Mesa Mark IIc+ Pre PCB

After building and using the single channel version of IIc+, I wanted to do a full blown two-channel version. Instead of expensive and not so easy to find optocouplers, I decided to go with readily available and cheaper non-latching small signal relays.

What makes the IIc+ sound?
There are a couple of interesting solutions not often found in amps:

  • Clean signal mixed with overdriven signal all the time. As you can see from the schematic, the only part of the circuit that gets cut off is lead circuit, clean signal path is always on. In lead bright mode, lower frequencies (and even some mids) of the distorted signal will be cut significantly by the 220nF cathode bypass cap. But clean signal added to the mix later will help bring back some of the lows that are not overdriven and flabby.
  • Pre-distortion equalization. You can fine tune your guitar response before the signal gets distorted, so you can prevent the sound from being flabby or hash. That also means that you’ll need some sort of post-distortion equalization. Most marks amps have the built in 5 band graphic EQ. I use graphic EQ pedal to shape the sound of distortion.
  • A lot of high-frequency shaping. There are a few capacitors going from grid to ground or to a cathode, or across the plate resistor. All of them cut very high frequencies. This prevents hi-end oscillations, but also removes some harshness.
Like always, I just can’t leave it be, so there are some changes to the original circuit, most of which can be omitted if not desired:

  • Already mentioned relay switching instead of optocouplers. I used Finder 30.22, but there are many other options, like Omron G5V-2, Takamisawa RY12W-K. There’s a separate power supply for switching circuit, to keep it isolated from audio stuff, but you could use heater supply to power relays.
  • Regulated high voltage power supply, using Supertex LR8 to regulate B+ to 385V. It’s a really cool little regulator. As long as you have at least 12 volts above the target voltage, it will regulate the voltage to 1.2 multiplied by the ratio of the two bias resistors. In this case, I got 180K and 560ohm resistors which produce 385V. It can take up to 20mA which makes it perfect for tube preamps. Click here to download the data sheet.
  • Separate master volume controls for Clean and Lead channel.
  • The last gain stage is converted to AC coupled cathode follower. There’s no need to boost the signal further because you will usually place some EQ/delay/modulation pedals after the preamp, so it’s important to provide smaller pedal-friendly signal not to fry them. Also, cathode follower acts as a buffer and outputs nice low impedance signal, like FX loop send.
Schematic and Layout

I drew these with my own DIYLC software, click to see the larger version.

Mesa Mark IIc+ Schematic
Mesa Mark IIc+ Board Layout (ver 11)

Click here to download printable trace mask for etching your own PCBs. Make sure to turn OFF the option to scale the document to page size when printing.

Click here to download DIY Layout Creator file of the layout. It’s not the latest version matching the rendered layout above, but that’s the latest I could find on my backup drive.

Wiring Options
Layout is drawn with flexibility and good performance in mind. I tried to accommodate the board to accept different component sizes, different tube heater wiring and different power transformers. So these are your options:

  • Tube socket is flexible to allow 6.3V or 12.6V heaters using 12AX7/ECC83 tubes or 6.3V using Russian 6N2P(-EV) tubes. Refer to the layout diagram to see how each of the three options should be wired.
  • Regardless of tube type or voltage, you can supply heaters with AC or DC current. DC requires few more components and higher input voltage, but can yield lower noise level. It’s advisable not to run DC heaters with 6.3V because three tubes will draw 900mA which will run the regulator very hot. You’ll need to mount a heatsink.
  • Heater elevation circuit (10uF capacitor and 100K/470K resistors) is optional and should be used only with AC heater supply. Refer to the bottom section of the layout diagram to see the difference.
  • Real or virtual center tap options. If your power transformer has tapped heater secondary you can omit the two 100ohm resistors that form a virtual center tap in AC heater mode and connect the real center tap where noted. You can also just terminate the real center tap and use the virtual one.
  • If your high voltage secondary has a center tap (300-0-300V), you can omit two of the four rectifier diodes that have their cathodes pointed to the HV pads and connect the center tap to the ground pad.
  • Separate secondary for powering relays, or sharing the heater secondary. If shared, two jumpers should be installed between heaters pads and two pads leading to the relay supply. In that case, note that relay voltage should match your heater voltage, so use the appropriate relays and regulator.
  • In front of each of the 5 triodes’ grids there’s room to install a small grid resistor to eliminate risk of blocking distortion and reduce risk of RF noise. Layout shows jumpers J1-J5, but you can put a small (10K, maybe even smaller) resistor that shouldn’t affect the tone noticeably.
  • If high voltage regulation is not needed you can always bypass it. Just omit the LR8 regulator, two biasing resistors and replace the diode with a jumper.

