 Mark IIIs contain either four or five 12AX7 tubes in the pre-amp section, depending on if they have the reverb option.Simul-class Mark IIIs usually contain two 6L6s in the inner sockets and two EL34s in the outer sockets for 15w/75w use.These are distinguished by either the absence of a marking, a black dot, or a black marker stripe above the power cord entry.