The EL84 output pentode was introduced by Mullard in 1954. To demonstrate its use and quality the Mullard applications lab produced an amplifier design. The now classic 5-10 high quality audio amplifier.
The EL84 has been very successful. With the renewal of interest in valve audio in the 21st century new manufacturing has been established in Europe and the far east. This exhibit is of Russian manufacture and branded as a Groove Tube. Rob says that a pair of these Sovtek EL84s were supplied in a Fender pro-junior amplifier.
When used as a pair in an ultra-linear push-pull configuration with 43% distributed loading the output could reach 10 W with 0.9 % total harmonic distortion. The valve operating conditions so produced were intermediate between a triode and a pentode. The operation of the distributed loading was to introduce negative feedback directly within the output stage.
See alsoUL84 & PL84 produced for designs using AC or DC mains without a mains transformer and a series heater chain. The PL84 was used for TV frame output service.
Robert Coleman writes: Groove tubes grade their audio valves on a scale of 1 - 10 according to gain. Clearly a large spread in manufacturing tolerances. They also classify the valves by colour and give on their website the types to use for different guitar sounds.
The end wall of the anode has gaps and the centre of the working face is where the extra heat dissipation afforded by the flanges is found and required. These valves run hot and our other Sovtek EL84 has discolouration on the label due to the heat.
The design is as a true pentode with three wire grids.
The thin glass tube envelope is 22 mm in diameter and, excluding the B9A base pins, is 69 mm tall.
References: Data-sheet & 1040. Type EL84 was first introduced in 1953.