It was developed to eliminate the need for a driver tube in radios, and has rather more gain than is usual in a power pentode, producing full output from a relatively small drive signal. This eliminated the need for one preamplifier triode in radios, making them cheaper to produce. As the EL84 itself is a 9 pin miniature, it was also cheap to produce and manufacturers were quick to adopt it in general use, and they are found in many old European valve radios.
In common with all 'E' prefix tubes, using the Mullard-Philips tube designation, it has a heater voltage of 6.3V. It is capable, when used at its plate rating of 300 volts maximum, of producing 17 watts output in Class AB1 in push-pull configuration.
Developed by Philips in 1953 for use in the British Mullard 5-10 amplifier, the EL84 came to prominence when used in Watkins (and later the Vox) amplifiers preferred by many British invasion bands of the 1960s. When overdriven, the EL84 power tubes of these amplifiers produce a distinctive chiming, articulate, treble-heavy sound when compared to 6L6 tubes more commonly used in American amplifiers of the era such as those from Fender.
Other equivalent tubes are the 7189, an extended-ratings version of the tube for industrial applications and the 6P14P (Cyrillic: 6П14П) produced in the USSR by the Reflektor plant, which is a direct equivalent of EL84/6BQ5. A slightly modified version of the 6P14P is currently being manufactured in Russia for Sovtek. An extended-ratings version of the 6P14P is also available - 6P14P-EV (Cyrillic: 6П14П-ЕВ) and is known among US guitar players as "EL84M" or the "Russian military EL84". While not necessarily a true "military version" of the tube (in fact it is more comparable to the 7189), 6P14P-EV are known for their low noise and durability. Large NOS (New Old Stock) supplies of the tube are available.
Current production of the tube takes place in Russia (Sovtek and Electro-Harmonix brands), Slovakia- Čadca (JJ Electronics), and Serbia- Niš (Ei). The Sovtek EL84 is often sold under their own brand name by other well-known electric guitar and guitar amplifier manufacturers - such as Fender or Mesa Boogie.
Also see 6P1P.