This time I opted for PCB-based construction to allow for easier assembly and for the other people easily etch their own boards. I’m usually not a fan of board mounted components, so only tube sockets are left on the board. There’s a separate daughter board in case you use board mounted pots, but it’s optional. I didn’t use it in my build.

I took number of steps to mitigate reliability issues that may be caused by board mounted tube sockets. The main issue is that mechanical movement of the socket caused by inserting and removing a tube may cause joints to crack and traces to be lifted.

  • Socket pins are bent inwards to ensure good mechanical connection
  • Sockets are epoxied to the board
  • There are many mounting screws and standoffs to ensure the board doesn’t flex
  • I glued a plastic standoff below each socket to reduce stress when inserting a tube (see photos)
  • I used a small piece of L-shaped wire to make better solder connection between each pin and copper trace leading to it (see the drawing below). That way I increase joint surface between copper and the pin
Pin soldering

All the components on the front panel are soldered to the main PCB, but filter capacitors and the transformer are connected to the board using non-soldered terminals. That allows for easier disassembly in case need to debug a problem or replace a component.

Parts Choice

I was very happy with russian military tubes in my SLO build, so I wired the board to accept 6N2P-EV tubes. They are near ECC83/12AX7 equivalents with 6.3V only heater wiring and internal shield between the triodes. They are low noise and long life and sound really good. To bring some of that warmth of JJ ECC83 tubes, I used a conversion socket that allows for ECC83 to be plugged into a socket wired for 6N2P. After some experimenting I settled with two 6N2P-EV in outside positions and JJ ECC83 for the lead circuit.

This time I wanted to experiment with poly film coupling capacitors (the first version was using paper-in-oil), so all the coupling caps are poly film. Again, I tried to stay away from electrolytics, so cathode bypass caps are 15uF poly film blocks and filter caps are 20uF 400VAC motor run caps. The only place where electrolytics are used is the supply for relays and heater elevation circuit. None of them should influence the sound. Capacitors in the pF range are mix of ceramic and silver mica, whichever I had in my parts bin.

As far as resistors go, I used a mix of Dale and Xicon 1/2W resistors for the most part. Plate load resistors are 2W KOA with the exception of the 5th stage where I used a 1/2W carbon comp resistor to add some mojo 🙂 Power supply uses 2W or 3W resistors.

Finally, the transformer is a custom wound toroid made to my specs:

  • 20VA core
  • 230V primary
  • 300VAC @ 40mA secondary
  • 12VAC @ 0.14mA secondary
  • 6.3VAC @ 1A secondary

* All voltages are under load

Need a Footswitch?

Easy, just add a mono jack in parallel with the channel switch and you can use any latching footswitch to toggle between clean and lead. Just note that for the footswitch to work, channel switch needs to be in the “Clean” position. Otherwise, it will override the settings from the footswitch. On my photos, footswitch jack is the one on the far right.

Want to Build a Single Channel Version?

