Abstract: A modification unit for a musical instrument amplifier includes a modification circuit and a connector for connecting the modification circuit to a tube socket in place of one of the tubes in the pre-amp stage of the amplifier. The pre-amp includes at least a gain stage with a gain tube and a socket for receiving the gain tube where the tube socket has at least one plate contact, at least one cathode contact and at least one filament (heater) contact. The modification circuit includes a jack for receiving an electronic signal originated by the musical instrument and a circuitry for modifying the received electronic signal to generate a modified signal.