Mesa Mark IIc+ Pre PtP
Introduction
Mark IIc+ has a reputation of being one of the most sought after amps today. Only 1500 of them were made back in the 80’s and these days they go for up to 4000-5000$ a piece. One of the players that made this amp famous is John Petrucci. Being a Petrucci fan I couldn’t resist building it, since I can’t afford to actually buy one 🙂 I decided to build a stripped-down version featuring lead channel only without fancy switching. Bass shift and lead bright switches are left in but as board mounted DIP switches. That way I can play with them to find the position I like and leave it like that.
Mods
The last preamp stage on the original IIc+ doesn’t do much as far as overdrive characteristics are concerned. It shapes bass response of the output stage which I don’t have. Voltage divider between 5th and 6th stage reduces signal level to just few volts to make it effect friendly. Since I’m building the preamp only I don’t need the last stage to boost the signal back. Instead, I converted the last stage to AC coupled cathode follower. It’s supposed to be a transparent buffer that will provide nice low impedance output that should drive any effect and long cables if needed. Since it doesn’t cut any bass, it will essentially have similar response like the original stage with deep pulled out. If you want to learn more about AC cathode followers, Merlin has a great article posted here. Large 15uF cathode bypass caps are replaced with 6.8uF poly caps. These were the largest I had and there’s no much noticeable difference in bass response between 6.8uF and 15uF.
Heaters are run at 12.6V to reduce radiation – less current means less radiation and therefore less chance of noise. Also, they are elevated to ~80V using a 220K:47K voltage divider right after the first filter cap.
Schematics
Circuit schematic – click for full size
Power supply schematic – click for full size
Click here to view hand-drawn layout I used to build the board. Note that it doesn’t include DIP switches and doesn’t show series resistors I ended up using for values I didn’t have. Cathodes, grids and plates are marked Cx, Gx and Px respectively, where x is tube index. T, M, B, etc markings on the bottom side are pot connections; e.g. T stands for Treble.
Parts
There are no electrolytic capacitors anywhere in the preamp! Power supply uses (giant) motor run (not motor start) poly film capacitors. As mentioned before, cathode bypass caps are also poly film. Coupling caps are mostly Russian PIO with a couple of poly film. Small capacitors in the pF range are mix of ceramic and silver mica type. Mica are notorious for being harsh when used as treble bypass/coupling caps so I used them only to shunt higher frequencies to ground. Resistors are mostly Dale and Xicon. 2W metail film resistors were used for plate resistors wherever I had that value. Transformer is a custom wound toroidal built to these specs:
- Primary: 220VAC, 30VA
- Heater secondary: 12.6VAC@0.6A
- HV secondary: 300VAC@40mA
- “Blind” winding: has only one tap and serves as a RF/EM shield
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Click to download parts list |
Construction
I used the same chassis like for Soldano Preamp, just a bit shorter and powder coated in cream. It’s a great chassis to work with because each panel may be removed separately. I wired the pots with the front panel removed. Tube sockets are wired before installing it back to allow easier access. Shielded wire is used on the input, from volume pot to the socket and to/from lead drive pot. For circuit board I used two fiberglass perfboards joined together. Doesn’t look as fancy as a nice eyelet board but gets the job done. As you can see below, chassis is packed tightly. Motor run caps are huge and don’t leave too much free space. A couple of times I used two resistors and capacitor to get the right value simply because I didn’t have it 🙂
Chassis layout – click for full size
As far as grounding is concerned, (almost) strict ground bus scheme is used. Thick bare copper wire is used as a bus and is grounded to the chassis very close to the input jack. Components are grounded to the bus in the same way they appear in the signal flow. This also applies to the filter caps. First cap is grounded at the end of the bus and the rest of them are grounded close to the tubes they are powering. All shielded leads are grounded on the input side. Shield on the lead that goes to the Lead Drive pot is used to ground the pot’s lug 3, to avoid using a separate ground lead. Tube socket center pins are grounded to the closest socket mounting screw. This maybe converges from the strict bus grounding scheme, but I reckon it’s fine since it only servers as a shield. Also, internal power transformer shield is grounded directly to the chassis at the same spot where safety ground is connected.
