Mesa Mark IIc+ Pre PtP
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.
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.
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.
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
|Click to download parts list|
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 🙂
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.
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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 🙂