At risk of this attracting the tan pants mafia, anomaly’s thread on the main forum had me looking at my early Mark IV and wondering what the differences are (if any) in the Mark IVA, A/B and B circuits. I know that “A/B” wasn’t a formal designation by Mesa, but I’ve seen different opinions on whether there are any differences between “A/Bs” and Bs, other than “A/Bs” using Mark III transformers.
I’ve got an “A/B” (number 5860). It has the Mark III transformer but having taken a look at the circuit and comparing it to pictures of Mark IV A and B circuits there seem to be some similarities to both and one area that looks like neither.
The first photo is a gut shot of my amp. I’ve circled green the parts of the circuit that look like the A circuit and blue the parts that look like the B circuit. There’s also a bit I’ve circled orange that doesn’t really look like either(?). The next few pictures (with green circles) are the A circuits and the other two (with blue circles) are the Bs, just to illustrate where the two circuits seem to differ the most (to my untrained eyes) visually.
I’m no expert, so would be interested to hear what it is that I’m seeing. I’m absolutely not about to suggest the A/B is somehow better than As or Bs - just genuinely curious about the circuits / components.
I’ve got an “A/B” (number 5860). It has the Mark III transformer but having taken a look at the circuit and comparing it to pictures of Mark IV A and B circuits there seem to be some similarities to both and one area that looks like neither.
The first photo is a gut shot of my amp. I’ve circled green the parts of the circuit that look like the A circuit and blue the parts that look like the B circuit. There’s also a bit I’ve circled orange that doesn’t really look like either(?). The next few pictures (with green circles) are the A circuits and the other two (with blue circles) are the Bs, just to illustrate where the two circuits seem to differ the most (to my untrained eyes) visually.
I’m no expert, so would be interested to hear what it is that I’m seeing. I’m absolutely not about to suggest the A/B is somehow better than As or Bs - just genuinely curious about the circuits / components.