The Mysterious Mesa Mark IIB+ | Analyzing a Factory-Modded Circuit 

Man I can't thank you enough for going through all that work. Now I can send this video to my customer so he can understand why I won't even attempt this mod. They are charging $400 at mesa now. I read that there are over 20 variations of this amp.. apparently they just made changes as they went and wouldn't change the schematic. The one I'm working on is from 85 and at least 50% of the resistors are different values than shown...please send help.. I'm not ok 😂

I have an 88-89' Mark III Blue Stripe combo. Got it in 99', sent the chassis to Mesa in Ca. for an over haul in 05', It was running fine but things were starting to scratch & crackle. Got it back in a month or so and it's still running today. I mention this because Brad's right. I was gonna do the work until I opened it up and was overwhelmed. I started calling shops and techs but no one would touch it with a 10' pole. The one shop that agreed to take it in admitted they knew nothing of it and it would be hit or miss. I thanked them for their time and called Mesa. I've heard horror stories from both parties, techs don't like working with the amps and Mesa gets amps that techs have butchered beyond recognition. So, this is for techs and Boogie owners. Techs, runaway when you see a Mesa coming up the driveway! Owners, let Mesa fix your rig. It will save you time and money, they don't charge anymore than a tech, your cost will be shipping if that applies. I'm a retired G.M. technician and I firmly believe when dealing with repairs of this type, you should take it straight to the source for repairs. O.e.m parts are superior in most stock cases and when you figure the time and money wasted after the guy down the street had to go to Jim's used cars & vcrs four times to get the right part that will last about a month (why do you think they have a lifetime warranty), cause you're gonna need it, then you have to pay Billy Bob down the street again. Take it to the source, the guy who feeds his family working on whatever you need repaired and be Brand specific. Don't take a Maytag to a Whirlpool tech. Sorry so long but I keep seeing people making the same mistakes over and over and throwing good money after bad. I hope this helps the techs and the owners, cheers! P.S. The Mesa portion may not apply now that Gibson acquired them.

I've owned one of these amps when I was in college and used it extensively... I can't imagine putting another gain stage in it. As I remember that amp, it had crazy distortion in it's stock iteration. It was also a very clean amp even pushing the EV/SRO 12" speaker. (I might add that it was crazy loud and weighed a TON!)

Each Mesa amp should have a tiny bottle of bourbon inside the chassis for the technician who has to take it apart and repair it.

That resistor on the first triode is a swamping resistor. It 'tames the beast" if you will. Stabilizes voltage gain, but lowers the gain in doing so.

As a Mesa 'fan' I found this fascinating. Thank you for the effort! Mike B has my Mesa MK2C+ on his bench being modded to C++ at this moment. I may send him a link to this for his 'amusement' :) Cheers

The Boogie looks up at you and says - "You'll be drinking early today"

Totally agree Brad . They've tried so hard to make it the only amp you'd need , but the result is a nightmare to fix as you have demonstrated , and not even that easy to use . Wait until you see inside a Mk5 . That is all of their most notable amp circuits combined into one amp . It took me 20 hours or more of repair time to change a couple of burnt diodes and traces in a MK5 worth about 20 pence . My customer encouraged me to keep going even though I was worried about not getting the thing back together , there being dozens of wires to unsolder before you can lift the pcb and do the actual work . And IMO you are absolutely correct in saying leave it in the studio .

Yep- this is the MkIIB version of the C+ mod. Great amp! Cheers!🤘🏻

It would be cool to try and get Randall on and talk about these.

@The Guitologist Love the repair vids, and I hope you can keep them coming. I'd love to see some more restoration/conversion vids where you turn old junk into guitar amps.

Really admire you for the effort you put in all the videos... Good job Brad! These kind of explainations are getting me really into electronics (it's very fascinating and I'm willing to start studying at least the basics, even if I got a job and other music related hobbies, so time is really not my friend). Also, you unintentionally helped me a lot with my Mesa DC-3, when you've gone through all the schematics in another of you "terror serie" Mesa repair... I simply did a mod to convert the effect loop from parallel to series, nothing special but your video helped me figuring things out (even how to initally get the board out of the chassy ahah). So again, thanks for what you do, and keep up the good job!

