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
Click on a thumbnail to play the video on YouTube.
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Click here to list all 7 related video clips.
Pictorial
Click on an image to see more details.
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 🙂
is your custom power transformer center tapped?.
Nope. I used around 80VDC tapped off of the main B+ to elevate heaters by connecting it to one of tube’s pin 9. Since I’m using 12.6V operation, all three tubes normally have 9pin unconnected, so that’s a good place to elevate.
What if one were to use 6.3v for the heaters?, would pin 9 still be used to elevate the heater w the voltage divider?. Another thing Is I’m gonna use a Hammond transformer, most seem to come center tapped therefore I would need to use fullwave rectification, but I’d rather use bridge like u have, meaning I would have to get a 150 – 0 – 150 yielding an 300rms vac correct?, or would I need – 600vac rms center tapped 300 – 0 – 300, what would u recommend?.
then I’d use two 100ohm resistors as a virtual center tap and connect them to both legs of the 6.3V secondary, something like my 3w power amp, check it out.
You can use either of those transformers….300VAC with the bridge or 600CT with two diodes, whichever’s easier.
how much is it safe to assume the HT will be after rectification?.. using a 300vac before rectification, i’m assuming 30mA are used to smooth the dc, and each 12ax7 is using 3mA correct?. i ask this because i was wondering at what voltage you had Biased the first stage triode at B+4, i see the resistor is 150k, meaning that in order to get a cathode bias resistor value of 1.5k like you have on V1, at 300V it would bring your plate current to 0.9mA w a Grid Voltage of -1.4V for the value to work. i only ask because i realize that in most cases your Vac will be higher after Bridge Rectification, and then u can factor in a %10 – %15 loss of Volts here and there, i worked some equations with an estimated 325Vac i assumed after rectification and managed to get the same result with some modification to the Grid Bias making the bias just a tad colder. i’m actually in the midst of designing my own preamp, but i’d love to tackle your mesa when i get a chance.
Dave.
B+ is around 400V (+-5V). Those 300VAC we’re talking about are under load (at least for this tranny I used, I always target voltages under load, so they bump the voltage up by approx 12%), so you’re looking at (at least) 300 * 1.41 ~ 425VDC after rectification. It will be even slightly more than that because I always slightly overspec the current rating of the transformer so voltage doesn’t drop as much as declared. I tweaked the first filter resistor to 10K to get to the ballpark of 400V on tube plates.
Hope that helps
sweet, that gives me around -1.8 Grid Voltage given the value of the cathode bias, meaning it’s biased a bit on the cold side but still warm enough to generate a good clean tone on the first stage if you were after that correct?.
did you install output tube there? because I only see 12ax7 tubes in your list. If no, what power amp did you use in the video?
no power amp, preamp goes to palmer speaker sim/di box and into the recorder
Hi Bancika! I found the video on youtube, and it bolw my mind! Congrats! Amazing sound! Now, i want to do one of these preamps that you post. I have 3 questions:
1º In the specs, you said the power transformer is 220 VAC – 400 VAC. The 400 VAC are RMS like the 220 VAC in the primary or the 400 VAC are peak?
2º The other secondary are aprox 12 VAC for the heaters. I saw several times VDC (like LM7812 and some capacitors) for the heaters. What do you think about that? Whic voltage is better?
3º In my city i cannot foud a transformer like that, and i want to use 2 transformers, one for the heaters and one for the tubes. Do you think that’s good?
Thanks!
Sorry about the delay.
1) The transformer is 220:300VAC RMS which gives over 400V after rectification
2) You can use the rectifier, I did that on Soldano preamp with a 14VAC secondary and a regulator. Just make sure that you heat-sink the regulator because otherwise it will shut down after a while because it overheats. Also, use a generous amount of filtering, I used 47000uF before and after the regulator.
3) Yes, that’s perfectly fine.
Can you give me full list alement for 2 channel preamp ?my email leon935@ya.ru
Don’t have it, sorry. Just go through the schematic or layout and write everything down. Not everything in life comes on a silver platter 🙂
Will it work with transformer 12v 0.6 A??? i cant find such transformer..
yes, you need approx 450mA for 3 tubes, so it will be fine.
