Two EL84's, the one on the right is an original Mullard & the other is a modern day Russian Sovtek substitute.
Like the EL34, the EL84 (a miniature power pentode) was developed & introduced by Philips in the mid-‘50s. Mullard soon followed suit with the manufacture of the EL84 and introduced their own design amplifier incorporating them - the infamous "Mullard 510 Model" - which used 5 valves to produce 10 watts (1 x EZ81, 2 x EL84, 1 x ECC83 & 1 x EF86).
Following this, every Hi-Fi amplifier manufacturer seemed to use the EL84, boosting the Hi-Fi market. At around this time musical instrument amplifier manufacturers, seeing the potential of this great little bottle on the B9A base, soon started producing amplifiers using them. The list would appear to be endless but Selmer were certainly amongst the first few. However to the best of my knowledge, it would be Vox that would push the EL84 to the top of it's stardom with their Vox AC30 combo. This amp incorporated no fewer than four EL84's in the output stage and utilised them to almost their design maximum ratings with a whopping 30 watts from the four valves! (Maximum 17 watts from a pair). The AC30 will go down in history.
Overseas the EL84 is known
as the 6BQ5 and is often printed with both numbers (EL84/6BQ5). The EL84 however
still lives on in its own right, and both Hi-Fi and the musical instrument amps
of today are still using them. They are currently manufactured by most of the
valve manufacturing companies in Russia, China, Yugoslavia etc. However I
haven't found one yet that still lives up to an original Mullard in performance!