Mesa Mark IIc+ Preamplifier

  
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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 ²s 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 bui
Mesa Mark IIc+ Preamplifier - schematic

ld 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...



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