the high voltage AC to high voltage DC, but in this particular example I am showing here another two pairs of contacts on the secondary side switch simultaneously with the first pairs of contacts to rectify the high voltage AC. Highly efficient synchronous rectification, up to 70 years ago!