Telling a Mark IV A from B

Love mine. Had it since bought new in '95. But after looking around this site, have no idea if it is an A or B. Searched and found lots of talk, but haven't found how to tell.

How would you tell my youngest son (8 years old) to find the difference?

Like I said, love it, but am curious......

If you download the manual from Mesa's website, you'll get the "B" manual.
You can tell from the back panel. The "A" has two effect loops, with the jacks arranged in a rectangular pattern. The "B" has one loop, with the Send jack positioned a bit higher and between the Left and Right Return jacks.
This is a "B":

If it has the old 'grainy' tolex it's definitely an 'A' - the majority of 'B' models have the newer 'smooth' covering. A quick way of telling roughly without looking round the back.

If it has the old 'grainy' tolex it's definitely an 'A' - the majority of 'B' models have the newer 'smooth' covering. A quick way of telling roughly without looking round the back.

Thanks guys, but I am still a bit confused...........

The back panel looks ALMOST exactly like the manual -- but there is a "ground A / B" switch under the fuse on mine and not on the manual.
So I guess that suggests a B?

There is the old school football feeling type tolex (not the newer smooth stuff.
Suggests A


Hmmmmmmmm

BTW - Serial number is 0071XX

I just found the satelite send info on the Mark IV thread. Looks to me mine is an early B. Odd about the Ground switch, though....

angelo said:

BTW - Serial number is 0071XX

I just found the satelite send info on the Mark IV thread. Looks to me mine is an early B. Odd about the Ground switch, though....

Click to expand...

It's not odd for the early B to have a ground switch and an attached power cord. My serial is lower than yours and is a B.

Boogiebabies said:

angelo said:

BTW - Serial number is 0071XX...

Click to expand...

It's not odd for the early B to have a ground switch and an attached power cord. My serial is lower than yours and is a B.

Click to expand...

Just an FYI: mine's a 0078xx and has both the attached pwr cord and ground switch....

The tolex is a good guideline to determine an A or B, but not definitive...as you can see, the picture posted shows a Version B with the old (Version A) tolex. I have a short head with the same tolex, also version B. I believe there was a short period of time in 1993 where they used the new Version B amp, but was still using the old tolex and SUS-4 cabinet.

Also, as mentioned, the early B's had the attached power cord, so that is also not a definitive indicator.

The effects loop is the way to tell. As mentioned:

Version A has two mono effects loops, one normal one switchable.

Version B has one effects loop with a mono send and stereo return. Version B also has a Satellite EQ jack (to send the EQ state to switch the EQ on a satellite amp), a Satellite Send jack (buffered output of one of the effects loop returns, as well as a Reverb jack, to switch the reverb on and off.

Note: some (or maybe all) Version A's also had a Satellite EQ jack, to activate the EQ on a satellite amp, but this jack was located under the chassis and not labelled.

Hope this helps. If someone posts a picture of the rear of a version A, to contrast with the posted picture of the Version B, the difference will be easily apparent.

Good advice, but there is an anomaly between the A and B transition. In Mid 1993 there were MK IV B's with A type effects loops, so not all can be determined by outward visual inspection.

My Mark IV got two loops, one switchable, one normal. No satellite option. Serial is IV40XX...So what do you guys think? Is it A or B?

So, now that my B version is settled. What might be the most noticeable differences in tone if I was to plug into an A?

Boogiebabies, what do you mean: "MK IV B's with A type effects loops"?

Do you mean that those amps have the updated "B" preamp circuit, but still use the "A" chassis, so they still have two effects loops, but no reverb jack...kind of a hybrid...?

Very interesting...

I don't understand why it's such a big deal!!!!


A-- 2 fx loop's ( normal, switchable), there is a satelite send and nearly all of them had a fix power chord.All A's had a sticker on the PT saying something about voltages or something.ground switch

Early B's had the stereo FX loop..had fixed power cables and even a ground switch.. This is a B..

New B's are unplugable power cable, no ground switch and sticker on the PT..


The Tolex doesn't really show if it's a A or a B becasue the mk IV pic in this thread is a B but it has rough tolex..But general rule of thumb is that the rough stuff is older..

cho said:

Boogiebabies, what do you mean: "MK IV B's with A type effects loops"?

Do you mean that those amps have the updated "B" preamp circuit, but still use the "A" chassis, so they still have two effects loops, but no reverb jack...kind of a hybrid...?

Very interesting...

Click to expand...

Yes, one of the guys on this forum has one.

Boogiebabies said:

cho said:

Boogiebabies, what do you mean: "MK IV B's with A type effects loops"?

Do you mean that those amps have the updated "B" preamp circuit, but still use the "A" chassis, so they still have two effects loops, but no reverb jack...kind of a hybrid...?

Click to expand...

Yes, one of the guys on this forum has one.

Click to expand...

So, how do I ultimately determine if my MkIV (S/N IV1840) is an A or B? Not that it matters that much, but now I'm curious

straitouttahell said:

Boogiebabies said:

cho said:

Boogiebabies, what do you mean: "MK IV B's with A type effects loops"?

Do you mean that those amps have the updated "B" preamp circuit, but still use the "A" chassis, so they still have two effects loops, but no reverb jack...kind of a hybrid...?

Click to expand...

Yes, one of the guys on this forum has one.

Click to expand...

So, how do I ultimately determine if my MkIV (S/N IV1840) is an A or B? Not that it matters that much, but now I'm curious

Click to expand...

It's an A. The B transition was in 1993 in the 6000 serial number range.