Vacuum tubes audio: 1/1/12

The EL84 (European designation - known as the 6BQ5 in North America) is a vacuum tube of the power pentode type. It has a 9 pin Noval base and is found mainly in the final output stages of amplification circuits, most commonly now in guitar amplifiers, but originally in radios and many other devices of the pre-transistor era.
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 a preamplifier triode in radios, making them cheaper to produce. As the EL84 itself is a 9 pin Noval, 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. 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.

It is not exactly clear which company first introduced the EL84/6BQ5. The 12-watt moderate end pentode was mentioned in the 1953 European Telefunken tube catalog. The EL84 was developed to replace the pre-war and early post-war tubes like the EL11 and EL41.

Most EL84 tubes that didn't meet the 12-watt plate dissipation test ended up as EL82 tubes with specked 9-watt power dissipation. Philips/Valvo followed in 1954 with their version of the EL84, introduced at the London Radio show in September of 1954 under the Philips Mullard brand.

As Telefunken introduced the EL84 in Europe, Tungsol offered the 6BQ5: a beam power tetrode very similar in specs to the Tele EL84. The only difference is that the cathode is connected to pins 1 and 3, whereas the EL84's cathode connects only to pin 3. In the 1960s the EL84 was used in nearly every European class-A table top radio, delivering 5.5 watt single ended or 12.5 watt in push pull at 6.5% distortion.