The EL84 is a thermionic valve of the power pentode type. It has a 9 pin Noval base and is used mainly in the power output stages of audio amplification circuits, most commonly now in guitar amplifiers, but originally in radios and many other devices of the pre-transistor era. The EL84 is smaller and more sensitive than the octal 6V6 that was widely used in the USA and around the world until the 1960s. An interchangeable North American type is the 6BQ5 (the RETMA tube designation name for the EL84).
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 (the EL84 requires less than 4.4Vrms for 5.7W output compared with 9Vrms for 5.5W from a 6V6 or 8.7Vrms for 11W from an EL34). This eliminated the need for a preamplifier triode in radios, making them cheaper to produce. As the EL84 uses a 9 pin Noval construction, 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 and other audio equipment. A single EL84 was used in low-cost equipment, and a push-pull pair for lower distortion and higher power.
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. Many guitar amplifiers routinely run EL84 tubes in excess of 400VDC, with the Traynor Guitarmate reportedly putting out 25w RMS with 2 EL84's in a push pull configuration and a B+ between 400-420VDC.
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 in 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.