In their search for new sounds, the band incorporated musical instruments such as the Indian tambura and tabla, and clavichord, vibraphone and tack piano into their work for the first time. The guitar sound on the album was more robust than before, through the use of new Fender amplifiers; the choice of guitars, which included Harrison using a Gibson SG as his preferred instrument; and the introduction of Fairchild 660 limiters for recording. With no expectations of being able to re-create their new music within the confines of their live shows, the Beatles increasingly used outside contributors while making the album. This included the band's first use of a horn section, and a selection of these sounds were then added to the musical backing of "Tomorrow Never Knows". The process was carried out live, with multiple tape recorders running simultaneously, and some of the longer loops extending out of the control room and down the corridor.