Very familiar with this circuit. Just before the holy Grail Mark II C + . My friend has one we worship it regularly. 🤟🐧🐃😎👍

Imagine Brad going through a 2C++.. He’d have a field day with that! 🤣😂🤣

Your attention to detail and knowledge is outstanding

I have a IIB . Just an amazing amp! Beats anything else I have from Mesa boogie ( IV, studio pre, quad)

V4A - The grid is fed by a 0.1uF cap which is flipp'in huge for a guitar amp. Therefore current flows to the cathode via the 100k resistor plus the full signal as a small part of that voltage. It has more bass than what passes through the 0.1uF cap therefore get rolled off by the cathode - it being a negative. So this is both a preamp stage and bass roll-off filter.

Wow Brad, that is a LOT of work! I hope there's a Boogie Head out there that appreciates you. I like Eric Johnson's approach: use a different amp for different sounds. Problem solved.

Good points about reliability, roadability and maintainability.

Thanks for the video. I own several Mesa Boogie amps including a MKIIB+ with the effects loop mod. Whenever I have any big problems with any of my amp that my amp tech fix, I send them back to Mesa for service. Thanks again.

O my God, Brad. No sooner had I finished watching your Mesa video, that one of my regulars delivered one for me to get going. Why Universe? Why? I've been a good man....OK, except for that one time, but surely my sins weren't that bad. :-(

There seems to be schematic problems with quite a few Mesa amps. Not sure if perhaps I've had amps people have worked on prior, but I've never seen any RA# stickers or "messy chassis" evidence. Had a Roadster come in that was one tough dog to follow using a Mesa schematic that had wrong values and so many surprises, I had a party when I finally traced it over and found the problem. I really thought it was going to be a permanent part of my bench for the first few days I had it. They packed so much into that amp, but nothing compares to a Roadking. I'd be just fine if I never had to work on one of those again! Those are something else!

On the early boogies I don't think it was so much paranoia as the hallucinogens and the weed floating around Northern California at the time 😎🤩

I remember your video on the more modern nightmare Mesa. I hate to say this really but it is a real shame that they sound so good LOL. Keep up the great videos my friend. Us loyal fans have not left and new loyal fans will come along with time as they did to begin with.

The real WTF to me is that heatsink on the back. It appears to be just decoration as there's nothing connected to it - in fact, nothing in that area!

It's my understanding, that the Mesa Mk II-B used the so called, "cascading effect" to overdrive the 12-AX7's in the pre-amp stage to obtain the amp's distortion where Jim Kelley amps overdrives the 6V6 power tubes to derive the amps distortion... which in my opinion has a more natural/organic tube distortion.

Dude I love these in-depth schematic videos, especially Mesas!!! Keep 'em coming!! Great vid!!!

Hey Brad first thank you for all the informative videos! I watch your videos on a regular basis. I do poke my nose around a few amps from time to time. I have come across one of these mark2 b this one has the date actually printed on the preamp board as 1983. I recently recapped the amp and installed all new tubes. The amp since went back to the owner. And came back about 6-7 months later with some weird issues. At the time of servicing I had a very hard time finding a schematic that was anything close to the amp I had infront of me. (None of the diagrams I have found either then or now we're remotely close to the component values that are in this amp. I know for a fact it was "virgin" to any techs hands up until mine. Amp is a regular gigging amp, used minimum of 2 times a week. Anyways I am having issues trouble shooting with the schematics I have found. My first question is would you possibly have any links to a mesa mark 2 B 60w no reverb?? 2ndly there is an auditable noise, very loud noise, like a squeal when standby is engaged that will taper off and die. Some kind of cap discharge finding its way through the speaker. The amp otherwise runs pretty good. I found some microphonics were happening I have narrowed it down to V2 circuit as far as to say it's not the tube not the socket. I then patched the guitar straight into the FX return and the squeal was almost constant I could wiggle the patch cord and make it go away for a bit. I then chopsticked a few ceramic caps around the v2 tube socket found 3 similar looking caps that are microphonic. The amp is very dirty inside and the caps are so old that the value markings are all but gone. They unfortunately are not readable. Maybe one is 180pf 1000v but it's a half guess.... I just am not confident with the value. The other two look to be the same caps but the values are gone. So...... Back to the schematics I don't see a variation of a schematic with these exact caps wired in any of the same location. I feel you aggravation with thus amp but I am curious after so much time has passed we're you able to find any other info on these mark2 amps?? Thanks again for the videos

Hey Brad, I tip my hat off to you, my good man. Thanks for the part 2 video and response to my question. 🤘🤘Brad rocks hard.