Video Clips
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Pictorial
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Result
The first thing I noticed is how quiet this thing is. There’s almost no noise even with both volume and drive knobs maxed. Also, it’s got noticeably less gain/distortion than my Soldano preamp. It’s expected as Soldano has a cold clipper stage and a heavily overdriven cathode follower. Those two also make Soldano sound a bit refined compare to IIc+ which is more raw. Less gain also means more useful tones. With Soldano, I set gain at 20% for rhythm and at 30% for lead. Everything above is too saturated for my taste. With IIc+, good rhythm tones start to happen with both volume and drive set above 50% and for liquid Petrucci-esque lead tones, both knobs need to be cranked up. That leaves more useful crunch tones across knob range.
Having tone stack right after the first stage is cool, but preamp alone without a graphic EQ pedal is useless. You really need to shape the frequency response after the preamp to get some usable tones out of it. Although I have all the parts for the EQ, I left it out intentionally because I plan to build it as a standalone unit. That way I can mix and match Mesa EQ and MXR EQ with Soldano and Mark preamps. Like with the original, bass knob needs to be set very low, 0-10%, to get useful rhythm tones without flabbiness.
After my experience with cloning two of the most sought after and mojo soaked amps of today – Trainwreck Express and Mesa Mark IIc+ I figured that for an amp to be famous it needs to be very bright. So if you’re out there, trying to design an amp that will be next “the sh*t”, make sure it’s bright 🙂
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Hi!
Great project.
You said that you will build the Mesa EQ? Did you? And what do you think where can i get the inductors for it?
About how much will this build cost?
-Thank you
depends on component selection (and where you are), but I think 150-300$ is the ballpark. And lots of hours.
Hello,
what do you think if i use 265V transformer instead of 300V ?
Regards
that should be OK.
OK, thanks.
Hiya Bancika!
Thanks for the treasure on these pages.
What did you use for EQ with this beauty?
Had you built your EQ unit by this stage or did you use an existing bit of kit?
I see the Mr EQ link. Would that foot the bill or would something else be better?
Cheers,
-Brendoon
Hi,
I use MXR 10 band EQ pedal, it’s great.
Cheers,
Bane
Amazingly awesome project.
I am thinking of integrating this baby upstream an AX84 OH power section and run into 12″ speaker.
should I go back to the original last stage?
thanks
what do you mean by “original last stage”?
alright, so for the price point i ended up purchasing a jet city amp jca2212c ($350US). i plan on tinkering with and modding this amp. i’d like to see about adding in some of those mesa designs as options to the amp. like, the mix of the clean with the lead channel. i notice in the jca amps there is way less filtering than in the mesa designs.the PT used in the jca is a 350-0-350. i’ll post the schematic link bellow. do you think its possible to get some mesa style distortion with a little tinkering? do you have any ideas that you’d like to throw my way? feel free to email me in case you’d rather not clutter your discussion feed with this topic.
http://www.subirimagenes.com/imagen-capture1-8519900.html
ramds3and@yahoo.com
So, you busted my chops with making the edcore transformer work…for no reason 🙂
well yeah. me building the preamp isn’t out of the question. it’ll have to wait until i have a budget for it again. i couldn’t pass up the price after researching the origins of the amps design (a.k.a. SLO 100). i would have spent a large junk on that preamp. 100 dollars more got me an entire tube amp combo that i can tinker with.
would using electrolytic caps ruin the design of this preamp?
Absolutely not. The original uses electrolytics. I use polys because I think they are better.
i’m seriously considering building this preamp. would a transformer with 150-0-150 work to make the 300 volts? here is the link to the transformer https://www.edcorusa.com/xpwr076 . i built a mesa bottle rocket with success then tried to build a mark iv preamp (just lead channel). it was a failure :(. after coming across your diy project it has given me new hope. i really appreciate the info you’ve put up here.
