Mark IIA
The Mark II introduced channel footswitching, and wasn't referred to as the "Mark IIA" until the Mark IIB was issued. It was also available as a head, which could be hooked up to a number of different speaker combinations, although a 1x12 cabinet was the most common. The preamp gain on the Mark IIs occurs after the tone controls and so, according to Mesa/Boogie, the IIA has a "tighter, more focused sound" than the Mark I. The Mark IIA's control panel was extended from the Mark I's to include a separate master volume control for the lead mode, and various push/pull switches including Pull Bright, Pull Treble Shift, Pull Gain Boost, a separate Pull Bright for the lead mode, and of course, Pull Lead. The 1/4" jack previously marked "1" was changed to just "Input," and "2" was changed to "Foot Switch." The Mark IIA was a great improvement over the Mark I, however it had a few major flaws that it received criticism for among collectors. 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. 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.
Read more about this topic: Mesa Boogie Mark Series, Mark II
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