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Interesting In Amp Modding


Steve Vai

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Greetings,

I'm interested in developing a preamp circuit or modifying a Marshall circuit so that it is similar to one of the modified ADA MP-1 preamp circuits, that I can use in an amp head. The one I have in mind is the Mod 4: http://home.lehighone.com:8081/adadepot/mo...OD4_Mark_II.zip

Basically what I want to do is create a version of this preamp that could be put on a PTP board, without any solid state components. So how would I go about adapting the schematic found in the link so it could be a new amp design?

Thanks!

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First of all, that's an entirely analog circuit that has nothing at all to do with MIDI (MIDI is a computer-controlled synthesizer sort of thing that entails a bunch of big chips all over the place - there's not a single chip anywhere on that board :D)

I went to the ADA webpage and downloaded the pile of schematics for the MP-1. The MP-1 has a solid state preamp that's NOT that schematic you posted - what you posted is a separate purely analog tube signal processing stage (that has nothing at all to do with MIDI) that they call the "tube breakaway PCB." I guess it's something they use to add tube distortion once they have the sound output.

The MP-1's preamp is a big complicated ugly solid state affair that couldn't really be ported over, I don't think - that bit you posted is more like a distortion pedal, or at least it looks like it :D.

Why is it you specifically want the MP-1's preamp? It's almost certainly a better idea to pick out a tube preamp to build rather than take a solid-state preamp and try to change it into a tube amp.

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First of all, that's an entirely analog circuit that has nothing at all to do with MIDI (MIDI is a computer-controlled synthesizer sort of thing that entails a bunch of big chips all over the place - there's not a single chip anywhere on that board :D)
Yeah, I understand that. I meant I didn't want to add anything with MIDI into the head design.

I went to the ADA webpage and downloaded the pile of schematics for the MP-1. The MP-1 has a solid state preamp that's NOT that schematic you posted - what you posted is a separate purely analog tube signal processing stage (that has nothing at all to do with MIDI) that they call the "tube breakaway PCB." I guess it's something they use to add tube distortion once they have the sound output.

The MP-1's preamp is a big complicated ugly solid state affair that couldn't really be ported over, I don't think - that bit you posted is more like a distortion pedal, or at least it looks like it :D.

Why is it you specifically want the MP-1's preamp? It's almost certainly a better idea to pick out a tube preamp to build rather than take a solid-state preamp and try to change it into a tube amp.

I want to make either a head or a rack preamp. Would it be possible to adapt this tube breakaway circuit to a 2204 preamp for more gain and a voicing similar to the MP-1? I like the sound of the MP-1, and I considered making a JCM800, but it won't be enough gain. If I could make a 2204 preamp modified to be closer to this MP-1 tube circuit, that would be ideal. I'll just ignore the SS parts of the MP-1 and focus on the tube board circuit. My goal isn't to copy the MP-1, but to make something with a similar gain structure and voicing.

Thanks for your response, it really helps clear things up.

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Man, I don't know... the 2204 and that bit from ADA don't have a lot in common. It wouldn't be a direct drop in preamp, and I just don't know enough about it to be able to tell you if you could turn it into one and what to do with clean, drive, and od2.

It LOOKS like the "clean" "drive" and "od2" inputs take a DC voltage to bias transistors to change the effective resistances that determine gain at different stages in the circuit (which is clever), but I don't know what ranges you'd want to use - I suppose you could figure it out to some degree just by looking at what transistor types it uses.

I'm really no expert at this, but that's how it looks to me - you should really try to get someone like lovekraft to answer you :D.

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You're re-inventing the wheel - if you want the MP1 sound, get an MP1 and use a tube power amp. A lot of engineering and tweaking went into making the MP1 sound that good, and you're not going to duplicate that by simply patching that tube section into a Marshall preamp somewhere - if it was that easy, I'd be rich and famous myself! :D

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You might want to look at www.ceriatone.com. They have schematics and sell main component boards for marshall 18W, JTM45, Plexi 50W and Plexi 100W amps, as well as a number of Fender amps, two Matchless amps, and a Vox AC30 minus the top-boost board. You could also check out ax84.com - they have their own line of amps.

Having just finished building my first actual amp, from a kit with a chassis and boards supplied, I don't think I'd have wanted to start from scratch on my first one, although I suspect I'd manage now.

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I was actually looking into the Ceriatone products about a year ago. I got an MP-1 to run into the Marshall's effects return, so I didn't get anything from them. I then bought a Peavey Classic 60/60 to go with the MP-1, and now the Marshall's just sitting around... So I guess I've ended up right where I started :D I was looking into the Plexi 50 board, but I don't think that will have enough gain. I'm looking for something with similar gain to a Splawn, VHT, modified JCM800s that were big in the 80s, etc.

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I went to the ADA webpage and downloaded the pile of schematics for the MP-1. The MP-1 has a solid state preamp that's NOT that schematic you posted - what you posted is a separate purely analog tube signal processing stage (that has nothing at all to do with MIDI) that they call the "tube breakaway PCB." I guess it's something they use to add tube distortion once they have the sound output.

