The vacuum tube is then known as a triode, tetrode, pentode, etc. Its formal name is bidirectional triode thyristor or bilateral triode thyristor. Eliminating a preamplifier triode in radios made them cheaper. De Forest's tube developed into the triode the first electronic amplifier. Inventions of the triode amplifier, motor-generator, and detector enabled audio radio. The detector and first audio stage were provided by a dual diode/triode combination tube. The driver may employ a cascade of transistors and a triode. Several radio amateurs have experimented with the possibilities of using VFDs as triode amplifiers. In a specialized circuit called "inverted triode", the roles of anode and grid are reversed. The principle of operation is identical to that of a vacuum tube triode. He would later help refine the triode vacuum tube. In addition to improving the triode vacuum valve, he developed directional loudspeakers. The wide-band regenerative receiver used an RCA 955 acorn triode. Lee de Forest placed a screen, added a "grid" electrode, creating the triode. It was the vacuum triode that made practical radio broadcasts a reality.