Dig it man, love the vids. Keep em comin! I think Mesa’s are great, I used one in the studio for a lot of things but always toured with a Marshall jcm800. Way more reliable and honestly never had an issue with it. I’ve had it since 2004 and it’s existed since 1985 lmao

The diode off pin 9 is to derive the relay DC power supply. The one at the relay is for fly back

You have an incredible amount of patience to trace out an MB schematic

I feel like MesaBoogie may be the antithesis to Hiwatt.

Mesa offered many options to their customers… so many options it was perhaps inconvenient to publish the schematics of the many variations and mods. Also, Mike B did things for customers and Mesa did not release a schematic for these mods. Consider it modded like one of the many Fenders that come across your bench… accessing the other side of the circuit board is one of the Mesa service practices as well.

HeyBrad, forgot to tell you this. True story. My first introduction to Mesa amps. I was young kid sweeping up at an electronics place after school, my first job. This really strange hippie guy comes dragging in a Mesa. He starts rattling off what is wrong and what he thinks is wrong. Owner looks him straight in the eyes and says “I’d rather have a rusty nail drove through my ball sack as work on a Mesa.” Guy pauses a second, then says “well about my amp, when can you look at it?”

I have a Mesa Boogie Mark II-B from circa '82.. it sounds great and had been my main amp for 30+ years.. but I got a Quilter ToneBlock 201 a few years back and the Boogie has been retired to the shelf.. I had heard about this mod but decided not to get it done.. kinda glad I left it stock.

Excellent. I was thinking to have my Mark IIC Modified to a Mark IIC+ by a good authorized guy near me instead of sending the thing to the other end of the country but I think Ill be asking for trouble doing it that way.

I did mesa boogie warranty for nearly 10yrs back between 2005 - 2012. I requested they send board layouts so you could locate components easier. They sent all they had which were Gerber files. If you've ever tried to follow traces on a double sided Gerber file...well, you know the pain!

I miss these good ol days brad

Thanks for the hard work Brad . peace

A glutten for punishment, all these Mesa repairs!

the world needs more people like you Brad. Thx for your sharing your knowledge and misery for our pleasure :D salute from argentina

Brad's channel should be required viewing for every electrical engineer doing analog design. And doing circuit verification(he feels your pain).

17:20 There is no way that V4A can work connected like that. The cathode is at a positive potential and the anode has no DC voltage on it at all.

Thanks invaluable breadcrumbs for anyone scratching their head with one of these.

If a highly skilled amp tech like yourself says that Mesas are hard to repair, I sure believe it! This one is a nightmare inside. I mean it sounds really great after you worked your magic on it, but so frustrating to get it to that point!

Regarding the weird cathode fed stage: it may well be the tube equivalent of a common base transistor stage although i would think the grid there would typically go to a power supply. It would be referred to as a common grid stage. The common base stage is primarily known for its ability to present a low input impedance by driving the low impedance emitter and converting to a high impedance on the collector. Not certain but i believe it also has unity gain. I'm only speculating...haven't done any research... Nicely done hand-drawn schematic! Time consuming but well worth it. Glass to see you appear to be warming up a bit to the challenge of the MESA BOOGIEMAN!

@ Brad,The right to left read/lay-out of the schematic may stem from the fact that Randall Smith is left handed.

Outstanding info buddy (as always) Hope it saves the sanity of some hardworking Amp techs out there and that they find this before they go insane. Take care and all the best. 👍✌😎

Great follow up to the first video what a crazy amp to have to work on

Watched this one a few times, by now. Classic. 🤙

Hi Brad, had commented a while back on a video about Jimmy Stokley and The Exiles from some pamphlets you had received, which turned out to be Exile from central KY. It was odd that I thought I might know who this G. Thompson was as he is living in Nashville now so I FB messaged him and he confirmed that yes that was his amp. It was Glen Thompson from my own home town in KY. He had a guitar shop when I was a kid and used to go there often and would buy strings, guitar parts and such from him and even sold stuff on consignment in his shop. Glen played at Renfro Valley KY and did various other things with them, I think running sound etc before moving to Nashville. He is a very good guitarist but I dont know for sure if he just went there to retire or if he went to do session work etc. but must still be into the music scene as he said in his msg that he had sold the boogie to Greg Martin of the Headhunters a couple of years ago. He said that he had the amp from the mid to late 80's which is about what you had stated from the original pot dates. Small world.

