Mark III red stripe - accurate schematic

[loylo69]

Hi!

I am a happy french owner of a Mark III red stripe combo, fully loaded with reverb, GEQ, simulclass and an EVM12L.
I bought it 2 years ago at a quite cheap price because the lead channel wasn't working.
I repaired it, a Vactrol LDR responsible of the channel switching was dead.

It gave me the chance to trace the circuit. The factory schematic of the black dot/stripe version aside, the other schematics I sourced on the internet seemed a bit wrong to me, specially the purple stripe version I found at sloclone forums. The blue stripe version that can be found here at the boogie board is way better, but I am not sure the R2 channel has been correctly traced.

I was able to lift up the circuit board and desolder the relay which hides some of the traces of the R2 circuit, so I feel confident that my red stripe schematic is pretty accurate.
First here are the preamp/lead/reverb sections. I still need to trace the poweramp and the power supply, so I'll upload an update of the schematic later.
I'd also like to compile the variations of the other stripes version on my schematic. I'll see if I can do it.
At last, I'd try to propose some mods that are my interpretation of the III+ mod, III++ mod, vol1.1 mod...

 

[Agustín Collia]

Wow, thanks a lot man! Seems quite accurate to me. Looking forward to the schematics for those mods, especially the III+ and Volume 1.1 mods. It would be very cool if Rhythm 1's volume could be controlled by one pot, and Rhythm 2 and Lead by the other pot. Thanks again!

 

[loylo69]

You're welcome!

Hope this can be useful to somebody.
I'd like to precise that I have no intention to screw the business of Mike B. and any other Mesa boys. In any case, I just can't compete with those electronic gurus, and I don't intend to. This will only be my interpretation of those acclaimed factory mods as I have never seen any of them, and don't want to disclose secrets.
I'd like to share this just for the sake of theorizing and sharing with the DIY community. As a matter of fact, I can't have the opportunity to send my amp from France to the Mesa factory to perform any mod, so I have to do (or try) it myself!

By the way, until I finish to trace the schematic, I'd like to share pics of my III red stripe with you. I have put it into a headshell I've built myself 2 years ago. Those combo are so heavy, it is easier for me to carry it weekly to my band's rehearsal room. :wink:
Even the logo is DIY! :mrgreen:

 

[chipaudette]

Thanks so much for this! I don't own a Mark III (I have a Mark IIC that was upgraded to a IIC+), but I've always been curious about the circuit differences between the various Mark III stripes.

If I may request, could you also trace out the power amp...especially the presence circuit (which is the feedback from the power amp output back into the phase inverter)?

With all the extra brightness attributed to the Mark III (and the variations in brightness/sharpness/harshness discussed in the different stripes), I've always assumed that the presence circuit must have been tweaked across the different stripes.


Also, if you're interested, one of the differences between the Mark IIC+ and the Mark III is in the lead circuit on the bottom left. If you look at the 270K resistor above V3B, the Mark IIC+ has a 1000 pF (aka, 0.001 uF) capacitor jumping around that 270K resistor. That cap acts as a filter to cut the very high frequencies (ie, the fizziness) of the high-gain lead sounds. The cap helps smooth the sound a little bit, which is good to some ears and bad to other ears.

I did know that they omitted this capacitor in many of the Mark III stripes, but I've never been sure if it is missing on ALL of the Mark III stripes. Your schematic shows that it's not there for the Red stripe, so that's cool to know.

I'm pretty sure that, if you send your Mark III to Mesa to get modified to a (so-called) Mark III+, they add this 1000 pF cap into the lead circuit to make it more like the IIC+.

Chip

 

[megavoice]

Welcome loylo69 in the "red"-group and thanks for your descriptions.
It would be nice if one of the Authorized MESAs could confirm all that here.

And thank you, chipaudette, for your comparings with the C+, as I`m also interested in this.
BTW, as I`m a LP-player I`m NOT AT ALL interested to tame down the pikes in the "Lead Mod", like probably a Strat player might do............. :wink:
Crancking up the volume and let the power section work mightily, for me it sounds almost perfectly.....

