In the early 1950s, Tandberg opened a branch plant at Kjelsås (in Oslo) to produce reel-to-reel tape recorders. Their first model was introduced to the market in 1952. Over the next decade, Tandberg quickly incorporated a number of leading-edge concepts; the model 2 Hi-Fi of 1956 had three tape transport speeds, allowing improved high-frequency response. The model 3 Stereo from 1957 was Tandberg's first stereo system which allowed playback of stereo tapes, however the ability to record in stereo was not made available until the introduction of the model 5 in 1958, which allowed for the connection of an external recording amplifier to record the second channel onto the tape. Their first full stereo system, the model 6 of 1960, featured 4 amplifiers, 2 for recording and 2 for playback respectively, giving the operator total control of both audio channels. In the 1960s Tandberg introduced the cross-field recording technique in the model 6X, allowing their recorders to handle higher frequencies than competing models. Tandberg licensed the concept to Akai, who utilized it extensively in the 1970s and 80s in their Akai and Roberts recorders. In 1964 Tandberg inaugurated a new method of language teaching, the language laboratory, and with it their first fully transistorized tape recorder, the model 10, a recorder designed to serve as the teacher recorder in language classrooms. However the first fully transistorized recorder marketed to consumers was the model 12 of 1966 which among other things featured low impedance microphone inputs, designed for their first dynamic microphone, the TM 4. In 1969 Tandberg finally abandoned the use of vacuum tubes in their recorders, and the 6X which had introduced cross-field recording to the world, the last tube recorder still manufactured by Tandberg, went out of production. That same year a new lineup of transistorized Tandberg recorders entered the market to fill the gap left by the preceding models.