Even now, many believe this is Eric's greatest guitar work. HIDEAWAY (1966) by John Mayall's Bluesbreakers- featuring Eric Clapton One of the first great rock guitar tones was the Blues Breakers 'Beano' album featuring a young Eric Clapton playing a Les Paul Standard through a Marshall amp. For me, the quintessential Plexi sound is a JMT45 turned up to 10, as heard from Clapton on the “Beano” album. The Plexi sound is synonymous with incredible volume and crunchy overdrive, as guitarists of the ’60s cranked the volume do achieve more distortion. These JTM45s got into the hands of Eric Clapton, Jimi Hendrix and Paul Kossoff, and created the famous “British Sound”. These changes resulted in more amp distortion, which became an increasing trend in the early days of British Rock and Roll. It used KT66 power valves (or tubes, depending on your persuasion) compared to the 6L6 valves of the Bassman, and higher gain ECC83 preamp valves, that the 12AY7s of its American cousin. However, due to the availability of components, he ended up making something that sounded quite different – The JTM45! This was the first Marshall amp, and one of the most iconic. In the early ’60s, Fender amps were expensive to import, so in 1962 Marshall decided to start building his own amplifiers to replicate the Fender Tweed Bassman. The store built a reputation amongst guitarists such as Jimmy Page, Pete Townshend and Ritchie Blackmore, all looking to get their hands on rare Fender amps from across the pond. Jim Marshall was a drummer and London music store owner. So really, when we talk about the Plexi, we mean early Marshall amps. The Plexi gets its name from the early Marshall amps that used a plexiglass front panel during the ‘60s, before this was replaced by brushed metal in 1969.
So, what is the Plexi and is there a definitive Plexi sound? What is a Plexi? I think of the Plexi sound as Hendrix through his Fuzz Face or Clapton on the “Beano” album with John Mayall and The Bluesbreakers. That is why there is a 100K resistor from cathode to ground, and no resistor from plate to B+.When I asked a colleague here at Andertons “what is the Plexi sound?”, he said “Van Halen I” - which brought up an interesting point. It is essentially a gain stage turned upside down. The output of the V2B cathode follower stage is the cathode, not the plate. V2 is the cathode follower stage V2A is a gain stage and increasing the cathode resistor from 820 ohms (warm and clean) to 8.2K will result in a cold clipper, which is then fed to V2B in a cathode follower configuration (V1A plate output goes directly to V2A'a grid), with V2A being a current amplifier, with less than unity gain stage (gain factor typically 0.9 by voltage) with low impedance output and high current drive, which feeds the tone stack and makes the tone controls more effective than if you feed the tone stack with a conventional gain stage. Two master volumes, actually, more or less following the model of the dual output level controls found on a Mesa Dual or Triple Rectifier. The Coco 50 appears to be based on a 1987 base model, with more gain and a master volume control. He's very good, and if he'll help you at all, he'll give very good advice. Maybe you should ask David Bray for recommended changes to make to his amp.