A couple of other types of Power Valves that you might see in some guitar amps are the 5881, and 6550, I seem to remember seeing a set of six matched 6550 Valves used in a Bass Amp once, I think it was the venerable Ampeg SVT, or I could be mistaken, Marshall used 6550 Valves in their Marshall Major 200 Watt amps.
One interesting observation is that, as guitar amplification evolved from the relatively low-powered models from the 1940, the Power Valves not only increased in power output, amp designers were using more of them in the power-amp sections of the amps, in the 40's most guitar amps used either a single Power Valve, or a couple of them, these early amps produced anywhere from 1 watt to about 5 or 10 watts at the most, during the 50's and 60's more powerful amps were designed and built due to the demands of guitarists wanting to be able to be heard clearly over the noise the drummer and crowds were making, Vox are a good example with their Vox AC15 and AC30 amps, the AC15 used two EL84 Valves to generate 15 watts of audio power, in response to requests for a more powerful amp, Dick Denney, who designed the AC15, designed the AC30 which used four EL84 Valves to generate 30 watts of audio power, the Vox AC30 circuit is much like the AC15 in other respects, the Vox AC30 was used by famous bands, like The Beatles, The Shadows, and The Rolling Stones to name a few, Queen's Brian May used a total of 18 Vox AC30 amps while on tour, the Vox AC30 has a very musical chiming sound to it.
Another thing to mention is that in the early days when Rock music was evolving, bands didn't have access to the high-powered P.A. systems that we have today, so they had to use high-powered guitar amps to be able to be heard, as a consequence, or maybe because bands could be heard better, the music became louder and heavier, apparently loud music tends to generate feelings of excitement in humans.
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Originally Posted by Chuck
