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

168 Responses to “Mesa Mark IIc+ Pre PCB”
  1. 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.

  2. 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.