2C Mode Coupling Cap Mod.

The manual states that 70% of 2C's had the Graphic, yet they still put in the smaller Coupling Cap from the non graphic to represent .. FAIL...

Every 2C+ ***** complains about the feel and tone of the original, all because of a simple Cap.. Who has a schematic?

There´s a link in here:
http://music-electronics-forum.com/t15431/

If you flip it to Mark IV mode, you activate that larger coupling cap from the GEQ Models instead of the smaller one. And you get PULL DEEP engaged. Mark IV mode is still a Mark IIC+ mode because the lead circuits were the same.

If that cap is your issue, then switch to Mark IV and enjoy!

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APEMAN said:

Hey there...
there are more differences between IV (=mode8) and IIC+ (=mode7) then the coupling cap. Look at the schematic...
Just my 2ct...

Click to expand...

You're right. Aside from the Coupling Cap and the Pull Deep, there is a 3K3 resistor removed from the chain around the old location of the FX loop which I'm not sure the effect of.

I still argue if you think the Coupling Cap is why you dislike this amp, try Mark IV Mode and pretend it still says Mark IIC+. If you still don't like the tone and feel, then the problems might be deeper than that cap unfortunately.

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The C39 mod does not cut the high frequencies.... on V4B stage. It actually enhances it as well as increases the midrange frequencies that seemed to be choked off. Since I have removed it I have noticed the amp is more forward in attack. At the moment I am using all 12AX7 preamp tubes and find it more desirable than the 12AT7 in V4 or V6. The two cascaded gain stages have a specific function, V5A is a lower gain stage but the load line is such that it is responsible for generating the harmonics (intermodulation distortion) Coupled to V4B which has the highest gain circuit in the Mark V is responsible for the soft clip due to overdriving half of the harmonically rich signal. This in turn couples the output though a low pass filter network but is also fed back to the V5A circuit though a 3.3 Meg resistor with a bright cap. This feed back is actually the positive feedback to V5 since it is in phase with the input signal. This can become regenerative and thus the reason behind the grid to cathode capacitor. It is probably more effective on V5A circuit than on the V4B circuit since the Miller effect capacitance would be lower on V5A vs V4B circuit. Miller effect is the parasitic capacitance between grid to anode being multiplied by the gain which is determined by the anode resistor. A small signal analysis reveals this miller effect capacitance and the additional grid to cathode capacitor add up creating a low pass filter that will also reduce the midrange frequencies and at the same time allow for the upper harmonics to be created as if it was behaving like a notch filter. (no empirical proof here but it is how it sounds to me, ice pick along with a lack of midrange). Removal of C39 seemed to restore the midrange and top end and at the same time has reduced the ice pick (this I cannot explain) as C39 is more of an oscillation control at very high frequencies typically in the MHz range (typically when the Miller effect seems to have the most impact). In addition to the low pass filter effect, C39 is also creating a stored charge and has an RC time constant between the cathode resistor and itself, this creates a pole in the frequency response and may as well be a crude form of local feedback path. Small signal simulation did reveal a slight shift in the response curve with the cap and without. I did not think it would be noticeable in the real world but for some reason it is audibly noticeable. I would actually have to scope the input and output of both stages and run a single frequency (perhaps do a sweep on frequency) to see what is actually happening to the waveform through the circuit. This I could probably do but it will reveal little of what is the real issue. Frequency Analysis would be more revealing but that type of gear is out of my reach as my company will not spend that kind of money. There is so much going on between the V5A and V4B circuit along with V3a to V6A not to mention the changes over the years to the CH3 tone stack (most responsible for the ice pick than the rest of the gain stages). I have been able to manage the ice pick with preamp tube selection which are all 12AX7 tubes but of different types.

As for the IIC+ EQ coupling cap, I doubt changing this component to a different value will result in more of a IIC+ tone as there are too many gain stages used in the Mark V that are altering the tone and character of the amp than the IIC+ or JP-2C have for the lead channels. Also a change in cap value will require a change in the shunt resistor otherwise you may wind up overdriving the FX loop.

If you do want to attempt this mod, an easier approach might be to disable the switching on the cap such that it always stays on the larger cap (rather than switching to the smaller one in IIC+ mode. It should be easy to achieve, but with the lack of switching network schematics around you might need to do some testing yourself to figure out where to make a change in the switching network.

Perhaps it is easier said than done. The IIC+ mode is far from the real deal anyway and change in coupling capacitor may not prove worth while. Assuming you can actually find the components, you can always add another cap in parallel to the 0.22uF (C99) and the same applies for the load resistor (100k, R209). Also there may be some parts on the PCB that are not on the schematic as some of the early models had them added after the fact. I am not sure where they are located, on the main board or the EQ board (much more work than needed to get to this one).

