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PostPosted: Fri Dec 01, 2017 8:08 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
Hi,

I was wondering whether anybody has a Mark III Purple Stripe schematic. Search turned up some threads with links but they are all dead. My Purple Stripe amp is a 60W with reverb but without the graphic EQ but a schem of the 100W would be fine as well.

Many thanks in advance
Stephan


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PostPosted: Mon Dec 18, 2017 3:48 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
I cracked the amp open yesterday because it needs service - lot of scratchy pots. Looks perfectly original inside but was also very dirty. It looks like it has never been serviced or modded in any way so a cap job is in order. I traced the amp along with the schematic that is available on schematicheaven and found the following differences:

Lead circuit:
- resistor from lead drive pot wiper to ground is 330k, not 150k, and there is a 120pf to ground which isn't on this particular schematic
- on V3b, instead of the .001 in parallel to the 270k Ra there is a .002 from the output of V3b to ground
- after the .047 coupling cap off V3b, there is a 220k II 250pf series network followed by a 100k Ii 500pf network to ground, i.e. like Mark II C+/III Red Stripe
- Lead Master pot reads 187k - it's probably a 250k audio. Value difference should be inconsequential since I never turn up fully.

Reverb circuit:
- the schematic is missing the 220k to ground after the 1M5 on the input but in the amp it's there

Master volume is a 250k (reads 209k), not 100k as in the schematic.

Presence circuit:
- the cap in parallel to the 56k resistor is .01 instead of .005 on the schematic

Looks like I only need to change the presence cap and the cap in parallel to the 270k Ra on V3b and I am pretty much in Mark II C+/Mark III Red Stripe territory. I will probably not bother to change the 330k on the lead drive to 470k as in the II C+/III Red circuit since I don't really need more gain than the thing already has. I will change the 1M5 on the reverb circuit to 470k as in the Mark IV and also the .003 cap from V4a anode to cathode to .002 because I want more and a brighter reverb. Probably putting in a 12AT7 helps, too, so I try that first before changing the circuit.

Cheers Stephan


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PostPosted: Thu Dec 21, 2017 12:54 am 
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Mark II

Joined: Wed Jun 07, 2017 9:37 am
Posts: 87
Location: Brooklyn, NY
So I gather you can find the mods online and have a local tech do them then? I've been toying with the idea of getting a Mark III.


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PostPosted: Thu Dec 21, 2017 5:52 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
Some mods you can still find but some are now difficult to find since pictures hosted elsewhere are no longer accessible. I do all amp modding myself as a hobby. I started doing this in 2001 and have modded and built several amps since then (my first complete amp build was in 2008).

I performed the following mods on my Mk III:

- changed the presence cap (C 516) from 0.01 uf to 0.005uf (Mk IIc+/Mk IV spec). This is a definitive improvement as it opens up the amp and makes the presence control work more gradually.
- changed the plate bypass cap on V3b (C 30) from 0.002uf to 0.001uf (Mk IIc+/Mk III Red Stripe spec). I like this better as it cuts a bit less highs compared to the stock value.
- put in the reverb glitch fix, i.e. a 120k resistor from the red wire of the reverb transformer to ground.
- changed the Mesa Black Shadow C-90 speaker to an Eminence MOD (no longer made; it is said to be similar to the V12 but with slightly more bass).

The speaker change eliminated the harshness in the top end. That amp now really sings in the lead mode, even at very quiet volume levels.

I will replace the electrolytic caps in the power supply but I am still waiting for the parts. I will definitely try a 12AT7 for the reverb tube and maybe also for the phase inverter tube but other than that I see no reason to tinker further.

Compared to the mods I did to other amps I would rate the above mods to be non-invasive as the part changes reflect what Mesa Boogie was doing in older and newer models, and I understood that the reverb glitch fix was an official remedy.

Cheers Stephan


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PostPosted: Fri Dec 22, 2017 12:08 am 
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Mark II

Joined: Wed Jun 07, 2017 9:37 am
Posts: 87
Location: Brooklyn, NY
Ok very cool. I'd love to get into this stuff at some point. The III+ modded amps I've heard clips of sound amazing. They are relatively inexpensive for a hand-made amp constructed in the States too. They seem like such a good deal.


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PostPosted: Sat Dec 23, 2017 5:40 am 
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Mark II

Joined: Tue Sep 08, 2015 3:35 am
Posts: 60
Agreed. I'd even go further than "relatively inexpensive"- I always wonder at how all Mk series amps are anything like as cheap as they are now, "+" or not.

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Too many Mesas and other amps, too many speakers, and far too many gain pedals and rack FX.


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