The EL34 is a power pentode type. It has an international octal base  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. Russian analog is 6P27S

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 in push-pull typically run anywhere from 425-500V plate voltage and produces 100 watts output. This configuration is typically found in guitar amplifiers, especially Marshalls and is quite hard on the valves.

The EL34 is a pentode, while the 6L6, which delivers a similar range of power output, is a beam tetrode which RCA referred to as a beam power tube. Although power pentodes and beam tetrodes have some differences in their principles of operation (the beam plates of the beam tetrode or fifth electrode (3rd grid) of the pentode, both serving to hinder the return of unabsorbed electrons from the anode (or plate) to the 4th electrode (2nd grid)) and have some internal construction differences, they are functionally closely equivalent. Unlike the 6L6, (EIA base 7AC) the EL34 has its grid 3 connection brought out to a separate Pin (Pin 1) (EIA base 8ET) and its heater draws 1.5 Amps compared to the 0.9 Amp heater in the 6L6. However, Sylvania (and possibly GE) marketed a tube as 6CA7 which was not only in a markedly different 'fat boy' envelope, but used a beam forming plate much like a 6L6. Examining the mica spacer on the top of the tube will confirm the lack of a suppressor grid. Although these tubes have similar (but not identical) characteristics, they are made very differently.

Matched EL34 Vacuum Tubes by Russian manufacturer, Svetlana
The EL34 was introduced in 1953 by Mullard and its parent company Philips and, although no longer made by them, is manufactured by JJ Electronic, Shuguang, Svetlana and Reflector (Sovtek, Electro-Harmonix EI Serbia / Yugoslavia 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.