Can you make the record Mesa mark IIc+ with eq(cut mid)???
all clips are with EQ set to mid cut (not too much to sound like pantera, but it’s definitely V shape)
can it sound like this http://www.youtube.com/watch?v=C1VgHT9K5l8&feature=related?
Do you hav the layout of msa eq from Mesa mark IIc+?
please help, what rl1 rl2 rl3 ? is it relays??? and the relays that you have written have 8 endings instead of 16!!!!!please help!!
Hello Bancika!!Should i make the power supply for this schematic http://diy-fever.com/temp/iic+i_full.pdf?What rectifier should i use for it?
Hey!!!Where are you?))))
Great sound, great clips. I have build an mkIIc+ / mk iv inspired amp recently and I think it is obvious that there are some intentional mistakes in the schematics repeated over and over. Do you have any ideas how to fine-tune the circuit to get near the famous liquid sound? Thanks, Pavel (Czech Republic)
I used schematic from SLOClone forums, some folks reverse engineered a real IIC+. Official schems are known to have errors, but this should be correct.
Cheers
Sorry man, i also can’t figure out how to connect 12ax7 from the pcb to the real tube … why you draw three different voltage and two different tubes? Mark 2c+ use 3 12ax7 at 12.6V so i have to use only the second wire schema? Still bit confused …
I made the board to be versatile and adapt to tubes you’ve got and the transformer. If you want to use 12AX7 in 12.6V mode you can do that, but you can also use russian tubes or run 12AX7 in 6.3V (that’s actually more common, but I prefer 12.6V because it draws less current, so less noise is emitted).
Cheers
Thank you man! This is simply what i was looking for! I’m not good in that stuff and i don’t understan a couple of things in your final schema (iic+i_full).
1) I can find the IN (is written…) but i can’t find where is the OUT 🙁
2) I don’t understand where i have to put the female jack clean/lead switch
3) You use the deep switch for bass shift and lead bright, i need a way to do that from the external panel with some
on-off switch (like the classic metal power on-off). Can you send me a way to do that?
4) In a couple of comments you say that some control are reversed … if i understand well that means that some controls at 7 o’clock are at the “maximum”. Is that right? Bit ugly … bit confusing … i really want to do that but i need help. I need a bit more clear schema to work with. Thank you man.
1) out is at the right side, comes out of the 22nF cap
2) put it next to the clean/lead switch. I’d use a switchable plastic jack that disengages the clean/lead switch when jack is inserted.
3) just connect wires instead of mini-switches that lead to the panel mounted switches
4) yeah, I bozo’d the daughter board, few pots are reversed, so I wouldn’t use it. The main board should be fine. Another reader took time to fix the layout, I’ll mail it to you (you’ll need DIYLC to open it)
Great preamp!! Do you think it would be possible to build one of these using a modified nixie PSU?
Why not, as long as you can provide around 400V.
That is great to hear!! I kinda thought it would work since I have built a couple high gain tube pedals (Soldano GTO Supercharger type clones) and they are quite similar except for the fact that they use the nixie psu… Shouldn’t be any problem getting the voltage… Anyway, thanks a lot for your input and I love all the projects you have done!! Great stuff man!!
hi Mr. i think i will try to build this preamp,,,in my plan i’ll use 6n2p vacume tube. in the power supply schematics above u connected the V3 pin 9 between R220K with R47k. but it ‘s different with board @ diy-fever.com/temp/iic+i_full.pdf. in this board if we use 6n2p vacum tube pin 9 must connect to the ground. what should i do,,would you give me advice.
thanks
just wonder i have a mesa boogie s.o.b i like it but i need more drive do you know any mods for this amp ?
i think you did a great job on this amp i will love to build one.
Hi Bancika !Im really impressed with your modes!Well done!I have same guitar as u and im a fan of Petrucci.Im russian and ilive in ireland.Could u tell me is it any chance that you can build same preamp for me?
Hi,
sorry mate, I only do this for hobby and I’m very tight on time, so I can’t take orders.
Cheers,
Bane
no problemmo amigo!How much it will cost me the parts?aproximetly?in euros
I’d say in the ballpark of 200e
Cheers
I have some questions…
1-How does it sounds on clean sounds??
2-Can I insert in the circuit a 6 band eq?