Thumbs up for moral support. You're doing the Lord's work!

Love all your work Brad. You turned me onto event 201. Sent me down a rabbit hole.

I love you Bud. My son and I leap out of our shoes to see any new video you put up. I love a....l.....l. Your videos.

Great video! You are a very dedicated tech, let me tell ya! You have a bit of OCD as well. I can tell. Man, early Boogie schematics are notorious for their errors. As you stated, would drive a tech crazy. Great work!

Great stuff as always Brad. 😀

Thanks for the knowledge

Its the "Deaf Louie" effects loop mod. I had it done to my MKIIB in 1989/90

I have a Mark II and I want to say thank you for your work. I found this video extremely useful.

You need the un-modded amp. With a before/after example, you could analyze (for instance) what noises they are trying to eliminate. The most complex part of the whole deal is chasing the signal from >in to >out.

Keep moving Brad. It’s all about motion.

It seems like they randomly tinker and trial and error components on these amps to the point where they sound great.

I watch the videos when you post them I love them ,as matter of fact you helped me when I was working on my old marshall 30watt amp I have ,the way you different diodes and have you should follow the route of each capacitor exx...you just thought me alot of things such as values exx..my dad was an electronic engineer so I grew up know things already but as far as guitar I've just recently realized it isn't really that ,time consuming though but you have to be patient, you cant go through an amp in 5 mins to figure out the issue you have to take your time and check each individual piece and sooner or later you will find the problem but anyway I love the videos keep them ,godbless you and your family

Bloody hell, tightly packed isn't it?

Regarding builders being paranoid or thinking their ideas are so special. Dean Zink told me VHT amplifiers are like that.

I have two mark IIB's within 60 serial numbers of eachother... reason I have 2 is just like you mentioned, can't really rely on it if it broke to be able to be fixed. I have two mark iii's for the same reason. i do wish the IIB+ mod was able to be documented though...

Thanks!

Havent seen the full video, but when i hear the fx loop mod, i think this is the mod to convert it to a 2c+ like. Kinda cool to see one, still i think the IIB has a good sound stock lol

Well done, the self drawn schematic! With the reverse cathode follower they managed to not flip the signal 180degr.? I think. Not to confuse you:-) Clever! I learned a lot from my original mesa schematic from a early MKll-B in 1983. Since then I was a fan of the Company. Although the "Mesa Sound" to me was allways to harsh and mid focused. Until the Lone Star Amps and others appeared. The acctual makes are top of the line small factoryproduction makes. "The Art of Engeenering" as they selfclaim:-)

That relay connection to the tube filament is probably getting power to run the relay coil.

Hey Brad I was just wondering, if this amp had been laid out any better, would it fit into a chassis that small? I've got a Mark IV, and it seems with the whole Mark series Mesa have been hellbent on cramming as much crap as they possibly can into a tiny little space. It almost feels like it's become a perverse running joke to them at this point. Great video anyways, thanks for making it. Hope you didn't drive yourself too batshit in the process. I feel like he's a bit of a mad scientist, there's a video on RU-vid of him working on the prototype of the Mark V:25 and it lends some insight into how he works. He seems to improvise a lot.

But I love Boogies (owning a few over the years). The only other amps I've had/owned are Marshalls (100 watt lead/75reverb). Most other amps I've tried didn't have a crazy gains you can obtain (in my experience) with Mesa products, not to mention they're loud as fuk and built like tanks. I still super dig your channel and have learned a lot over the years about things I never thought of. Thanks Brad for another excellent video. Peace !

There actually was a company called Mitchell who was ripping off Mesa’s amps a few years before this amp was made (I think late 70s if I remember correctly). I think I read that Mesa had to send them a cease and desist order. Perhaps that had something to do with some of your observations.

My advice as an original owner of a 1986 Mark 3? Call Mesa and talk to Mike B (good guy who will return your call).Do it before Gibson fires him.Having said that,I can find no one in my area that will work on this amp.Absolute nightmare and not worth the trouble.I have no choice but to ship it to Mesa from NY for service.My old Marshall 4104 and antique Super Reverb look and sound better every day.I don't think it was paranoia of Smiths part.I think Mesa made mods on the fly and it wasn't documented.The Mark 3 has about 6 revisions!

