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diy MESA pre as 3rd channel on guitar amp? - Need Advise!
diy MESA pre as 3rd channel on guitar amp? - Need Advise!
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Old 9th December 2009, 11:50 PM   #1
Rswilliams is offline Rswilliams
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Default diy MESA pre as 3rd channel on guitar amp? - Need Advise!

Hello, my name is Ryan, I am a full time student at UCSB, and looking for advise on an ambitious project idea. This is my first post here, and after some searching it seems like there are a lot of very knowledgeable people on this forum. I would like to pick your brains on my project idea to make my ďDreamĒ guitar amp.

I recently acquired a 1988 Carvin XV-212C guitar amp off of craigslist for a great price. It is a 6l6 powered, 100watt, two channel amp with Celestion G12's. It is the combo version of Carvin's X100b head made famous by Steve Vai and Frank Zappa. I absolutely love this amp, but the one thing it is lacking is a good high gain channel.

I have been playing guitar for 8 years or so and recently joined a band. We play many different genres of music, often including modern hard rock like Tool, early Godsmack, Metallica and originals that require a Modern Mesa Dual Rectifier style sound.

The Clean channel on the Carvin is absolutely stunning, I play a Gibson LP studio, and have never heard a clean tone Iíve liked more. The Gain channel, sounds great with lower gain, and blues tones, something I use quite often.
However, it does not do high gain well. Due to the cabinets open back design, poor condition and sheer weight, I am planning on converting it into a homemade head and separate 2x12 setup.

Here's Carvin's link to info on the amp, and details on it's design and signal path for those who may be interested ("Design" pg. 5): http://www.carvinguitars.com/manuals/X-AMP.pdf

THE PLAN: I would like to build the complete Mesa Dual Rectifier high Gain preamp and incorporate it as the 3rd footswitchable channel in my Carvin amp. Iíd build a separate chassis, and incorporate it into my custom head enclosure. I need to figure out where to tap into the existing input and channel switching circuit (before V1), and would like to have the mesa preamp tap back in to share the effects loop and reverb before heading to the power amp. Iíll also need to build a custom footswitch to add the 3rd channel. How possible is this, and what do I need to watch out for?

WHAT I AM ASKING: I have the schematics for the Mesa Preamp, and friends in the electrical engineering department at UCSB who are exited to help me build it. However, I need to figure out how to incorporate it into my amp like I described above. I would love it if someone is willing to help me figure this out. The input end and channel switching will take a little figuring out, but I am confident it should be simple enough. How to incorporate it into the power amp, fx loop and reverb has got me a little more worried. I know after the carvins 2 channel preamp with shared tone controls and V1 (mesa pre will have itís own), the signal then goes to the graphic eq, fx loop, then the reverb before heading to the PI 12ax7. Can I simply tap into the circuit at the fx loop and have it share the fx, reverb and PI to power amp? If someoneís willing to help me figure out, that would be amazing! Hereís a pdf with the schematics.

http://www.carvinmuseum.com/pdf/amps/xamp-revk.pdf
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Old 10th December 2009, 02:26 AM   #2
PB2 is offline PB2  United States
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I was interested in building the Mesa Dual Rec preamp also, however you seem to want to switch back to the Carvin which complicates things. If you build the full Mesa preamp, and then go to V1 which is the main input of the Carvin you'll have far too much gain. It would be conceptually simpler to build the entire Mesa pre and then just use your Carvin as a power amp, speaker cab. However, it is obviously a lot more to build and you would lose the Carvin input channel sound.

Do you plan to plug the guitar into your new preamp, then bypass it - perhaps with relays when you want to use the Carvin pre?

I would do it by having a daisy chain output right at the guitar input jack of your new pre which would go to the Carvin main input. This way you have the same signal going to both preamps. Then just have relays or FET switches to pick either the Carvin main pre, or your external Mesa pre output.

I found Mesa schematics on the web and studied them for a while. It is a highly complex design - seems like it wants to have a Fender or a Marshall sound on top of whatever new ones they added. Consider that you can choose SS (Fender) or tube (Marshall) rectifier, 6L6 (Fender) or EL34 (Marshall) tubes, and the pre also has signs of both.

I'd like to see the Mesa schematics that you have; can you provide a link to them?

Also, BTW wouldn't you want a 4 X 12 cab with that much power?

Pete B.

Last edited by PB2; 10th December 2009 at 02:35 AM.
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Old 10th December 2009, 02:51 AM   #3
Rswilliams is offline Rswilliams
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here's the link to the schematic I was planning on working from: The Free Information Society - Mesa Boogie Dual Rectifier Electronic Circuit Schematic

And yeah, that is exactly the way I envision the two to interact. I would take the signal to both directly off of the input jack and rig up a relayed A/B switch to act as the channel switching via footswitch to either the Mesa or Stock Carvin Preamp. Seems simple enough to rig up. I don't want to tap into the Carvin Preamp at all, so I would have the complete Mesa pre with it's own V1.

What I need to figure out is how to tie in Mesa's output back into the signal chain before the fx loop, reverb, and power amp stage. This is where I am totally lost. Can I literally just spice into the carvins input to the fx loop circuit?

So Basically what I'd like to happen is this: Single input jack to the whole amp, A/B switching between both complete pre's, then the same shared effects loop and reverb circuits as well as full power section.
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Old 10th December 2009, 04:15 AM   #4
PB2 is offline PB2  United States
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Note the FETs Q2 and Q5 which are selecting Lead or Rhythm, you could add another FET and change the switching to control 3 rather than 2. Or it is probably better to set the Carvin to Lead and then add another SPDT relay or FET arrangement to select the Carvin or Mesa Lead. You obviously have to check all the other logic to make sure that everything plays correctly but this is the basic idea.

You could also switch at the top of the Master volume if you want that control to work for both the Carvin and Mesa. Actually, you probably don't want this since the two may require different settings of the Master volume.

Pete B.
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Old 19th January 2010, 02:26 AM   #5
pzung is offline pzung  United States
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one way to figure it out is find a schematic that already has three channels and copy their switching system. preferably a three channel carvin since all their amps will have a similar topology.
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