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. The American RETMA tube designation number for this tube is 6CA7.
In common with all 'E' prefix tubes, using the Mullard-Philips tube designation, the EL34 has a heater voltage of 6.3V. According to the data sheets found in old vacuum tube reference manuals, a pair of EL34s with 800V plate voltage can produce 90 watts output in Class AB1 in push-pull configuration. However, this configuration is rarely found. One application of this type was in "Australian Sound" public address amplifiers commonly used in government schools in Australia in the 1950s, using four EL34s for ~200 watts. More commonly found is a pair of EL34s running Class AB1 in push-pull around 375-450V plate voltage and producing 50 watts output (if fixed bias is used), while a quad of EL34s running Class AB1 inpush-pull typically run anywhere from 425-500V plate voltage and produces 100 watts output. This configuration is typically found in guitar amplifiers.
The EL34 was introduced in 1953 by Mullard and its parent company Philipsand, although no longer made by them, is manufactured by JJ Electronic, Shuguang, Svetlana and Reflector (Sovtek, Electro-Harmonix, Tung-Sol and some brands), amongst others. Some firms make a related tube called an E34L which is rated to require a higher grid bias voltage, but which may be interchangeable in some equipment.