 

[loylo69]

Thanks Chip. I'm glad to discuss matters related to amp circuits.
No problem, I planned to trace the poweramp section, as promised on my previous posts. In fact, I already draw it on paper, I have to sketch it on computer. :wink:

I agree with you, I feel that the harshness usualy described about the Mark III may be largely due to the poweramp, especially the feedback loop. And also the snubber cap that you mentionned in the lead circuit.
About this particular cap, it is indeed omitted in the blue/green stripe models. But it is there in the red stripe, at a lower value (500p) than in the IIC+ (1nF). In fact, in the IIC+, this cap is placed accross the anode resistor of V3B (between V3B anode and C+ voltage) whereas it is placed between V3B anode and ground in the mark III (C30 on my schematic, corresponding to the number on the PCB). But it has the exact same role, that is to avoid high pitch oscillation in higain territory and further more cut high frequencies, as you mentionned.
I've never tested other value (neither no cap nor 1nF ) in C30 place of my red stripe. But I feel that 500p is a good compromise, preventing HF oscillations without taming too much high frequencies. It retains the aggressiveness of the sound.

Concerning the poweramp, the cap numbered C516 on the PCB (accross a 56k resistor in the feedback loop) is a 10nF cap in the Mark III, whereas there is a 5nF cap at this location in the IIC+. Changing from 10nF to 5nF is a well known advice for a III+ mod, which is documented on the boogie board. I'm pretty confident that the III+ mod from the Mesa factory change this cap to 5nF, along with the 1nF cap in the lead circuit.
I performed this mod on my red stripe. In my experience, the 5nF cap allow me to raise the presence knob as high as 8/10 without harshness. In the meantime, I've read that a lot of Mark III users are forced to cut the presence all the way down, or almost, on their stock amp.

 

[loylo69]

Thank you megavoice.
I definitely hear you about the need of pikes in an aggressive lead tone. That is why I don't feel the need to mod C30 to IIC+ value as I've just described above. But I dig the cap mod in the feedback loop.
Just out of curiosity, would you mind sharing your settings?

 

[Agustín Collia]

It will certainly be very helpful to me! Your interpretation will be very welcome. I live in Argentina, so I clearly don't have the opportunity to send it to Petaluma either!
I have a Blue Stripe myself, one of the latest Blue's I believe. The work you've done with the wicker and with that headshell in particular is simply stunning! You clearly have a gift with that, not to mention the logo. GTS here also does some very impressive headshells.
Seeing the pictures, have you already performed the Volume 1.1 mod? Where did you move the footswitch jack?
Thanks a lot!
P.s.: I saw you asked megavoice for his settings. I'll give you my settings just in case you find them useful. http://www.chain-metal.nl/mark3/front.php?bg=1&t=Studio+Settings&p=60&d1=80&d2=60&d3=25&d4=45&d5=20&d6=80&d7=45&p1=&p2=1&p3=&p4=&p5=1&p6=&p7=&g1=85&g2=65&g3=25&g4=55&g5=70
At lower volumes I tend to run the presence at 6, and Volume 1 and LD Drive as described above. At stage volume presence is about 4, Volume 1 at 7.5 and LD Drive at 7.

 

[chipaudette]

I agree with you, I feel that the harshness usualy described about the Mark III may be largely due to the poweramp, especially the feedback loop. And also the snubber cap that you mentionned in the lead circuit.
About this particular cap, it is indeed omitted in the blue/green stripe models. But it is there in the red stripe, at a lower value (500p) than in the IIC+ (1nF). In fact, in the IIC+, this cap is placed accross the anode resistor of V3B (between V3B anode and C+ voltage) whereas it is placed between V3B anode and ground in the mark III (C30 on my schematic, corresponding to the number on the PCB). But it has the exact same role, that is to avoid high pitch oscillation in higain territory and further more cut high frequencies, as you mentioned.

Ah, I had not noticed C30 going to ground. Very good! Thank you for pointing it out.

 

[megavoice]

Thank you megavoice.
I definitely hear you about the need of pikes in an aggressive lead tone. That is why I don't feel the need to mod C30 to IIC+ value as I've just described above. But I dig the cap mod in the feedback loop.
Just out of curiosity, would you mind sharing your settings?

OOOOPS ! Never heared of a mod in the Feedback Loop ?????????