Even if you are after the single channel version of the preamp, I still suggest using this PCB layout for simplicity (and it leaves room for future upgrade to dual channel). For the single channel operation you can omit some parts from the board and replace others with jumpers. Firstly, you don’t need the 12VAC @ 0.14mA secondary on the power transformer to power the switching circuit. You can omit the whole switching power supply section in the bottom-right part of the board – two electrolytic capacitors, one regulator and one bridge rectifier. You can also omit the clean volume pot, channel switch and indicator LED as well as the three relay protection diodes (marked D1, D2, D3 on the layout). Finally, we want to replace the three relays (RY1, RY3, RY3) so that the circuit is wired in the permanent lead mode. Instead of RY1 we need a jumper that goes between the 22nF cap and 680K resistor by connecting together the two outer pins on the right side of the relay (looking from the component side of the board), and do the same for RY2 and RY3.


Click on a photo to see more details

206 Responses to “Mesa Mark IIc+ Pre PCB”
  1. Tony says:

    Hi Bancika,

    Is there any chance you still have the diylc files so I could edit them slightly? I want to make the board deeper so I have space for a cutout so I can have the valves horizontal and fit it in a 1u rack.


    • bancika says:

      I managed to find a relatively up-to-date version of the file and uploaded to the article. It’s not 100% in sync though.

      • Tony says:

        Thanks that’s a massive help. I remember asking you about these boards years ago on sloclone and seem to remember you saying there was an issue with the daughter board?

        I might be misremembering but thought I’d best check.

        Thanks again

  2. Thomas says:

    Hi. Thanks for your documentation to this project. I got a few questions. I built the circuit according to your schematics on the PCB, although it does amplify (and surely sound like a mesa mark), the amplifier oscillates severely at high gain. How do you improve the stability of the amplifier?(I mean practically on the board layout) Should I connect the ground planes with vias? Should I place the components as compact as possible or should I separate them?

    • bancika says:

      Hey. I intentionally build my version on a larger board to leave some space between gain stages and components to avoid oscillations. Your board looks neat and packed on a small footprint but that can be problematic with such a high gain circuit.

  3. Dheer Chhabria says:

    Hi, do you have the PCB files or eagle files for this project

  4. Ben says:

    Hey, I know this may seem dumb but what are the dimensions of the pcb. Thank you and can’t wait to try this

  5. MaryV says:

    Hi Bancika,

    finaly ive first powered up this preamp. But I have problems with HV section.

    1) HV voltage after rectifier is 422V (voltage goes up in time in this spot)
    2) voltage on the 560R and second filter 22uF cap decreas from 376V after powering up to 366V after few minutes. Voltage slightly decrease in time….
    3) 180K resistor in the voltage devider near the LR8 goes very hot so Ive changed both resistors in this devider to 5W rating….180K still hot. I also heatsinked LR8

    Ive changed LR8 for new one….without succes.

    What should it be? Bad transformer?

    Thank for help?


    • bancika says:

      I doubt it’s transformer that’s causing the resistors to get hot. I used 2W and had no issues. Maybe you have wiring issues, or short or flipped LR8 backwards…or something is drawing current. Or maybe wrong resistor value somewhere. Could be a lot of things.

  6. joheynjoe says:

    Hello firend

    Thank for useful informations.

    I want to build a heavy metal tonned amp. Which one of your amps should i choose?

    I am really a new very beginner and i dont understand electronics. What do you advice?.

    Can you share part list and steps for Mesa Mark IIc+ Pre PCB or Soldano Preamp MK2 as excel?

    Thanks friend,see you, healthy days:)

    • bancika says:

      Hi. I do not have any information other than the one posted on the website. Both amps are classic and great. I prefer Soldano, but that’s just me. Neither is good for beginners…you need experience to build high gain amps from scratch. Better look for some kits with PCB and detailed instructions.

      • joheynjoe says:

        Hi friend

        Thanks for your kindly reply.

        Where should i start from? Any advice for me? I will build it with a friend who is better with electronics.

