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.
Mods
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.

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.
Construction

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.

Pictorial

Click on a photo to see more details

Video Clips

Click on a thumbnail to play the video on YouTube.

Click here to list all 6 related video clips.

Comments
102 Responses to “Mesa Mark IIc+ Pre PCB”
  1. Shep says:

    I read through your tips. I read merlin’s grounding scheme section 15.9. I’ll list the link below. It shows that even with the devices having the same circuit reference to chassis ground there is still a ground loop contained within the preamp. I had them grounded at separate points originally. It was loud. Grounded at the same point it was quieter. And after lifting the chassis ground it was pretty much gone completely. I was thinking of grounding the preamp chassis with the mains ground and using the preamp output as a ground reference to the actual amplifiers chassis. So the preamp chassis is still safe and the ground loop is completely eliminated. What do you think?

    https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.valvewizard.co.uk/Grounding.pdf&ved=0ahUKEwiaxZnfndjQAhUS82MKHS-EA0cQFggaMAA&usg=AFQjCNHCXyjFjTXFIMbesWNh9ePvxDFWrg&sig2=CBpXvlUu6YhXLZKvJnyFWA

  2. Shep says:

    I did some tinkering just to see if lifting the chassis ground would get rid of the hum completely, it did. Making sure the ground reference is grounded in the same location in both devices helps alot, but ultimately, I think it would need some sort of isolation transformer or something. Not to sure what the best approach for an artificial ground lift would be. really freaking awesome preamp bancika, thanx.

  3. Shep says:

    Did you switch to a lesser gain tube?

  4. Shep says:

    Found another problem which might be beneficial knowledge for you or anyone planning on making this pedal. If your plugging into a power amp or effects return on an amp. You need to know where the circuit to chassis ground is located in the amp/poweramp. Whether it’s near input or output. If you make the pedal with circuit to chassis ground at input and your power amp has a chassis ground at output it will create a giant ground loop in your room. Hence why I heard such a loud hum. Now the hum is tolerable. And the ground loop is still there but is controlled. As long as the preamp and amp your using are both grounded at output or both at input.

    • bancika says:

      thanks for reporting. Ground loops can be an issue with any preamp/poweramp, or even pedals. I always ground at the input. Btw, did you try changing tubes? My first build was very noisy, and simple tube swap fixed the issue.
      Cheers

  5. Shep says:

    Found a problem. I had bypassed the 680k right before v2a. So now the tone is awesome and I can dime the gain. All that I I’ve got now is a continuously loud hum while I’m not playing. Like heater or cap hum

  6. Shep says:

    The issue is feedback when I’m not playing anything. And with gain all the way up its like a crazy fuzz pedal. Like there is too much gain. I can still get some pretty decent tone out of it, though, even with those bugs. I can always make sound clips if you want to hear.

    I haven’t calculated expected voltages. I’ve just been using your schem as a guideline. All of the other voltages in the circuit are at least +-5% v if not closer than that. Some are spot on. Only off voltages are v2b plate/cathode and v3a plate cathode. Checked resistance values for plate and cathode resistors, they checked out. I’m going to double check the values and check the grid to ground resistors. I have a feeling it has to do with something in between v2b and v3a. Because every where else is fine except for those two stages and they are both about 40v to 50v lower. I’m going to recheck the resistors for ohms and coninuity within the circuit. Check caps and check coupling caps for dcv after the cap. I’ll get right back to you.

  7. Shep says:

    Hi, been about two years since I built your pedal. I’m back in it trying to do a little debugging. Just a quick question. On my build I get about 40 to 50 less volts on my plates for v2b and v3a. Every where else is very close to the same. Could that voltage difference be a problem?

    Thanx

    • bancika says:

      Do you have any issues with the preamp? How do the other villages on those triodes look? You can try to calculate expected voltages using curves from the data sheet. Did you measure plate and cathode resistors to make sure they are correct?

  8. Bro says:

    Hi Bancika

    Based on your schematic and wiring, you choose 6N2P as cathode follower on the output. I was wondering, can i use 12AX7 as cathode follower, without changing the plate voltage magnitude i.e 385 V ?

    • Bancika says:

      Sure. 12AX7 are used in bunch of amplifiers that run plate voltages that high…Soldano’s, Mesa Recto and many more.

      • Bro says:

        thanks Bancika

        based on 12AX7 datasheet that i’ve got, maximum allowed plate voltage is 300 V, i’m afraid that giving 385 V directly to the plate (as in cathode follower) will damage the tube ;D

        • Bancika says:

          I think that’s referring to voltage of the plate relative to cathode voltage. In this case, cathode is at around 135V, so relative voltage is around 250V. Could be wrong though, but I wouldn’t worry. Check out Soldano SLO schematic.

          Cheers

  9. Carlos says:

    Hi, I have a question for you, I want to make an amp using your pcb and adding a power amp with 2 el84, like 15 watts or so, can’t find anything on the web that’s been proved to work fine, would you guide me where to find those schematics or web page, or something that you did by yourself? Thank you, and great job, it sounds awesome, I’m looking forward to have one of my own.

