Tube-type radios require two sources of power: 1 to heat tube filaments and another (a high voltage DC) to establish the flow of electrons from filament or cathode to plate for the tube to operate. Until about 1926 radios had to be operated from batteries. Operating tube filaments on AC produced hum. The high voltage supply was also obtained with batteries. Radio batteries were bulky, messy, and expensive and required periodic recharging or replacement. Also, batteries could provide only moderate amounts of power which limited the volume of sound that could be produced.
In the mid-1920's "B eliminators" were developed that could replace the high volt- age batteries. These consisted of a trans- former that stepped up the AC line voltage and a rectifier and filter that converted the stepped-up AC to a steady high voltage DC supply. In 1926 tubes with filaments that could operate satisfactorily on AC were developed. Filament voltages were obtained by stepping down the line voltage with a trans- former to give the voltage needed to operate the tubes (1.5 - 6.3v). Plug-in electrical sets were now practical in which the electrical requirements of the set could be obtained entirely from the AC line.
Around 1933 a method was developed to obtain operating voltages that did not require a power transformer. Tubes were developed whose filaments could operate on higher voltages. A string of them in series could be connected directly across the 110v AC line. A ballast tube or line cord with built-in resistor was also usually required to reduce the line voltage to that required by the string of tubes. The high voltage supply was obtained simply by rectifying the AC line directly. This gave a high voltage of about 110v DC.Output tubes were developed to give adequate audio power with this volt- age. These transformerless sets were used mainly in small, inexpensive table radios. These sets were common from the mid-1930's until the end of the tube era in the 1950's.
In some rural areas and in automobiles, no AC power source was available. In the mid-1930's a method was developed for obtaining the high voltage supply. A vibrator was used to chop up the DC battery current to create pulsating DC. This was stepped up by a transformer to give a high voltage alternating current. The AC was then rectified into DC by a tube. Alternately, the vibrator itself (synchronous type) could switch to 2 halves of the AC signal to give the high voltage DC current.
Between the 1920's and the 1940's other line currents were sometimes used, e.g. 32v DC or 110v DC. Radio sets were manufactured to operate from these voltages, employing at least one of the methods mentioned above to obtain proper supply voltages. In the late 1950's, in an effort to eliminate the vibrator in automobile radios, a series of tubes was developed that could operate with the voltage supplied by the car battery directly as the high voltage supply, i.e. only 12v DC. No output tube could give adequate power under these conditions so transistors were used instead. Such hybrid radios lasted only a few years, being replaced by the now familiar all-transistor radio.
Since portable radios have always been popular,radios with tubes
that could operate satisfactorily off batteries continued to be made until
they were replaced by transistor radios in the late 1950's.