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. 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.
With ultra-linear operation the screen grids were connected to taps in the in the output transformer primary winding. 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. The EL84 found use in many classic valve guitar amplifiers, classic and current valve hifi amplifiers including: -