My Lead standard settings are almost the same like most of the others have on their`s:

Reverb: Full
Presence: 6
Volume In: 7
Treble: 7
Bass: 1-3
Mid: 1-3
Master: as high as possible, as THE POWER SECTION will give you the most countoured and non-buzzing sound finally
Lead Drive: 6
Lead Volume: all depends

All pots pulled and mostly played in Class A mode.

 

[loylo69]

Thanks to you all for sharing your settings and for your kind words.
You both are running your presence knob higher than many users seem to do, as far as i can remember.
Here are my latest settings: I recently changed from a V GEQ, a bit like yours Agustin, to this one with tamed 6.6kHz slider along with raised presence. I feel it's too soon to say if I'll keep these settings or come back to the previous one.

OOOOPS ! Never heared of a mod in the Feedback Loop ?????????

In fact it's just the C516 mod I described above. But from different sources here on the boogie board, it seems that C516 in the blue/green stripes models has already the same value as the IIC+ (5nF), whereas it is a 10nF in previous mark III versions.
It is consistent with the description of the different stripe versions of the Mark III on wikipedia:
"Blue Stripe (1988 - mid 89): The fourth revision was the "Blue Stripe" Mark III which featured a blue marker stripe above the power cord. The amplifier was voiced so bright, it is considered to be the most aggressive Mark Series Boogie ever introduced. The power amp was also altered to mirror that of the IIC+."

Seeing the pictures, have you already performed the Volume 1.1 mod? Where did you move the footswitch jack?
Thanks a lot!

It was just the R2 master volume mod. But since then, I changed the place of pot and put it in place of one of the two 4 ohms output jacks, so I can put the footswitch jack back to its place.

But I want to do the vol1.1 mod with a stacked pot, controlling R2/Lead gain with the inner pot and R2 master volume with the outer pot. I want to these controls back into the footswitch jack's place. I have an idea to relocate the footswitch jack at the back of the chassis without drilling a new hole. I'll try it and report it back to you if I succeed.

 

[megavoice]

Just a little correction:

The Bass Pull ist mostly pushed in, I use it pulled only from time to time.
The Lead Master I`m pushing in when playing louder or very loud, as the Treble becomes too stitchy, but when pushed in I`m missing somthing as too much frequences are cut down........

I`m also astonished about the fact that so many guys have their Presence that low. I also tried, but for me, and the more I did, the more "juce", smacking and clipping I missed, and the "heart" faded away ????

Note all !!!
The Mark amps are nothing else but an old Fender amp with a pimped added Lead circuit.
This I`ve been told by my tech when he had taken a look inside the schematics.
And the "Normal Mode" is, when all knobs are pulled out. There is no boost. Pushed in, the knobs cut frequences away and in general "only" worsen the Sound.
Of course there seem to be lots of userers who can work with and find useful sounds at the end, but I`m thinking, these Push/Pull function had been only a sales-gag.

 

[Agustín Collia]

Please do report back when you succeed! I'm not so worried about R2 Master, but I'll definitely perform the Volume 1.1 mod!
I for one can't seem to run the Presence below 5, 4 at the most... It just sounds... dull, lifeless (then again I'm using humbuckers, but still).
I haven't found the sound to be "worse" when many of the knobs are pushed in, but well, to each his own.

 

[chipaudette]

Just a little correction:
The Mark amps are nothing else but an old Fender amp with a pimped added Lead circuit.
This I`ve been told by my tech when he had taken a look inside the schematics.
And the "Normal Mode" is, when all knobs are pulled out. There is no boost. Pushed in, the knobs cut frequences away and in general "only" worsen the Sound.
Of course there seem to be lots of userers who can work with and find useful sounds at the end, but I`m thinking, these Push/Pull function had been only a sales-gag.

Whoa, whoa, whoa. Ease up a little here...let's not get to crazy with too many conclusions all at once :wink:

It is true that the non-lead portions of the pre-amp in the Mark series is derived from the classic blackface Fender pre-amp. It is true that pulling the "Pull Bright" and the "Pull Shift" on the bass knob makes the circuit more like a Fender. But, after that point in the circuit, there are a number of changes that are important:

1) Vol1 Setting: The existence of a master volume on the Boogie means that you run the "Volume 1" *much* higher on the Boogie than on a non-master Fender blackface. As a result, a different feel on the clean channel. Try running your Boogie with Vol1 on 3-4 and tell me it sounds the same as when it's on the more typical setting of 6-8

