Easily identifiable by their acrylic glass (a.k.a. Plexiglas) front panel, which earned them the nickname, "Plexi"s. These now have significant collectors' value. Amplifiers from the 1970s onwards can be distinguished most easily by their brushed metal front panel, and are known as "ali panel" Marshalls. After 1973, in order to streamline production, the inefficient hand wiring was discontinued and Marshall valve amplifiers were switched to printed-circuit-board paths. The result is a similar sound that some enthusiasts dub as more "sterile" sounding although this is a hotly debated topic within the musician community. Also, much of the tonal debate between the plexi- and aluminium-panel Marshall amps comes from the fact that in 1974 Marshall's US distributor had them change all of the amps sold in the US and Japan over to the much more rugged 6550 instead of the EL34 output tube. This produced a much different sound than the EL34--a sound perceived as less smooth and more metallic when overdriven. This change was brought about due to reliability problems with the EL34's, and the 6550 generally allowed the amps to make it through warranty without problems. The circuit changes required to switch the amps were very minor, and it was easy to change from the 6550 to the EL34 or vice versa by changing a few resistor values, moving the tap for the feedback loop and rebiasing the amp. The mystique surrounding the Plexi series only increased due to the roar first heard on Van Halen's 1978 eponymous debut.