C battery (vacuum tubes)

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In electronics, a C battery is any battery used to provide bias to the control grid of a vacuum tube. Until the early 1930s this was common practice in valve (tube) radio sets but was largely superseded by grid leak resistors or voltage divider biasing.

Grid bias batteries are still manufactured today, but not for radio use. They are popular in schools and colleges as a convenient variable voltage source in science classes. The most popular battery is the 9 volt type with taps every 1½ volts that accept a banana plug.

The alphabetic designation of these batteries is derived from the historic identification of the elements of the vacuum tube. Initially, the only such device was a diode with only a plate and cathode. Following the direction of electron flow, these electrodes were identified as "A" and "B" for the cathode and anode (plate), respectively. Later, when the control grid element was added to create the triode tube, it was logically assigned the letter "C." Subsequent addition of the "screen grid" and "suppressor grid" to improve the performance of the triode, resulting in the tetrode and pentode, respectively, did not require an extension to this series of batteries. These grids were either resistively-biased from the existing batteries, or were connected to ground or the cathode.

The "C" (grid) battery is not to be confused with the C cell. In devising the lettered sizes for single-cell "batteries," it seems that there was a conscious effort to avoid "B" due to its association with a much higher voltage, and single-A was also avoided, but there was apparently no concern for possible confusion with the use of "C" as a size.