That should work. Just terminate the center tap and you can use bridge rectifier on two outer taps.
it’s safe to say i should ground the CT, right? i’m researching grounding schemes now. after researching this topic, i realize it may have been the fatal flaw in the mark iv preamp i tried to build.
No,you need to isolate CT and not connect it to anything. If you ground it you can use the transformer as 150v, not 300v. Tape if off and use outer taps as though the niddle doesn’t exist. Check this out http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/ietron/rectbr2.gif
so, the circuit is grounded to earth ground from the wall main? i know this seems like a dumb question but i’d like to be 100% sure of what im doing. the following layout is what gave me the idea to ground the ct. as well as some other threads i read.
http://vintageamps.com/plexiboard/viewtopic.php?f=28&t=106733
You can ground CT if you have 300-0-300 and go with two diode full wave rectifier. Since you don’t, you need to go with 4 diode voltage double rectifier. Forget about grounding for a second, that is a separate issue. You need to choose rectifier configuration that works for you. In this case you want to have a 0-300 transformer or 300-0-300. If you have 150-0-150 you can make it 0-300 just by forgetting that it has a CT. Plus/minus 150 is relative to CT only if we say that CT is 0 and we ground it. You can call it 0-150-300 and then it’s obvious why we can ditch the ct.
Bancika,
This is the tone that i have been trying to create for years. I plan on building this but I want to add the switching so that i can have the clean option. Any suggestions before I start this? Would you be able to check my work if I send you schematics?
Hi Bane,
First of all, many thanks to you for putting up such a valuable resource for DIY guitar and amp tinkerers everywhere. It’s an excellent resource.
I want to ask you what you think the advantages are of using motor run caps here as opposed to electrolyrics. Is there some secret-sauce tome mojo them that justifies the massive size? I’d appreciate your thoughts.
Regards,
Michael
No secret. Poly caps have much lower esr which makes them better for any audio application, signal path or power supply. Also, they should last a lifetime, so no recapping, no drying out and degrading over time. You can get solen fast caps which are designed for audio but they are much nore expensive and about the same. Not sure if they offer any benefit compared to cheap motor run caps which are widely available and cheap…
Congratulations on your very interesting project ! I am considering building a similar arrangement into a 3u rack box. As the box is oversized I would like to build an inductor based 5 band EQ into the same enclosure (possibly Mesa MK 4 style). My question is would this work right at the end of the signal chain (between the cathode follower and the output socket. I would probably build a seperate zener regulated power supply to get the -30v supply required by the circuit. Kind regards Shaun
Yes, it should work.
Thank you, will let you know if it works out. My only concern is that it may be too hot for the rest of the effects chain. I guess there is only one way to find out 😉
Cheers Shaun
My schematic is modified to deliver pedal friendly output. In that case you don’t need +-30V rail, +-9V should do. I plug mine into a delay powered by 9V and it doesn’t clip.
Cool – I will send you some pictures when I’m done. Thanks for taking the time to consider the options for me, but as always a little fine tuning/modification will probably be necessary. I am using a home made JTM45/JMP50 hybrid as the main amp and am hoping this project will give a vintage VS modern flexibility.
Keep up the good work Shaun
Hi Bancika
Nice project
I build now PTP but i use turets board
My question is :
Two points A in the diagram leave as is?
I do not connect to anything?
Thx
They are connected together, it’s network that mixes in some of the clean signal to distorted signal. Adds mojo 🙂
Hi Bancika,
a short question – what is the exact voltage delivered by B2, B3 and B4?
Do I assume right that if theres 300 VAC * sqrt(2) = 420 VDC right after the rectifier? And the voltage drops to 384 VDC after the 10K resistor (B2) and then theres again two 1K drops, 3,6 V each? (B3, B4).
And what’s the deal with the B1? Is there any point it connects to the preamp cirquit?
Thanks for Your time!
Please refer to http://diy-fever.com/amps/mesa-mark-iic-pcb/
That’s a newer version of the preamp with all voltages listed.
