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. lee says:

    hello Bane,
    isn’t three 6n2p-ev is 3x340mA=1020mA,is 1A little low?the heater need 1.1A-1.2A to heat the tubes

  2. James says:

    Ty for sharing!! I have a C+. Please elaborate on that mod of placing a resistor before each grid to eliminate the risk of blocking distortion. This is the first time that I have ever heard that germ. What is it? Is blocking distortion good or bad? Ty for your time! James.

  3. Andrej says:

    Hi! I’m just getting into DIY pedals and amps. My question is how much did it cost you to make this amp? You can email me about it.

    Thanks in advance for the answer, Cheers!

    • bancika says:

      I built it many years ago, can’t tell for sure. It really depends on which parts you buy and how much it costs to get them in your country. You can probably build it for as low as 200 USD/EUR or as high as 400 USD/EUR if you buy more expensive parts.

  4. hello bane, how are you
    First of all I speak Spanish and sorry for my English
    I have a question and it is that
    I don’t have much knowledge of electronics right now, just some physics laws and a bit of practice in soldering but I have never worked with PCBs even though I am about to study electrical engineering at university next year (I am just 18 years old) , but I want to make this preamplifier before and my question is what knowledge of electronics do I need to be able to build this preamplifier.
    thanks a lot
    I wait your answer

    • bancika says:

      You need more practice in soldering and generally building things rather than electronics, but you also need some experience building tube stuff. Building it is easy, getting it to work properly is not that easy. You need to make sure that all voltages are as expected, debug the circuit if you have issues (noise or it doesn’t work at all) and similar…

  5. ryan says:

    Hi Bane,
    What are the maximum currents on 12V and 6.3V and 300V secondary of the transformer to the preamp work well ? I don’t have low current transformer.

    • bancika says:

      there’s no maximum rating, use whatever you can. With higher current you might need to drop some voltage for heaters if using DC unregulated heater supply as higher current transformer will output a bit more voltage (due to less sag under load).

  6. ryan says:

    Can this transformer specs work well? :
    230V primary
    300VAC @ 40mA secondary
    12VAC @ 1A secondary (i can’t find 0.14A one)
    6.3VAC @ 1A secondary

  7. Matt says:

    Hello Bancika

    I’m curious about the power transformer, was it based on the 105 pt Schumacher produced for Mesa and if it was have you changed any values of it. What’s the current value vefore applying load to it?

    Cheers !

  8. hyu says:

    Hi Bancika
    I want to build a 12ax7 12.6v version.I have a question about the power transformer, now i only have this transformer (2 outputs and 1 input) :
    -220V primary
    -300V secondary
    -12VA secondary
    Can i use this transformer ? which transformer should i use? 2 outputs in the secondary or 3 outputs in the secondary?

    • bancika says:

      What are the current ratings of the secondaries?

      • hyu says:

        the current ratings of the secondaries are 3A, i can get another transformer with higher current

        i jump the heater and switching pad and wire it into 12V secondary output

        this is my diagram,can you check it for me :

        • bancika says:

          The board is designed to work with two or three secondaries, whatever you have. If you have two secondaries you can jumper together the heater circuit and the switching circuit. Take a look at the bottom of my diagram. You can use AC or DC heaters. If you choose to use AC I advise using heater elevation as I drawn it. For DC I would use a separate rectifier for the heaters, as I drawn it

          • hyu says:

            what is the minimum HV tap of the transformer? Can i wire two identical transformers with only one output in the secondary in parallel ? :12V secondary and 220V primary and 5A instead 2 or 3 outputs secondary transformer ?


          • bancika says:

            You can wire two transformers back to back as you drew, but the output voltage will not be 220. Without any load, 12V transformer will output probably close to 14V, so ratio of the transformer is probably approx 1:16.

            The first 5A transformer will not be loaded even close to those 5A so it will output probably 13-14V which you need to drop to 12.6V with a power resistor to feed the heaters. But you can use those 13-14V to feed the second transformer and get approximately 200-220V. That will work but is less than the designed voltage of 400V. If you can get a 220-9V transformer for the second transformer it would be better as the ratio is approx 20. That should get you to 260-280VAC that is close enough for the target 400V DC.

          • hyu says:

            I found a transformer with HV specs: 220V – 220V – 240V
            so 220V+220V = 440V in series ,is it too high ? is 240V HV tap enough?

          • bancika says:

            You can use 220+220 in series but you need to drop extra voltage with some power resistors. You will have 150-200V to drop, which would produce some heat. But you need to play with the values…say that you have 200V extra and current load of the 3 tubes is 6mA…you would need 33K resistor that will need to dissipate 1.2W of power. I would use at least 5W resistor, maybe 10W to be safe.

  9. Rune says:

    Hi Bancika!
    Anything I need to know if I want to add another preamp tube to Get the full preamp in there, one triode for the last stage with the deep pull/gainboost switch and then one triode for your ac cathode follower for effect friendly signal?
    Larger fuse? Transformer is 300-0-300 with 6,3v paralell on heaters.

    • bancika says:

      Just make sure that you have at least 1.5A available on the 6.3V secondary. 4x300mA = 1.2A, and another 300mA to be safe.

  10. Nicolas Jarpa says:

    Hi, great site my friend. Can I use both ecc83 or 6n2p without any changes other than the heater’s wiring? Thank you

  11. Dheer says:

    Hi, I was wondering if you had the schematic files for this built. Like an eagle project with the libraries and all. Thanks!

