The strings on the new recording of I Promise are undoubtedly Mellotron. Each tape loop on a Mellotron is a recording of actual musicians playing the note, which is why it sounds a lot more real than any sort of synthesizer. However, the attack on each note is very unnatural in a way that telegraphs Mellotron (particularly at 2:04). The strings also have the Mellotron’s characteristically stable vibrato, although that can be found on vintage String Synths as well. Nigel’s production here leaves the strings quite clear, so we can get a very good assessment of them.
Jonny at the 1996 Pinkpop Festival, using his Fatar Keys Sl-161 to trigger his E-mu Classic Keys module (visible on the right, on top of the Roland Space Echo).
I Promise was only ever performed in 1996. On that tour, Jonny got all of his polyphonic keyboard sounds (including strings) from his E-mu Classic Keys sound module. The module’s samples include a Mellotron as well as vintage String Synthesizers. Its string sounds can also be heard on performances of Lift from that year, as well as Fake Plastic Trees, Street Spirit, and Nobody Does It Better.
Jonny used the E-mu unit for strings on every performance of I Promise. The the strings are a little buried on most bootlegs, but when they’re clear they do sound similar to those on the album – both the playing of the part and the sound of the strings.
That said, the band certainly had their Mellotron M400 by the time the recording of I Promise was attempted. On OK Computer, Radiohead predominantly used their Mellotron for its choir sounds, but its strings do make some appearances (they’re layered softly with Thom and the Mellotron choirs on the “rain down” section of Paranoid Android, for example).
Given sound quality of the strings, as well as the the strong Mellotron “attack” combined with the more organic sustain (but with stable vibrato), I think we can safely assume that Jonny used the real Mellotron (rather than the Classic Keys) for the recording of I Promise.
Jonny playing the Mellotron M400 during the recording of In Rainbows.
Note that it was not until touring in 1997, after OK Computer had been recorded, that Jonny added an Akai S3000XL, with samples of the band’s own Mellotron, to his setup.