Check the listing for details. MXR M-133 Micro Amp Pedal. Condition: New. Listed at 105.99 USD. The MXR M133 Micro Amplifier pedal adds a preset amount of gain, using a single control. With a guitar, this lets you boost your signal for lead work, adjust between two different guitars with unmatched output (i.e., humbucker to single coils), or it can supply a permanent boost in a long effects chain where signal drop-off is a problem. .