    • Bancika says:

      I’m not aware of such a project. You can either slap together my IIc+ with AX84 power amp (it’s 15-20W), but are well documented, or you can start from the original IIc+ schematic and convert it to 15W, which will require some knowledge.

  10. Jonathan says:

    Hi bancika
    can you tell me what value of choke or resistor a can use i can’t find the lr8

    • Bancika says:

      1-2K resistor should be fine. Depending on what your input voltage is, you might want to drop more to get it around 400V.

  11. Pasqua86 says:

    Good morning
    I successfully made a 50W amplifier with a push-pull 6L6 using this pcb.
    I also added a reverb driven by a 12AX7 taking the signal across the resistor to between 150K and V3A V3B

    Only one thing:
    To drive properly a pair of 6L6 I had to replace the 4k7 resistor between V3A and V3B with a 47k. otherwise the volume coming out was little.

    I hope it will be useful to all people who want to build an amplifier with this beautiful PCB.

    Thanks Bancika!

    • Bancika says:

      thank you for the report. Great to hear there’s another IIc+ roaring out there. You are right. My design it geared towards a standalone preamp and output is set to be FX pedal friendly. If you want it to drive a power amp, you’d need either to tweak it like you did, or leave it as is, have FX loop where preamp output is, and then boost gain after the FX loop.

      Cheers

  12. jason says:

    hi, this is an awesome project and the sound clips are very inspirational! thank you so much for publishing this information for us. i do have a couple questions for you.

    i’ve been reviewing your schematic and noted several differences from your layout diagram concerning component values. for example the schematic shows all power supply caps as 20uf but the layout diagram shows 1 33uf cap and 3 22uf caps. also in the ac heater option, the schematic shows to use a 68k resistor, but the layout shows 100k. i’m inclined to strictly follow your schematic but was hoping you could clarify what component values i should use.

    my second question(s) is in regards to the heater options. if i use the virtual center tap, would i wire it using your “ac heater” option (using the 10uf cap and 470k, 68k, and 2 100ohm resistors as per schematic)? if i understand correctly, this would require only one 300v secondary being connected from the transformer to the board. what voltage is this virtual tap supplying? i assume 6.3v but again, would like to clarify before building.

    i’m new to amplifier building and i’m learning so much. it seems the more i learn, the more questions i have! its people like you who inspire me to delve deeper into the world of electronics. keep up the great work!

    god bless,

    -jason

    • Bancika says:

      Hi Jason,

      thanks for the comment and thanks for the analysis. I don’t remember why I put different values. Could be just a mistake or I updated the layout with values when I built it. Either way, those values are not crucial and either of the two options will work fine. 100K gives slightly higher reference voltage than 68K. 33uF caps will provide more filtering, but 22uF is plenty. I used 22uF caps because motor run caps are huuuuge and I couldn’t afford more space. If you use electrolytics, you can use any value.

      second question – yes, AC heater assumes two things – virtual center tap (two 100ohm resistors) and DC elevation. As for the 300v secondary, there is only one 300V secondary, no matter which version you go for. It provides high voltage for tube plates. In AC heater mode, we just use some of that voltage to elevate heaters above the level where they are suspect to AC heater noise. Normally, AC voltage for heaters floats (alternates) around the ground, but we want to lift it higher, say 50V. Heaters will still “see” 6.3V, but that voltage has a higher reference point. 470K and 68K resistors form a voltage divider on B+ to create reference voltage. If your B+ is say 400V, those resistor values will give you 50V of reference voltage. If you use 100K resistor instead, it will give you 70V of reference voltage. Either will work great.

      Hope it makes sense.

      Cheers,
      Bane

      • jason says:

        Thanks for the fast response and the explanation. i have re-read it a few times and its starting to sink in. i will need to do more reading on voltage dividers but your explanation has been very helpful. any recommendations for educational resources, online or books, pertaining to tube amplifier circuits?

  13. Marko says:

    Hi Bane,

    I’m populating this awsome board, the sound is DT… 🙂 excelent!!!

    Little help!

    What is the value of bridge rectifier on your PCB board?

    Thank you,

    Marko

    • Bancika says:

      Relay supply rectifier is DB101 (50V 1A). You can use the same for heaters if you choose to use DC heaters, but I’d recommend the 1.5V version, part name B80C1500 or similar. Although, DC heaters are not crucial. DC elevated AC heaters I did on my build work just fine (shown on the layout as “AC Heater Operation”)

      Cheers

  14. Aharon says:

    Hi bancika,im gathering the parts for the pre,i have most of it but I can’t seem to find the supertex LR8 locally,eventualy i’ll get it but can i use a choke instead of the regulator?
    Thanks for the project
    Aharon

    • Bancika says:

      you can omit the regulator and put a resistor or choke instead. It’s optional
      Cheers

  15. Zan says:

    Hi Bancika
    I made PCB version
    And where is 47nF DropOrange Capacitor ?
    On Anoda V3a ?
    THX

  16. michael says:

    Hello,

    have you build the 5 band EQ? I’m just trying to get it running, but have some problems.

    Regards,
    michael

  17. Zan says:

    Hi Bancika
    Way You not use on front panel 2 switch ( deep and bright ) ?

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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.