Heck, the Extreme mode sounds much closer to the JP-2C than the other two voices in my opinion. Not sure how close the JP-2C is to the IIC+ for which it was based on.

I have also read that some are having issues with the Extreme mode being muddy in character. What year where those made? Mine is a 2012 model and sounds quite robust and no mudd on extreme. I am currently running 12AX7 in all positions as I found the 12AT7 seemed to be a bit dark for the speaker I have in the combo. Still I never found the Extreme mode to be muddy by any means but it could be my amp too.

What is the capacitor value supposed to be ? 2.2uF? 15uF? You could always short the two caps together that will lower the shunt resistance down to 9.9k and result in 15.22uF capacitor. Not sure what the end effect would be.... Perhaps tack on a 1k resistor between the two caps vs a short.

I do not think that a 0.047uF will improve the IIC+ mode (this is what I found on schematics available, also the Mark III and Mark IV have the same capacitor value but the huge difference between the earlier models and the current Mark V is the output level of the FX loop which is derived from the EQ. What may work well for an instrument level may not be all that great for the line level circuit of the Mark V. A smaller capacitor will reduce the lower frequencies effect on the output. Perhaps the statement was such that they originally had a different capacitor that effectively was similar based on the current circuit when compared to designs of yesterday but was not actually the same value unless it was stated as such. I like the Mark V as it is, even more so with the 12AX7 in all positions. As it seems one small change in component value may require other parts to be changed to have the same effect. I do not find the response of the IIC+ mode of the Mark V to be an issue, I like it as is.

bandit2013 said:

I do not find the response of the IIC+ mode of the Mark V to be an issue, I like it as is.

Click to expand...

+1

Thanks for everyone's efforts . If the V 2C is not the same as the original, then it's a let down that Boogie claims it's reissue worthy and your original is dirt; which is apparently not the case.

Boogie never claimed that they're dirt.

Have you tried it with the 12AT7 in it..? Hell of a lot closer.
May be your answer.
If you want F1 old mark type performance, thats the tube.
Otherwise its like having regular fuel in a race car and wondering why a the prev champ has just lapped you, then going to comment at the car manufacturer..
With that beast in V4, it sounds and plays much more like my red stripe III. Which is said to be real close to IIC+.
Allows you to dial it much more without usual flub limits or harsh high frequencies even with extra massive organic, rich harmonic distortion that it's dialling parameters then allow.. I'd at least reserve judgement until you've tried that.

That analogy is lame.. 12AT7's have no gain. They make a circuit biased for 12AX7's sound dull and with less volume.. If Boogie wanted that tube they would have specified it..

The best bang for your buck is to install a JAN/Phillips 12AT7 in V4. As V4 has the highest gain and is a follower stage for the main distortion driver tube circuit. This cuts out Ice pick tone on CH3 only. Distortion characteristic is actually much better, more controllable, precise and actually gets the Mark V lead channel much closer to that of the JP-2C or IIC+. Both of those amps do not rip your ear drums out as they are not generating shot noise with a moderate gain setting at gig level. Also improves feel and characteristic and has plenty of harmonics to mess with for pinch off or tapped. The only cost is one tube and does not require any circuit modification.

The analogy may be lame, and at best less understood if you have not tried it or understand the basics of a tube amp and triode circuit. And yes the 12AT7 has less gain than a 12AX7. My Mark V does not sound dull at all and is still very bright but I have more control over feed back than I had prior with the 12AX7. I did take it out and install a 12AX7 but felt the 12AT7 sounded better. There is nothing wrong with using a lower gain tube in a 12AX7 position as long as the tube characteristics are not too much of a jump. We are not talking 12AY7, 12AV7 or 12AU7 here. This mod may not be for everyone and was offered as a suggestion to tailor your Mark V tone and it works too.

