 The reason for using a fetron was to address some of the problems associated with microphonic 12AX7 tubes in a high-gain situation; its use was later discontinued as newer production tubes were able to withstand the extreme conditions within the amplifier.The Mark IIB is credited as the first guitar amplifier with a tube-buffered effects loop.However, the loop was placed between two critical gain stages, and tended to overdrive some instrument level effects, and also caused volume pedals to act as remote gain controls for the lead mode.