Mesa Mark IV A/Bs

J

Jimty

Active member
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.

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you probably already know, but the amps in those top two pics were worked on at some point. some of the orange coupling caps have been replaced on both, some filter caps, etc. looks like the top one had work done where you circled. that stuff doesn't look stock to me.

Per a conversation i had with Mike B, 15 years ago, those 2 ic's that are mounted upright near those 3 axial caps, in the mark IVa's were problematic. they were getting too hot and causing issues. the solution was to add a heatsink to one of them, or sometimes you could get away with bend them apart really far. Looks like as the circuit progressed, they were phased out all together by the full B revision. Thats all i have to add :dunno:
 
my current mark IV sn: 1733.
I bought this from mesa hollywood. They worked on it before they sold it. they added a larger piece of metal to the ic's existing heatsink so it dissipates the heat better. They always upgrade the coupling caps on v5 from 400v to 630v, as seen in most of these pics. I changed the filter caps and a few resistors in 2022.
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JimAnsell said:
you probably already know, but the amps in those top two pics were worked on at some point. some of the orange coupling caps have been replaced on both, some filter caps, etc. looks like the top one had work done where you circled. that stuff doesn't look stock to me.

Per a conversation i had with Mike B, 15 years ago, those 2 ic's that are mounted upright near those 3 axial caps, in the mark IVa's were problematic. they were getting too hot and causing issues. the solution was to add a heatsink to one of them, or sometimes you could get away with bend them apart really far. Looks like as the circuit progressed, they were phased out all together by the full B revision. Thats all i have to add :dunno:
Click to expand...

Thanks Jim. Yep, re the work on the top amp (which is mine), the seller pointed it out in his advert and then I connected with the tech who did the work before pulling the trigger. He’s an experienced guy who used to work for Matamp, and he confirmed he’d recapped the amp (including replacing the orange drops with the yellow caps) and done some clean up work on the board due to old caps leaking. Values of the new caps he fitted are stock so far as I’m aware. All that being said - it sounds great!
 
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mine is IV6030 the original owner had a local tech do the recap , did it in a funky way but it works. This is the 3rd Mark IV I've owned and the R2 channel on this one is almost as gainy as the lead, blows away the R2 on the "A" I had and is hotter than the
B" I previously owned. I'm going to send it in to Mesa at some point just to make sure everything is ok, but this one sounds menacing, even on the lead channel drive at 7, treble at 7 and lead gain not even pulled at 7 is gainy as hell. I have NOS tubes in this one
Mullard Blackburn V1
JJ high gain V2
RFT in V3
Shuguang square getter V4
Sovtek LPS in the PI
Since this picture was taken I had a very good local tech, change the lead drive pot, the power chord and the power chord for the fan. He gave it a once over and said all is ok (authorized Mesa tech), but wouldn't be a bad idea to send it in and have them change the relays and any other caps etc they need to to get it back to "stock"
IMG_5112.jpg
 
kmanick said:
mine is IV6030 the original owner had a local tech do the recap , did it in a funky way but it works. This is the 3rd Mark IV I've owned and the R2 channel on this one is almost as gainy as the lead, blows away the R2 on the "A" I had and is hotter than the
B" I previously owned. I'm going to send it in to Mesa at some point just to make sure everything is ok, but this one sounds menacing, even on the lead channel drive at 7, treble at 7 and lead gain not even pulled at 7 is gainy as hell. I have NOS tubes in this one
Mullard Blackburn V1
JJ high gain V2
RFT in V3
Shuguang square getter V4
Sovtek LPS in the PI
Since this picture was taken I had a very good local tech, change the lead drive pot, the power chord and the power chord for the fan. He gave it a once over and said all is ok (authorized Mesa tech), but wouldn't be a bad idea to send it in and have them change the relays and any other caps etc they need to to get it back to "stock"
View attachment 296702
Click to expand...
I’ve heard of some techs mounting the (normally) vertical caps on top of the big blues like this. Interesting to actually see it.
 
Jimty said:
I’ve heard of some techs mounting the (normally) vertical caps on top of the big blues like this. Interesting to actually see it.
Click to expand...
I'm curious as to why I have 4 of them when there are 3 on the board that were replaced
 






Here's pics of a "B" i had. Nice amp but nobody in town would work on any of my Mesa's.
Once Gibson bought Mesa i lost all confidence in the brand for service.
 
shar-vell Dan said:






Here's pics of a "B" i had. Nice amp but nobody in town would work on any of my Mesa's.
Once Gibson bought Mesa i lost all confidence in the brand for service.
Click to expand...
that's a really clean one, I hope you didn't sell it just because you worried no body would work on it
 
Ya, as far as I know Mike B is still working there, not sure if he's taking IV's in for service at the moment though. I'm really suspicious of the work they did on mine, I can't say conclusively that they ripped me off, but I just feel that's what they did, cause I looked at the inside of this amp before I sent it off and there were no caps leaking that I saw... Unless they started leaking on the trip there which I doubt, and it's gotta be turned on for that to short circuit anything, right.

Cool pics anyways. You're A/B just looks like a B revision to me, at least 99% so... they probably did make some minor changes to the circuit over time, but nothing that would result in a tonal shift IMO. Mike B said that "the Transitional Mark IV doesn't really exist, there's just a A revision and a B revision". His words, not mine.
 
Why ? Because have 2 of the best Engineers [not just tube swappers.] in town and when they refused that means im at the mercy of shipping it to Petaluma or Nashville or where ever to be worked on my whomever for however long and i don't play that game.
Most shops here in town wont even buy or take a Mesa b/c they're time consuming over engineered with redundant parts and some of ther parts you cant even get anymore.
Sound great yeah but nothing i don't already have.
AND i have no faith in Gibson as an electronics company.
Ive been thru many buyout aquisitions. They tell you "nothing will change. Only one thing changes and thats everything.
 
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