The most deafening amp I've ever owned: 100 watts from four EL34's. It does have a master volume so theoretically it's possible to get decent tones out of it with slightly lower SPL's as well. Having said that, the more you crank the master volume, the fuller and more balanced the sound gets. Remember to use hearing protection - this is no joke. It's interesting how well this amp handles blues, aggressive metal or anything in between. Also worth noting is that this baby loves to be boosted: a tubescreamer or a treble/clean boost pedal really turns this beauty into a rock'n'roll / hair metal workhorse or even a furious metal beast.
This particular amp is one of the "better ones" being a vertical input model from the early days of the JCM800 series. So what makes it better? First of all: durability. The input jacks, pots and tube sockets aren't mounted on the circuit board, but only on the chassis, which really makes the amp more robust and service-free than the later (say, post-1985) JCM800's. These early JCM800's also had more power supply filtering, which results in a punchier and fuller sound when cranked whereas the newer models actually sounded worse when run at or close to full output power. The amp has been fully serviced (including new electrolytic capacitors) in April 2012.
Specifications
Marshall Amps Info & Schematics
s/n: 01567P (February 15, 1982 is written on the inspection tag)