 The new footswitching system relied on a relay, which made an audible popping noise when switching modes.The reverb circuit was also noise-ridden on some models.The IIA and IIB, and some late-model Mark I amps, used a JFET-based device called fetron in place of the input stage 12AX7 (V1), and included a switch for configuring the amp for either Fetron or 12AX7 operation.