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 Post subject: Mark III to Mark II mod
PostPosted: Fri Feb 23, 2018 12:33 pm 
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Bottle Rocket

Joined: Fri Feb 23, 2018 12:27 pm
Posts: 3
Hello,

I'm searching the last few months to make a mod to my mark III purple stripe. Specifically I'd like to mod my purple stripe to a mark II c+. Any ideas of what I have to change in my amp?

Thanks!!!!


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PostPosted: Fri Feb 23, 2018 3:00 pm 
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Mark II
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Location: East Coast, New Jersey
One of the guys on the forum drew up a IIC+ schematic

http://forum.grailtone.com/viewtopic.php?f=27&t=74680

You could find a Mark III schematic and compare the two.

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PostPosted: Mon Feb 26, 2018 3:24 am 
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Bottle Rocket

Joined: Fri Feb 23, 2018 12:27 pm
Posts: 3
Thanks for the info!!!
I was wondering if anyone had modified the amp in the past so he could share some info on what i have to do exactly.


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PostPosted: Tue Feb 27, 2018 5:59 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
Here is what I did to my purple stripe:

1. Change C30 from 0.002uf to 0.001uf.
2. Change C516 from 0.01uf to 0.0047uf (this cap is in the feedback circuit in the output section)

Check whether the output of the lead circuit has the 220k II 250pf to 100k II 500pf voltage divider. My amp had it but the Mesa Mk III schematic does not have it.

I would not bother changing the 470k to ground (coming from the 3.3M series resistor) to 680k, unless you want exact IIC+ spec. You will lose clean headroom, and it will not make a noticeable difference in the lead mode because it is parallel to the 100k resistor to ground that is part of the voltage divider (see before).

Cheers Stephan


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PostPosted: Wed Feb 28, 2018 6:27 am 
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Bottle Rocket

Joined: Fri Feb 23, 2018 12:27 pm
Posts: 3
OK thanks a lot for the info!!!
I'll do it and be back with the result!!
Thanks!!!


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PostPosted: Wed Feb 28, 2018 6:19 pm 
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Mark IV

Joined: Fri Jul 01, 2011 4:09 pm
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Location: Looking over the shoulder of the guy who lost his apples - not that he really ever had any to lose.
The Last Jedi wrote:
Hello,

I'm searching the last few months to make a mod to my mark III purple stripe. Specifically I'd like to mod my purple stripe to a mark II c+. Any ideas of what I have to change in my amp?

Thanks!!!!


Can't be done.
Most notably because the IIC(+) and Mk-III have different pre-amp circuit boards.
Never mind the PT etc....


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PostPosted: Mon Mar 05, 2018 8:05 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
gts wrote:
Can't be done.
Most notably because the IIC(+) and Mk-III have different pre-amp circuit boards.
Never mind the PT etc....


I guess you meant by "can't be done" "can't be done exactly", and if so, I agree. Different circuit boards have different stray capacitances.

I also agree that power transformers have an influence on the tone, and it is well known that the transformers on the Mark IIC+ were different. But the mods suggested will get the purple stripe in the ballpark. This may be all the OP wants/needs.

Cheers Stephan


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PostPosted: Mon Mar 05, 2018 4:39 pm 
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Mark IV

Joined: Fri Jul 01, 2011 4:09 pm
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Location: Looking over the shoulder of the guy who lost his apples - not that he really ever had any to lose.
darkbluemurder wrote:
gts wrote:
Can't be done.
Most notably because the IIC(+) and Mk-III have different pre-amp circuit boards.
Never mind the PT etc....


I guess you meant by "can't be done" "can't be done exactly", and if so, I agree. Different circuit boards have different stray capacitances.

I also agree that power transformers have an influence on the tone, and it is well known that the transformers on the Mark IIC+ were different. But the mods suggested will get the purple stripe in the ballpark. This may be all the OP wants/needs.

Cheers Stephan


Perhaps but he didn't ask for "ballpark" he stated "Specifically I'd like to mod my purple stripe to a mark II c+"
Which is why I posted it can't be done.
And even a "ballpark" mod doesn't come close. The way a C+ responds to your playing is very different than any MkIII will ever respond be it modded or otherwise.

Fwiw a lot of folks out there are misinformed about what Mark series amps can be modified to a C+ and which can't (and why).
Best to give him accurate info and not continue to perpetuate the mis-perceptions/ misinformation out there....


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PostPosted: Thu Mar 08, 2018 4:48 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
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Location: Europe
gts wrote:
Fwiw a lot of folks out there are misinformed about what Mark series amps can be modified to a C+ and which can't (and why).


From what I have read the IICs can be upgraded to a IIC+, which is what MB have offered as a factory upgrade. I also know that the first Mark IIIs (no stripe/dot/black stripe) had the same power transformers so they would be the next best candidates. Everything else is too different to be modded to be "exact IIC+". Please feel free to confirm or correct.

