The EL34 is a thermionic valve or vacuum tube of the power pentode type. It has an international octal base (indicated by the '3' in the part number) and is found mainly in the final output stages of audio amplification circuits and was designed to be suitable as a series regulator by virtue of its high permissible voltage between heater and cathode and other parameters.
These are known as "Fat Boy" tubes because 6CA7/EL34 tubes made in the USA have much larger glass bulbs than their European EL34 equivalent. These have large plates with 3 holes and are known to sound very good and to be very rugged.