2) Effects Loop: To do an effects loop, you take the already pre-amplified signal and then beat it back down to guitar level to send out the effects loop to pedals. Then, when the signal comes back, you re-amplify it. It's like having a whole second pre-amp after the first one. That changes the sound. (Oh, and the "Pull Deep" knob affects the frequency response of the effects loop...just showing how much the effects loop can matter depending upon how one chooses to implement that circuit)

3) Short-Tank Reverb: The reverb sounds quite different on the two amps...the tank itself is quite different (medium vs long) and the circuit is a bit different. The effect is that the Fender blackface reverb is long, luscious, and sproingy. Mesa, much less so. Anyone who owns a blackface (or silverface, or similar re-issue) Fender knows that the Reverb is so key to that Fender sound.

4) Circuit Voltages: Even for the parts of the pre-amp circuit that are "the same", the Boogie runs near 400V whereas a blackface fender (at least my reissue) is running around 260V. Bigger isn't better, but bigger is different. Got any old guitar pedals made before they all had voltage regulators? Ever felt that they sounded differently when the battery was old. It's because they did sound different. Now, transistors aren't tubes, but tubes do respond differently at different voltages.

5) Presence Knob: The boogie has a presence knob that you can use to affect the high-end sound (and feel) of the amp. The Fender has no such knob. Circuit-wise, both amps do utilize negative feedback to linearize the power amp's response (which is what the presence knob affects) but Fender chose to use a fixed value (instead of a knob) which is fairly dark. They get their brightness from other places in the circuit. Mesa gives you the presence knob. The presence knob is a huge difference.

6) Simulclass: Fender's are all Class AB...only Boogie has Simulclass. The power amp matters...especially at high volumes.

7) Transformers: Ever looked in the back of the boogie. The transformers are HUGE! This helps keep the boogie strong and solid, even at high volumes. The Fender, by comparison, has small transformers. This gives it a certain "sag" when the power draw (and the output levels) get high. Some folks love the sag. Some prefer the more solid strength of the Boogie. It's just different. And an important one.

So, that's my little speech help inform one's decision as to whether a Boogie is "nothing but" a souped-up Fender. I whole-heartedly agree that that Boogie is built upon Fender DNA, but IMO, the differences are big. It's like saying that Americans are the same as the English because Americans used to be English subjects. We're friends, we can speak a similar language, but we're not at all the same.

Whether it be US/England or Boogie/Fender, I prefer to say that while the shared roots are clear, time has allowed an ocean's worth of differences to separate the two.

Chip

 

[megavoice]

Just a little correction:
The Mark amps are nothing else but an old Fender amp with a pimped added Lead circuit.
This I`ve been told by my tech when he had taken a look inside the schematics.
And the "Normal Mode" is, when all knobs are pulled out. There is no boost. Pushed in, the knobs cut frequences away and in general "only" worsen the Sound.
Of course there seem to be lots of userers who can work with and find useful sounds at the end, but I`m thinking, these Push/Pull function had been only a sales-gag.

Whoa, whoa, whoa. Ease up a little here...let's not get to crazy with too many conclusions all at once :wink:

It is true that the non-lead portions of the pre-amp in the Mark series is derived from the classic blackface Fender pre-amp. It is true that pulling the "Pull Bright" and the "Pull Shift" on the bass knob makes the circuit more like a Fender. But, after that point in the circuit, there are a number of changes that are important:

1) Vol1 Setting: The existence of a master volume on the Boogie means that you run the "Volume 1" *much* higher on the Boogie than on a non-master Fender blackface. As a result, a different feel on the clean channel. Try running your Boogie with Vol1 on 3-4 and tell me it sounds the same as when it's on the more typical setting of 6-8

2) Effects Loop: To do an effects loop, you take the already pre-amplified signal and then beat it back down to guitar level to send out the effects loop to pedals. Then, when the signal comes back, you re-amplify it. It's like having a whole second pre-amp after the first one. That changes the sound. (Oh, and the "Pull Deep" knob affects the frequency response of the effects loop...just showing how much the effects loop can matter depending upon how one chooses to implement that circuit)

3) Short-Tank Reverb: The reverb sounds quite different on the two amps...the tank itself is quite different (medium vs long) and the circuit is a bit different. The effect is that the Fender blackface reverb is long, luscious, and sproingy. Mesa, much less so. Anyone who owns a blackface (or silverface, or similar re-issue) Fender knows that the Reverb is so key to that Fender sound.