        Can i get your material list for 5W Soldano SLO

        See you, take care:)

  7. MaryV says:

    Hello Bancika,

    Im just finishing this project and have one question….i want to connect output of this messa preamp to Behringer eq700 pedal. Then to return of FX loop of my Peavey 6505 for live using…will this setup work?

    I also want to be able to connect preamp with eq pedal direct to my sound card…. Is there solution for this?

    Will switch with two resistors (4k7 for pedal and sound card output and 47K for feeding 6505 output section in v3b cathode follower schematic section) work here?

    Thanks for this great project!

    • bancika says:

      I think if should work in all those scenarios. It has output level similar to a hot pedal running at 18V supply. So in first two scenarios, just start with output volume low to avoid clipping in the EQ pedal and take it up while the sound is still clear. When it starts distorting, you’ll know it’s too much and back it down a notch.

  8. jack says:

    what are the black ic on the layout on the right side by switching and heater ct

    • bancika says:

      see the markings on them, those are bridge rectifiers. Sounds like you didn’t really understand the write-up and the layout, I suggest trying some simpler build for starters.

  9. Sérgio says:

    Thanks! I am building a mark IV based and with the drive at max I have a lot of feedback. I think there is a lot of gain in the preamp section or at the drive.
    Best regards

  10. Francisco Berrios says:

    Hey man! Thank you for sharing with everyone this build 🙂 I have a few questions.
    1) Im going to build this iic+ preamp while also adding the graph eq, but when i was looking at the sloclone schem i found that they are taking the “EQ IN” from the Anode of the V3B, whats the difference between doing your method and the boogie method? Is it a gain thing? You just wanted “line lvl” output voltage there or something? As im building the preamp only with line out after the eq should i just use your method?
    2) Why are you using less voltage than the original desing for the Anode resistors?
    Well, thats it, thank you!

    • bancika says:

      1) my preamp is designed to be standalone unit that can be used with any FX pedals. If I took the signal out of the anode, it would probably fry pedals that come after it because peak-to-peak voltage would probably be around 200V. That said, I’m almost certain that in boogies the EQ circuits is overdriven because the signal is too hot for those transistors to handle. Whether it’s intentional or not, I do not know. Another benefit of the cathode output is the lower impedance. I would use it in any stand-alone preamp. Anode output only makes sense if you are driving more tubes directly.
      2) not sure what you mean. Which resistor is lower voltage?

  11. Sérgio says:

    Hi, nice job very good.
    Do you have any issues on lead channel with high gain noises?
    Best regards

  12. fw says:

    Hello, your version based on schematic or on real amp reverse drawing?

  13. Konstantin says:

    Hi Bancika,
    One cool project for sure!🤛
    I’m wondering where the chassis grounding point might be located. Can’t find one in the pics, is there one at all?

    • bancika says:

      Hey. If you look at the top-view photo of the finished amp, there are two points where something is grounded to the chassis. In the top-right corner there’s a green/yellow wire with a lug that is bolted to the chassis using one of the screws that keep the IEC socket in place. The signal is grounded at the input jack, since I used the regular non-isolated jack…

    • Konstantin says:

      Hey again,
      so I was thinking, why not cut the ground bus between 1 and 2, then jumper the relay center contacts to restore the ground.
      Otherwise aren’t we creating an unnecessary loop?
      And I’ve seen that you’re using the shielded cable at both ends. Common practice recommends to connect the shield on the one side only, amp builder Lee Jackson does this at his Youtube channel.
      Have you tried to test the effects of 1 sided shield connection?
      Thanks 🙂
      Definitely gonna build this with your board and a 5 band EQ!

      • bancika says:

        Normally I ground the shielded leads only on one side. Two of the leads are grounded that way. I honestly can’t remember why I decided not to do it for the other two as well, it was so long ago 🙂
        But in principle I agree, less loops you have – the better.

  14. Serg says:

    Hello friend ! great preamp! the best sound! Have a question, can I connect it to a sound card? Thanks!