So Brad, love your channel. You mentioned in there to “steer clear of Mesa amps or leave them in the studio where they belong...” And then you said something about an amp that was easy to repair. What would you suggest? I have 2 C++s and an early Rev F Dual Recto (like you did in another vid). NOTHING sounds like these amps- ESPECIALLY the C+s. They are amazing. What amp would give me that kind of gain and be “easier to repair”? Thanks for your input.

I would love to see inside of the foot switch since I wouldnt even slightly supprised that it has some "extra" goodies inside other than the switch

Hello. I like your work on MESABOOGIE. In what state are located?

Mike B works (or worked) out of the mesa Hollywood store on sunset.

Exactly why I stick.to my old 50s and 60s Harmony Amps

Randall is indeed the Anti-Leo!

Brad, you should write a book on amp schematics and history like the beato book (with perpetual 40% discounts, lol) and bring out your own boutique amp designs and mods, like morgan friedman etc. It shouldnt be too hard for you to find your niche in the boutique amp market thanks to your youtube channel based reputation and subscriber goodwill.

You Rule Brad!!

Look at the bright side Brad at least it’s not an old Heath Kit amp. As open ended as those were the mods some people came up with definitely left some scratching their heads.

@The Guitologist You got BALLS dude... hahaha But saying that, Thanks for the schem!!

You're extremely dedicated, reverse engineering the schematic. One thing you have to remember is that when the IIA's and the early IIB's were new amplifiers, they were made literally one at a time based on the customer's order. You can't compare this to Fender amps or Marshalls. Today will call that "boutique" amps. You should talk to Mike Bendinelli, I met him in person and Randall Smith back in the 80s at the factory. I have played a Mark IIA - continuously - since December 1983. It has never missed a gig. I recapped it in 2015 - for the first time.

Thank you Brad for the informative videos, I don't care what RU-vid thinks. BUT Now it all makes sense why Chibson bought Boogie!.. If it breaks buy authentic (a new one). Like Apple's war on repair policy. ((:

Thanks for the video!

Thanks Brad

Mesa has backline all over the country to support their artists but if you're endorsed. So yeah definitely keep the Mesa at home if you can't afford another one or can be the only hands on it.

Have you ever come across any Groove Tubes amps? I had 2 Soul-O 75s and they were a spaghetti mess on the inside but the best overdrive IMO. Had to get rid of it because no tech around here could solve it's problem. After switching it on (off standby) it would play normal for about 30 seconds and then lose gain like there was an aggressive compressor inline. The harder you picked the cleaner the signal. Still trying to track another one down because it was the best high gain amp I've ever played. Very Brown Sound-ish but with a fat switch that made it huge in the low mids without making it flubby or boxy. If your curious about the amp's tones I can send you a link to a recording.

I had no idea how complicated early boogies are, damn! Great video btw.

Great video man. Who would want an amp so shrouded in mystery? such as as this Mesa, or any other Mesa me-thinks. Appreciate you burning the midnight oil, and shedding some light.

Might Mesa have included/sent G. Thompson(?) an updated schematic showing all the mods? I don't know if factories did that kind of thing when they did "custom" mods. Follows it could have been lost in the history/life of the amp (3+ decades old). Or just Mesa being Mesa (seems more fitting as the big brains that work on these apparently agree). DT

When I sent my 1984 Mark II C back to Mesa, in early summer 1984, to have reverb added , they had it for weeks. I eventually called to get the status and was told they were backlogged and so had sent my amp to a local repair shop they had contracted for mod work and so mods were taking longer to turn around. When I got the amp back, it had the '+' mod added...so I called to ask about that and was told it would make the amp more stable.......but it was way more picky about preamp tubes afterwards.

Hi. Whats the year of this amp? Thanks

Hello sir, i was wondering if you could recommend a soldering iron set up for the at home guitar guy who wants to try and change pots and pickups and such. I would really appreciate it. thanks for your channel.

Most bands now take modeling preamps and run them straight to FOH, it’s way more more reliable and consistent compared to any tube amp, tube amps are for the studio.

Hey Brad, What s up with that cool meter on your variac? I've got meter envy!

11:30 that 1M resistor is probably wired as an anti-pop resistor for the bright switch... it would make no sense in series with the bright cap when active!