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.