  15. MaryV says:

    Hello sir,

    great design, thank you for sharing. One question – all cathode capacitors (not in cathode folower part of schematic) could be low voltage rated (63V,100V), ok? I want to order some poly caps and 15uF poly in 630V rating is very expensive and hard to find…



  16. Avega says:

    Hello, incredible work! It sounds are really good, Congratulations! I would love to do one, but I have doubts with the components and I do not want to fail in the attempt, is it possible to supply a more detailed list of components? Thank you!

  17. Mauricio says:

    Hey man, cool project!!! What are your thoughts on connecting this preamp to a solid state power amp? I’m building a hybrid amp

    • bancika says:

      Sure, why not. Just don’t overdrive the power amp.

    • Rafal says:

      Hi guys,I see that is many post here questioning connecting this awesome sounding preamp with power amp.I have done this preamp and it sounds great.Only one inconvenience is(in my opinion only) the corrector is common for both channels, everything other is great thought by author , good work by You bancika.I connected this preamp to few poweramps and I really liked the soud it makes.But 3 weeks ago I have bought Ashton 50 W combo just for some money,good deal.I played through this combo few days because of som new stuff obviously,but I realised that I can connect this awesome preamp to power amp by line in socket.Well,I have recorded the soun,if anyone wants to her this beautiful preamp.Greetings

      • bancika says:

        Congrats on your build, glad to hear that you are enjoying it. It is a versatile piece of gear and can be used in few different ways. In the meantime I have moved back to SLO sound and now my main toy is this beautiful thing 🙂
        Totally different sound than IIc+…both good but SLO is fuller and “juicier” to me.

        • Rafał says:

          Hello again Bancika!I Ihave recorded some clips on Youtube with your perfect preamp.I have made some mods is well because of my pickups,they expected to take of some bass caps.Anyway I have tried that beautifull thing with couple of power amps.Sound depends of pickups and kind of guitar You use,but I felt in love with that nearly finished your SLO modified preamp.I cant wait to hear the sound.And im very thankfull for share the pcb board,it suits to any preamp to sold I mean fo example marshall to soldano..Well inspsired stuff.For me that small thing is much enough to play stuff from blues to heavy styles.Guitarist from my band cannot stop to play on this toy 🙂 Once again thank You so much for share that!!!

          • bancika says:

            Hi Rafal. Thank you so much for the comment. It’s folks like yourself that make all the effort worth while. The IIc+ is very versatile preamp and I love it. But I love the SLO even more 🙂
            Hope you’ll enjoy it.

  18. Andrew says:

    Hey man! Amazing job on this thing I cannot wait to get started.

    How loud is it if Im using the power amp of a 60 watt combo? Thing it

    • bancika says:

      Loudness will depend only on the amplifier you are plugging it into. The preamp itself will output line-level signal which is sufficient to drive any guitar power amp.

  19. Eune says:

    Hey! Sweet work on the Mesa preamp!
    Do you got a layout/schematic/pcb for implementing the Graphic EQ into the amplifire, also with the power section of the PCB, every part is easily available for purchase, so shouldnt be impossible to do.
    Thanks in advance!

    • bancika says:

      no, this is all I have. You can probably get the rest on sloclone forum.

  20. Raphael says:

    I’m a newbie guitarist and I was thinking about building a preamp to play some distorted stuff like Metallica. Do you think it could produce these tones? The other thing… I’m pretty green in electronics, do you have a detailed list of components needed for this build (with voltages and wattage)? Can I connect it to a solid state power amp? I’d really appreciate your answer.

    • bancika says:

      Yes, it can do metallica tones.
      I don’t have the list of components.
      Yes, you can connect it to any power amp.

      • jack says:

        What is the 2-250ka that is attached to ry3? Is that a dual gang pot. I’m thinking of building the single channel version. How do the 250ka hook up in the single channel version. And the 470k resistor coming off the heater supply connects to b3 right.

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.