Cheers Stephan


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PostPosted: Thu Mar 08, 2018 11:19 am 
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Mark III

Joined: Sun Aug 21, 2016 5:23 pm
Posts: 221
Location: Hardwood Heaven
For a long time there were spare preamp boards to be used in these projects, but they have run out so now it's basically just IICs that can be upgraded.

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PostPosted: Sat Mar 10, 2018 1:54 am 
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Mark II

Joined: Tue Sep 08, 2015 3:35 am
Posts: 62
The best MkIIIs - I've had almost all the flavours, from Black Dot to Green - can get close-ish to MkIIC+, but are better viewed as great amps in their own right. They're good value, and are amenable to improving mods in both circuit and tube complement. The Black Dot Export is my own current favourite, having had a thing for Blue Stripes for some time. But it's still not a IIC+.

Since the boards necessary for conversion are exhausted, gts' comments are fair IMO. If you want a IIC/IIC+ then start searching, and be prepared to pay up. The reason for their perceived value isn't the recorded sound that we've all heard - it's the feel of the amp, in the room, that makes you play a certain way, which in turn leads to the great sound we've all heard on record/live.

Incidentally, the II series Marks (of all kinds) breathe differently from IIIs, as well as from each other; yet I'd still say I can get closer with my favourite Coli IIB than with any III, albeit boosted and at some considerable volume....!

If you have - or want - a great MkIII, they're cheap enough to buy and sell a few times in succession, in search of the one that best fits your playing. They're fine amps, hold up well once properly serviced, and are still under-valued enough to allow you to spend a bit more on good tubes (to sub in until you find your preferred gain structure).

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PostPosted: Sat Mar 10, 2018 6:18 pm 
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Joined: Tue May 05, 2015 9:07 am
Posts: 134
I tried a Mark lll+ red stripe and a Studio Preamp side by side a couple of years ago. The Studio Pre was closer to the feel of a c+ than the lll+. There are deals out there. I got the wide chassis factory c+ for $1660. I paid $1595 for the llc DRG and road case + about $700 for the c+ upgrade, new caps and shipping. There is a difference between the 100 and 105 transformers and between the HR's and Simul's but I'd be happy owning any version of the c+.

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Mark lll Black Stripe SRG
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PostPosted: Tue Mar 13, 2018 8:35 am 
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Mark II

Joined: Thu Feb 22, 2018 11:37 pm
Posts: 57
xdg999 wrote:
I tried a Mark lll+ red stripe and a Studio Preamp side by side a couple of years ago. The Studio Pre was closer to the feel of a c+ than the lll+. There are deals out there. I got the wide chassis factory c+ for $1660. I paid $1595 for the llc DRG and road case + about $700 for the c+ upgrade, new caps and shipping. There is a difference between the 100 and 105 transformers and between the HR's and Simul's but I'd be happy owning any version of the c+.


What was the pre through?


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PostPosted: Tue Mar 13, 2018 9:28 am 
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The Studio Pre was run through an old Zenith 6V6 stereo amp that was converted to a guitar amp.

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Mark llc+(HRx3,DRx2,HRG,DRG upgrade,HR upgrade)
Mark lll+(DRG 105, DRG X101)
Mark lll Black Stripe SRG
Mark V 25
Dual Rec (1994 Rev Gx2)
Mini Rec


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PostPosted: Wed Mar 14, 2018 11:19 am 
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Mark II

Joined: Wed Jan 31, 2018 8:32 am
Posts: 90
I’m pretty sure you could modify the preamp of a III to a IIC+ without much problem and get 95% close to a IIC+ easily enough. They’re not some mythical creature that have a hidden mystery component inside and some are definitely better (and worse!) than others in that they vary a lot in resistor and cap type. They also vary a lot in values and different components if they were an original factory C+ or an upgraded C. It will take some minor PCB mods on a III to do this, at the least disconnecting some components and maybe running wires to other bits of the board but it's definitely possible! I’m planning on doing this though to my recently aquired III Early Black Stripe and will report back. I would imagine the reason the III+ mod doesn’t replicate this turning the III into a clone of the IIC+ preamp because the III+ mod is more simple component value changes or cap additions rather than any PCB mods which would be time consuming and costly for Mesa.

For as close a conversion as possible, the more difficult bit to sort is the output transformer and the power voltage rails which are different on the III vs IIC+. If you have an early Black Stripe III with the IIC+ transformer I don’t see this as impossible though. You’d need to add a filter cap and mod the PCB slightly for the different voltage rails but it’s certainly achievable.

It wouldn’t be a ‘verified’ IIC+ but would be near enough identical. I’ll try it out and see what happens :) give me a few weeks though with work commitments at the moment though.

Jon


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