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One of the first amplifying radio receivers, the De Forest Type RJ6 receiver made by the De Forest Radio Telephone and Telegraph Co., from an advertisement in a 1915 trade journal. It was a one-tube radio that used the Audion (triode), the first amplifiying vacuum tube, invented by Lee De Forest in 1906, both as a detector to rectify the radio signal, and as an audio amplifier. The Audion is mounted on the front panel so the operator can see if the filament is lit, and adjust the filament current by its brightness. The Audion is mounted upside down, with the delicate filament hanging down so it won't sag and touch the grid. The large dial on the right is the tuning, the multiposition switch in upper center adjusts the plate voltage, and the multiposition switch in lower center adjusts the filament current. The single Audion did not produce enough audio power to drive a loudspeaker, so the headphones at left are used for listening.