with tremolo, which would fit in the boot of his car, and one of the most famous Marshall amps was born, the "Bluesbreaker" amp. This is the amplifier, in tandem with his 1960 Gibson Les Paul Standard (the "Beano"), that gave Clapton that famous tone on the John Mayall & the Bluesbreakers' 1966 album, Bluesbreakers with Eric Clapton. The Plexi and the Marshall stack The amplifiers from this era are easily identifiable by their acrylic glass (a.k.a. Plexiglas) front panel, which earned them the nickname "Plexi". In 1967, Marshall released a 50-watt version of the 100-watt Superlead known as the 1987 Model. In 1969, the plexiglass panel was replaced by a brushed metal front panel. Other early customers included Pete Townshend and John Entwistle of The Who, whose search for extra volume led Marshall to design the classic 100-watt valve amplifier. Ken Bran and Dudley Craven, Marshall's developers, doubled the number of output valves, added a larger power transformer and an extra output transformer. Four of these amplifiers were built and delivered to Townshend, and the Marshall Super Lead Model 1959, the original Plexi, was born in 1965. At the request of Townshend, Marshall produced an 8×12-inch cabinet (soon replaced by a pair of 4×12-inch cabinets) on top of which the 1959 amplifier head was placed, giving rise to the Marshall stack, an iconic image for rock and roll. The size of the wall of Marshall stacks "soon became an indicator of the band's status", even when rendered obsolete by improved PA systems; indeed, many of the "ridiculously huge arrays of heads and cabs" included dummies. Still, most modern 100-watt heads have roots in Marshall's design, even though they often contain many more features (or different valves, such as the more American-sounding 6L6 valves). Another valve change At this time, the KT66 valve was becoming more expensive, as the M-OV Company faced greater competition from Mullard. Hence, another valve change was made, with Marshall starting to use European-made Mullard EL34 power stage valves. These have a different overdrive character than the KT66s, which gave Marshalls a more aggressive voice still. In 1966 Jimi Hendrix was in Jim's shop, trying the amplifiers and guitars. Jim Marshall expected Hendrix to be "another American wanting something for nothing" but to his surprise, Hendrix offered to buy the amplifiers at retail price if Jim would provide him with support for them around the world.