    • bancika says:

      The schematic is in the article, but not in eagle format, as I use my own software for schematics and layouts 🙂

  12. Jason says:

    Just wanted to update that after all this time fiddling with it, and in my arrogance, I didn’t have the lead drive grounded and when I did it was grounded in the wrong spot. I’m happy to say it works perfectly now. I have 289v on b+1 plate using a center tapped 450v center and everything is cool to the touch even after playing for hours. I added lots of extras and used 3m pots on the lead drive and volume stages. It has so much gain that it oscillates but before that it almost sounds like a triangle big muff. I got a guild thunderstar lead that I removed the reverb from and am going to I’m going to mount it too so that I can switch between preamps by footswitch. My GF put wheels onto the base of the guild thunderstar lead so when I’m finished I’ll be able to push and go without having to hall a 70lb head around. Thanks again for your innovation and time. I appreciate the encouragement to solve the problems and am happier that I was able to do it without any major help.

  13. Jason says:

    Heya Bancika,

    I really appreciate that you’ve shared your work. I wouldn’t have been able to so this otherwise this is my first build. And I finally got around to getting this to work. I used p2p on fiberglass. Haven’t added switching yet because I’ve been running into an issue.

    For some reason the drive pot isn’t working but the middle pot has taken its place. I have rewired the tone stack several times and I seem to be missing something. Perhaps I have it wired backwards and the ground is going into the middle and thus replacing the drive pot? What are your thoughts? Thanks again.

    • bancika says:

      It’s hard to tell, I suggest debugging with a multimeter set to continuity. Ensure that all connections from the schematic are there in real life…and those that are not there are not made 🙂

  14. Jason says:

    Heya. Thanks for doing this. It has been fun studying this and learning about electronics from your site. I have been studying this and your single channel version and have been working on a hand drawn layout of the preamp that is p2p on a turret board like fender style amps. I have finished it and was wondering if you could look over my it for mistakes I’m pretty sure everything is correct because I compared it to the pcb version but its possible that I missed something. I also have a question about the relays: I noticed in your updated schematic it states that you can use a single dpdt relay for ry1 and ry2, does this mean that on that schematic the relays have been changed from dpdt to spst? It kinda looks like that in the schematic but I want to be sure. I’ve also been thinking about using optocouplers since they would be easier to mount onto the turret board. Thanks for your time. Hope you had a good holiday!

    • bancika says:

      Hi Jason,
      I used two relays instead of one just to make the layout nicer and separate input stages from later stage. A single DPDT relay is equivalent but a bit harder to achieve a nice layout because lead input and output will both converge to the same relay.
      You can use optops, furthermore the original mesa amp uses optos. I switched to relays because they are cheaper and readily available, that’s all.
      Unfortunately, I do not have time to get involved in projects, but I suggest using my DIYLC app for layout drawing then using some of tools there to make sure that it adheres to the schematic.

  15. Tony says:

    Hi I finished up this build in a 2u rack.

    It’s all working fine for the most part but I have some pretty loud hum of the volume control is above half way.

    I also noticed a few pots are wired backwards on the layout. Treble & mids. I corrected the treble and will do the mids later but the mids seems to have a very small range?

    I used 6n2p-ev on all positions but I’m thinking of changing v2 for 12ax7. I tried it with an adapter and liked the results but it makes it too tall to close the chassis so need to rewire.

    The only other problem I have is after a while the regulator shuts down muting the amp.

    I need to monitor voltages but I think they’re OK 421v after rectification but I’m guessing it’s raising after some time otherwise it wouldn’t be shutting down unless maybe its thermal related as it only does it with the chassis closed up.

    • Tony Beggs says:

      Hi Bane, I’ve solved all of the issues. the regulator seemed to be going into thermal shutdown. I attached a clip on heatsink and haven’t had an issue since.

      Im now thinking of maybe adding a buffered loop in like the metro one. I was going to try before the masters but the peak to peak voltage is too high as the board can only handle 60vpp so ill have to do some more measurements.

      The only reason i’m wanting to add a loop is to use it 4CM with my gsp1101.

      If you have any suggestions let me know.

      Thanks for a fantastic project. Ill report back with some clips soon once i’ve finalised the build… Just added a gyrator based 5 band EQ and its sounding amazing.

      • bancika says:

        Hi Tony. Great to hear that you solved the issues.

        As for the FX loop, not sure you need one as the output from the preamp is low impedance and relatively low peak to peak. The voltage divider at the cathode of the last stage attenuates the signal, making it pedal friendly.

  16. Jason says:

    Heya, I have some questions about this amp. Namely, can I use push pull pots instead of relays and I’d like to ask if the transformer from an old carvin x100b I have laying around would work. Please email me if possible thank you.

    • bancika says:

      I wouldn’t go with push pulls for two reasons:
      1) you need to switch the circuit in 3 places, but push pulls typically have only two poles, so you’d need to leave only one volume pot instead of two.
      2) lead dress would be much worse if you have a push pull pot where both the input and output from the LEAD circuit are close together, it’s asking for noise and feedback.

      As for the transformer, yeah you could use it with some adjustments of resistor values in the power supply. You’d need to drop some voltage both on the high voltage and probably on the low voltage secondary (due to lower heater current).

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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.