The MP-1's preamp is a big complicated ugly solid state affair that couldn't really be ported over, I don't think - that bit you posted is more like a distortion pedal, or at least it looks like it :D.

Why is it you specifically want the MP-1's preamp? It's almost certainly a better idea to pick out a tube preamp to build rather than take a solid-state preamp and try to change it into a tube amp.

Check your facts... the mp-1 is most certainly NOT a solidstate preamp. It's a tube preamp. Agreed, there is an SS voicing for clean sounds, somewhat similar to roland jc120, but the distortion channel is a completely analog tube stage. In fact, the "tube breakaway section" is about the whole amp in itself. The main pcb contains an input buffer, power regulation, the whole midi thing , supports for the fancy front panel leds, etc.. but it has nothing to do with the signal processing itself.

If you want to tweak the sound, the small tube breakaway pcb is where it's at.

That's why adadepot sells the separate drop-in tube boards: mod mk4, mod 3.1, 3tm,... you replace the stock board, et voila! A whole "new" amp.

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Why don't you check yours?. If you download the schematics pack from adadepot, there's a "preamp" schematic that's a part of the whole business that's an enormous pile of opamps and diodes. Every input into the tube PCB comes off of that pile of opamps. Changing the tube PCB will, of course, change the sound drastically, because at line voltages decent opamps are almost completely transparent. The fact remains, however, that a real tube preamp doesn't have opamps in the signal path, or transistors in the tube stage.

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The fact remains, however, that a real tube preamp doesn't have opamps in the signal path, or transistors in the tube stage.

So I guess say....a jcm 900 isn't a "real" tube amp then. Lots of amps have opamps in the preamps section. That doesn't make them ss. IMO, a tube amp derives it's gain from a tube stage, which is sometimes spiced up a little by transistors (the marshall jmp1 for example); while an ss amp gets it's gain from transistor stages. I'll stick to my guns and say the ADA mp1's overdrive channel does not get it's gain from those opamps. The 'pile of opamps' is most likely the ss channel.

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It depends how much of a purist you are. A "real" tube can mean that there are absolutely no semiconductors (including op-amps) in it. But then, if you're such a purist as to not want any semiconductors in the signal path, forget about using any pedals or signal processors of any kind in your rig or else there will be no point in being **** about using a "real" tube amp.

Anyways, onelastgoodbye, when you refer to "gain" I assume you mean "distortion". Usually, the op-amps in these programmable tube pre-amps are just used to buffer, sometimes equalize (for voicing), and adjust the signal to optimal levels for the tubes to use. So the op-amps don't clip the signal or directly contribute to the distortion; the tubes provide all the distortion or soft clipping. So, unless you're really **** about it, you can say that the MP-1 is a "real" tube pre-amp if all the distortion is actually supplied by the tubes even though the signal goes through a few op-amps.

On the other hand:

The op-amps are usually used to replace the first few tube stages of a pre-amp where very little distortion occurs, but those early tube stages can add a certain warmth to the signal, nevertheless. And many tube amp experts will say that the pre-amp tubes in these first stages are the most important.

So decide yourselves what you consider as a "real" tube pre-amp. To me all this is only semantics.

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The JCM900 is a hybrid design at best (and a poor one, IMHO - doesn't sound much better than the early Valvestate trash) - it's definitely not an all tube amp, with those silicon diodes making most of the preamp distortion, any more than a Tube Screamer is! As for the MP1, it's not an amp at all - it's a MIDI controlled multi-configurable rackmount preamp, so the argument is pretty much moot. Stop worrying about components, and concentrate on results - if it sounds good, it doesn't matter if it's made out of old post-WWII Asian transistor radios, and if it sounds rubbish, having a few tubes in it isn't going to help. :D

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I've decided to sell the Marshall and use the money to buy a Traynor Bassmaster, and mod that into a Plexi (or something gainier). So I'll probably make a thread for help on that, but I have a lot of info now as it is.

Thanks for all the help everyone! I learn so much here :D

did that and i love it.

personally i just did the whole push pull pot mod that i found on a peavey. where you can parrallel [already wired that way] or cascade the two sections. i did this on my buddys amp and its pretty cool. depending on what you want out of that thing heres a couple of easy things.

on most of the traynors they had both cathodes going to the same cap and resistor.

fix that keeping the same 820/220uf [thats what was on mine] for channel one.

i used the stock marshall values for channel two. [later i dropped it to 1.8k/1uf]

change the coupling cap on channel one to a .56 is what i have in there now and it sounds awesome.

channel two Ccap .0068. originally it came with a .022uf on both sides and i went with the .0022uf but it was a little brighter than i wanted..

hows it sound?? Channel one with a lespaul Ledzeppelin

channel two with a strat. [and lets be realistic a booster or fuzz to get a little more drive] Hendrix.

keep in mind this doesn't mean you can plug an ibanez jem into it and it sounds like jimi. but if you know the songs and have decent technique you should be able to conjure up those classic tones.

ciao from the rubber room

ed

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