John Frusciante’s MXR Micro Ampaccess_time First seen circa 2002
Chronologically looking, John first used the MXR Micro Amp as part of his pedalboard during the By the Way tour. It appears that he used it all the way until his departure from the band after Stadium Arcadium.
As far as usage and impact on the sound, unfortunately, there’s doesn’t seem to be a single interview out there where John goes into specifics about the Micro Amp.
What is most likely is that he used it to crunch up his clean tone. Most noticeable perhaps is the song Can’t Stop where the guitar is driven almost to the point of breakup. You can hear this often even in his solo performances during RHCP concerts, where the guitar sounds clean but as if being pushed to the limit.