2A3 tube – 1931 magnavox concerto phonograph repair

A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output. A typical triode audio power amplifier will have a driver that provides voltage gain, coupled to a triode (like 2A3 and 300B) or a pentode or kinkless tetrode such as EL34 or KT88 connected as a triode, connected to the loudspeaker through an audio transformer in a common cathode arrangement. The triode is biased to Class A operation by applying a suitable negative bias voltage to its input control grid , or by raising the cathode potential with biasing components. Magnavox, a company founded by Peter Jensen and Edwin S. Pridham in 1917, was the sole manufacturer of “moving coil” horn loudspeakers, which the two had invented. Located originally in Napa, and later in Oakland, California, the company made public address systems under the Magnavox “Telemegafone” name. These were manufactured from the late teens through the mid-1920s. Also manufactured in the Oakland plant in the 1920s were European-styled telephones, as well as portable electric heaters.

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