One last thing about the Mark V and IIC+. I had been looking at all of the available schematics online just for sake of curiosity. One thing was clear between the IIC+, III, IV and V. All four have the same exact components on the drive circuit, however they may use different tube positions. Also it appears the same number of triode circuits are present for the lead channel (CH3 on the Mark V). The only difference being with the Mark V. That difference is what separates the three amps from the V. The Mark V is the only Mark series amp that has the 5BEQ before the FX loop send/return. Not to mention that the earlier models do not have any NPN transistors and are referenced from -30V vs the Mark V eq which is referenced from +24V and uses a combination of three NPN and one PNP transistor. The driver for the 5BEQ (differential amplifier circuit) is quite different than the earlier models. Quite difficult to equate one to another if the topographies are not identical. I believe attempting to get a IIC+ voice of the Mark V would be difficult as it also lacks the same power supply. Last time I looked the Power transformer was small relative to the JP-2C (assuming it was reverse engineered from JP's IIC+ amp). You can get the tone very close but the feel and potential of the IIC+ will not be present in the Mark V (again assuming the JP-2C is a good representation of the real deal). I personally never played though a IIC+ so I have nothing to base this on. I have owned a Mark III but that too had a smaller transformer than the JP-2C but I believe it was bigger than the Mark V. That Mark III was one heavy amp to lug around despite its smaller size (combo amp). I am no IIC+ fanatic so what may have been used in its construction to bring to discussion would better be served to the IIC+ gurus. In short the Mark V pales in comparison to the JP-2C so I would assume the same would be true with the IIC+ models.

bandit2013 said:

One last thing about the Mark V and IIC+. I had been looking at all of the available schematics online just for sake of curiosity. One thing was clear between the IIC+, III, IV and V. All four have the same exact components on the drive circuit, however they may use different tube positions. Also it appears the same number of triode circuits are present for the lead channel (CH3 on the Mark V). The only difference being with the Mark V. That difference is what separates the three amps from the V. The Mark V is the only Mark series amp that has the 5BEQ before the FX loop send/return. Not to mention that the earlier models do not have any NPN transistors and are referenced from -30V vs the Mark V eq which is referenced from +24V and uses a combination of three NPN and one PNP transistor. The driver for the 5BEQ (differential amplifier circuit) is quite different than the earlier models. Quite difficult to equate one to another if the topographies are not identical. I believe attempting to get a IIC+ voice of the Mark V would be difficult as it also lacks the same power supply. Last time I looked the Power transformer was small relative to the JP-2C (assuming it was reverse engineered from JP's IIC+ amp). You can get the tone very close but the feel and potential of the IIC+ will not be present in the Mark V (again assuming the JP-2C is a good representation of the real deal). I personally never played though a IIC+ so I have nothing to base this on. I have owned a Mark III but that too had a smaller transformer than the JP-2C but I believe it was bigger than the Mark V. That Mark III was one heavy amp to lug around despite its smaller size (combo amp). I am no IIC+ fanatic so what may have been used in its construction to bring to discussion would better be served to the IIC+ gurus. In short the Mark V pales in comparison to the JP-2C so I would assume the same would be true with the IIC+ models.

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Just one nitpick: I think it's more accurate to say the Mark V is the only amp to have the effects loop moved after the GEQ, as the graphic EQ still comes at the same place in the circuit path. Instead of happening right before the GEQ, it was moved after all three channels join back together, before the OUTPUT/SOLO controls (And can therefore be Hard Bypassed to achieve a signal path without the additional circuitry if needed). So this ordering isn't likely to affect the tone with the loop empty, but will affect how some effects behave in the loop, as the'll be affecting the EQ'd signal, instead of EQing the affected signal.

Your other differences in the GEQ/5BEQ are all worth considering though.

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bandit2013 said:

<snip>

In short the Mark V pales in comparison to the JP-2C so I would assume the same would be true with the IIC+ models.

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This is an interesting comment based on the youtube comparisons which are out there. Are there differences? Absolutely. The IIc+ mode on the V does not have the flexibility of a full on IIc+ amp (or JP2C for that matter), but nothing would seem to indicate that the Mark V pales. This is all my opinion, of course. The one Dream Theater cover clip, (https://m.youtube.com/watch?v=0UKpBsgeHAE) with the guy playing Pull Me Under really sealed it for me to not to drop for the JP2C, the difference in tone (for that particular sound/song) just wasn’t enough for me. I ended up getting the TC-50 for something different since a Mark and Rectoverb are in the house already. The versatility of the Mark V is not to be underestimated. It really sounds cliche, but after owning it nearly 6 years I’m still discovering new tones buried in those 9 modes.

In that clip (I just listened again thru crappy iPad speakers up to my ear) and did slightly prefer the JP2C, once again. But, at a gig or with band practice, this difference will not be noticed. I tend to dial out a bit of fullness when playing with the band anyway so as not muddy up with the other guitar and bass. All my opinion of course! YMMV, as always...

I haven’t done the C39 mod and at this point probably will not since the Mark V with the AT7 in V4 for the past several months has been just completely awesome.

Mace

At a gig or band practice the difference is noticeable. And the V pales.

Recorded tones are different. The V and JP2C would be on par with a real c+ in a recording. Throw them in a mix and they get even less distinguishable.

The V is more versatile, you're right, mace. But the JP2C sounds better. And it's noticeable.

imho