4) Circuit Voltages: Even for the parts of the pre-amp circuit that are "the same", the Boogie runs near 400V whereas a blackface fender (at least my reissue) is running around 260V. Bigger isn't better, but bigger is different. Got any old guitar pedals made before they all had voltage regulators? Ever felt that they sounded differently when the battery was old. It's because they did sound different. Now, transistors are tubes, but tubes do respond differently at different voltages.

5) Presence Knob: The boogie has a presence knob that you can use to affect the high-end sound (and feel) of the amp. The Fender has no such knob. Circuit-wise, both amps do utilize negative feedback to linearize the power amp's response (which is what the presence knob affects) but Fender chose to use a fixed value (instead of a knob) which is fairly dark. They get their brightness from other places in the circuit. Mesa gives you the presence knob. The presence knob is a huge difference.

6) Simulclass: Fender's are all Class AB...only Boogie has Simulclass. The power amp matters...especially at high volumes.

7) Transformers: Ever looked in the back of the boogie. The transformers are HUGE! This helps keep the boogie strong and solid, even at high volumes. The Fender, by comparison, has small transformers. This gives it a certain "sag" when the power draw (and the output levels) get high. Some folks love the sag. Some prefer the more solid strength of the Boogie. It's just different. And an important one.

So, that's my little speech help inform one's decision as to whether a Boogie is "nothing but" a souped-up Fender. I whole-heartedly agree that that Boogie is built upon Fender DNA, but IMO, the differences are big. It's like saying that Americans are the same as the English because Americans used to be English subjects. We're friends, we can speak a similar language, but we're not at all the same.

Whether it be US/England or Boogie/Fender, I prefer to say that while the shared roots are clear, time has allowed an ocean's worth of differences to separate the two.

Chip

Well, thank you for all these explenations and differenciations,..... the most I like is that of the "Presence" function.
I cannot comment all this and will keep distance carefully, as I`m NO TECH and unskilled with electronics...
Hope you`re right with all that....... :wink:

 

[loylo69]

I totaly agree with you Chip! I couldn't have found better words than yours! :wink:

I`m also astonished about the fact that so many guys have their Presence that low. I also tried, but for me, and the more I did, the more "juce", smacking and clipping I missed, and the "heart" faded away ????

I for one can't seem to run the Presence below 5, 4 at the most... It just sounds... dull, lifeless (then again I'm using humbuckers, but still).

You seem to both share the same feeling about the presence control. And I agree with you two, it sounds lifeless and dull below a certain amount of presence.

It may be because this "presence" network is not designed the same way as the traditional one (in Marshall, Soldano...).
In fact, the traditional presence is just a treble cut filter. As it is a negative feedback loop (NFB), the signal is substracted to the open loop forward signal, resulting in a treble boost.

On the mark amps (after Mark I), the "presence" control is mostly an overall NFB control: with presence pot at 0, the NFB is maximum whereas there is almost no NFB with presence pot at 10. And there are complex RC networks in the loop, sothat it isn't just a treble cut filter (resulting into treble boost, because the signal is substracted, remember? ).
With the "Presence" pot full-up (almsot no NFB), the signal is reduced very slightly in treble and a bit more in bass .
Turning down the "Presence" increases NFB and reduces gain at both extremes. At some point, there is a small Mid-Cut appearing.
With the "Presence" at almost 0, it goes to a very dark tone.

At the end, if you prefer the raw and aggressive sound of a non-NFB amp (like the red channel of the Dual Rectifier), you may want to run the "Presence" of the Mark III as high as possible. On the opposite, if you prefer the tighter, darker, more polished feeling of an amp with more NFB, you may want to run the "Presence" of the Mark III as low as possible.

 

[megavoice]

Nice discoverings.......

But turning the Presence, above 6 a.s.o., too high sound will start crumbling, losing contour and get worse.....
It`s just like balancing on the edge of a vulcano.... :lol:

 

[Agustín Collia]

Yes, I mean, as always, it's a matter of balance... Presence at 10 (in a Mark III that doesn't have the mod loylo previously discussed) will certainly feel like somebody is stabbing your eardrum with an ice pick.