B+1 doesn’t go anywhere on the first version of the preamp. You want your first (or last, depends where you stand) node to be close to 400V. Others will be a bit lower.
Cheers
Hi Bancika, I’ve got a question. There are pots marked A and B in the schematic. Some manufacturers use marking A=log and B=linear, but others A=linear and B=log. I think it should be A=log and B=lin in this case, but not sure about it :-). Please answer to my e-mail, if you can. Thank you a lot. I like your work, it is so inspiring :-).
yeah, A=log, B=lin
Cheers
sorry…there is a third question…in the parts list it shows 2 DIP switches…but i don’t see them in yor photos
i’m guessing that this is the switch off of v2b and v1a?
Hey!
I so want to build this…two questions though…
one:
can i get a clearer layout from you?
two:
for ease of use in a rack…i want to pick the output at the rear…but its going passed the filter caps…is that going to cause an issue, or should sheilded cable be used?
check my newest build that has PCB layout http://diy-fever.com/amps/mesa-mark-iic-pcb/
Re: DIPs, see this photo…on my second build I used jumpers like on computer mother board, but you can use panel mount switches or push/pull pots like the original. They do pull bright and pull deep
http://diy-fever.com/wordpress/wp-content/gallery/markiic_pre/guts_right.jpg
I am now looking for the transformer that is nearly have the same spec as yours. Do I just have to match the voltage or does the current have to be the same too?
You can get with more current but you’ll probably have to drop more voltage with resistors because it won’t sag as much under load.
I’ve found this http://www.tube-town.net/ttstore/product_info.php/info/p4908_Ringkern-Netztrafo-56VA.html does it give much different when put it in your schematic without voltage drop?
That’s very close. You’ll need to drop some heater voltage. I used 0.56ohm to get to perfect 6.3V and you’ll probably need some resistance on 300V tap as well (1-3K)
why don’t use 12.6V heater voltage just like yours?
If I were to take the first 2 stages, basically the clean channel, and take where it ends(A), would I be able to say make that the out?, and pretty much run a cable from that to the fx return of my 6505 head using it as a power amp, would that work?, or would I have to forcefully use an AC cathode follower. And from what i see you’re using 5 gain stages (1 and 2 for clean, 3 4 and 5 for lead, plus the last triode as the CF. Right?. I appreciate all Ur help. Thank you.
you could
question, what does the “A” mean in the schematic between the first and second gain stages seem to connect to a Cap and Resistor. thanx
connect it to the other A 🙂
wow, how’d i miss that.. ha, thanx. what would be the purpose of that, or the terminology for it so i can learn more about what it’s purpose is.
the first A is the end of the clean channel in the original amp (they have switching, I didn’t have that), but that clean channel is always on, it just gets overpowered by lead channel when lead is on. That means that you always have certain level of clean sound mixed with distorted…one of those things that defines mark sound I guess.
Ok so let me see if I have this straight. If I were running 12.6v I would connect one leg to pin 4 and the other to pin 5. Now since My secondary heater supply will be 6.3v I will have to connect pin 4 and 5 together with one cable and connect that to one leg, and the other leg to pin 9, right?, now if that’s right. I should still elevate the heaters at pin 9 of the cathode follower, meaning pin 9 of v3 will have 2 cables connected to it. One from the heater supply and the other from the voltage divider elevating it to ~80v am I right?. And about the 100ohm resistor, it would act as a humbucker of sorts, reducing hum since I will be using 6.3v which is noisier by nature than 12.6v. Am I right?.
The 100ohm resistors would drop the current from 6a to 6mA right?.
And I think the 150-0-150 with bridge rectifier will be used, since that PT is cheaper than the 600vrms. So where does all this leave pin 9, can I leave it disconnected or still use it to elevate the heaters to 80v in order to work better w the cathode follower?.
pin 9 will be connected to one leg of heater secondary because you’re using 6.3V mode.
As far as 100ohm you got it wrong, there’s no dropping of current or voltage, just balancing hum between two “sides”.