Schottky diodes are also used as rectifiers in switched-mode power supplies. Some preforms are also used in diodes, rectifiers, optoelectronic devices and components packaging. Rectification can be controlled (uses Silicon Controlled Rectifiers (SCRs)) or uncontrolled (uses diodes). TRIACs are a subset of thyristors and are related to silicon controlled rectifiers (SCRs). With the availability of affordable silicon diode rectifiers, alternators were used instead. This is in relation to solid state relays, such as triacs and silicon controlled rectifiers. A set of rectifiers (diode bridge) is required to convert AC to DC. The guitars were played through Mesa Boogie and Dual Rectifiers heads connected to Marshall amplifiers. These locomotives utilise silicon rectifiers for conversion of AC power into DC. These locomotives utilise silicon rectifiers for conversion of ac power into dc. It is widely used in the triggering circuits for silicon controlled rectifiers. These locomotives utilise silicon rectifiers for conversion of ac lower into dc. These tubes were eventually replaced by SCRs (Silicon Controlled Rectifiers). Also devices with mercury arc rectifiers or vacuum tubes were in use. Simple DC power supplies can be made with rectifiers.