3-Can I have this sound?—>(the first I know that’s a chours) http://www.youtube.com/watch?v=C1VgHT9K5l8&feature=related
Thanks 😉
This version doesn’t have clean channel. You can use a EQ pedal like me or just add the mesa EQ at the end of the chain…and yes, it can nail that tone. You’ll just need a tube power amp and v30 loaded cab.
Thanks! Ican’t sounds clean using the gain and drive at 1 or 2??
It’s clean-ish but still not super clean. It’s got 6 gain stages 🙂
hi guys.. can u tell me about the transformer that u use in this project?
specs are listed above in the article
WOW!!!, what a fantastic preamp!!!! I’m from the UK and currently use a Marshall JMP1 preamp and Marshall el34 Poweramp which I love, but have have always wanted to get near to that illusive Petrucci lead tone..mmmm.. yummy!.. And I don’t have the kind of money to buy a Mark V!!!
I would love to build this preamp, but I have never done anything like this before. What’s the best advice you can give to a complete newbie?? What are my chances of taking this project on and completing it without electrocuting myself?
Many thanks, and once again fantastic work.
Sorry to bother you again. might you possibly be able to send me the DIY layout File of the PCB. that way i can make some changes for the lead channel and print out a bill of materials.. thanx
i found this, “http://www.mouser.com/ProductDetail/TE-Connectivity/4000-01E07K999/?qs=1eQvB6Dk1vjrX3lMNrzltQ%3d%3d” i don’t know if it would work. seems similar to your custom spec transformer. except mine is 120VAC, since this is the North American Standard of 120V.. i know that in Most European Countries the standards for voltage are somewhere between 220V-240V. would i be good to go with that?.
i’ll take your advice on not using the daughter board. i’ll wire straight to the pots. i’m curious to know how one would go about figuring out how many volts, or the specs that go into choosing a power transformer for something like this. i’m sure it involves the three 12ax7’s, but i’m sure there’s also something to do with how you want it to operate?.. and i correct or totally off. maybe using transformers back to back would be my best bet, but honestly that’s the only part about this whole thing that has me confused. I’ve been doing research on VAC, AC, Ma… DC… and it just confuses me.. wouldn’t there be an article or something that would explain how to go about choosing a power supply for a project like this, i mean i know you already did all the calculations for the voltages and stuff.. but how did u do it?. hope you can understand and have the patience to help me out. thanx.
Wow. that sounds amazing. i would love to build something like that but i don’t know where to start.
i’ve been trying to get around to understanding schematics but it seems there’s more to just that. I’m pretty good with a soldering iron, i’ve done several mods to my Peavey 6505, but nothing of this magnitude. I’m well aware of the dangers involved in dealing with high voltages running through these amps. but i’m really set on building this. but like i said. i’ve been trying to understand the schematic but i don’t know where to go from there. i have a marshall mg 250 dfx with lots of guts for the taking(Resistors,Caps..Etc). i’m trying to make it as simple as possible by trying my hand at the single lead channel version you have in the vids. any advice that would point me in the right direction would be highly appreciated. thanx.
you could try etching the PCB, it simplifies the process. I made a PCB layout for both 1 and 2 channel versions. You can omit the switching and have only lead channel if you want to.
diy-fever.com/temp/iic+i_full.pdf
Just don’t use the control daughter board, some controls are reversed. Use lug pots instead, it will be more reliable anyways.
Cheers
Awesome thanx. i’m planning on getting a set of boards made for me with the trace you made if that’s ok with you, since i’m not experience in etching my own. should have them in a week if i order them today.. would you happen to have a parts list in any other format?. another question i have is the power transformer. what do you reccomend. i’ve been searching through angela.com. and they have alot if different ones. thanx for the help i appreciate it. you might get tired of me as i will be coming back seeking your help from time to time. thanx.
ok, so i got excel for the parts list. i’m gonna have to etch my own pcb board since the manufacturer will take too long. i’m also thinking of getting a rack chassis for it. still unsure about the Transformer though. i’m from New York City by the way. maybe that would have something to do w the voltages?. thanx.
Just don’t forget about the daughter board. It’s not correct (unless you don’t mind reverse pots or install some pots from the other side of the board, which would be ugly).
You need around 280-300VAC before rectification capable of delivering around 10mA DC. I get trannies custom made for me. If you go with switching I’d get separate secondaries for heaters and switching. If custom is out of the question, I’d use back to back transformers.