• We would like to remind our members that this is a privately owned, run and supported forum. You are here at the invitation and discretion of the owners. As such, rules and standards of conduct will be applied that help keep this forum functioning as the owners desire. These include, but are not limited to, removing content and even access to the forum.

    Please give yourself a refresher on the forum rules you agreed to follow when you signed up.

Understanding the IIC+

Chronos

Veteran
I'm starting to fall in love with this amp sim and can get great results, but would like to get a better knowledge foundation of it since it along with other Mesa amps can often be a real struggle to dial in.

One thing I would like to understand is about the Gain Boost on the real amp. The Wiki page says it is not modeled, but I notice a gain boost when using the fat switch on the sim and this seems to be common practice for Mark series users here as well. Is this a similar effect from the original? Is the Gain Boost on the real amp a simple increase in the input trim, and if so roughly how much would it be?
 
I'm starting to fall in love with this amp sim and can get great results, but would like to get a better knowledge foundation of it since it along with other Mesa amps can often be a real struggle to dial in.

One thing I would like to understand is about the Gain Boost on the real amp. The Wiki page says it is not modeled, but I notice a gain boost when using the fat switch on the sim and this seems to be common practice for Mark series users here as well. Is this a similar effect from the original? Is the Gain Boost on the real amp a simple increase in the input trim, and if so roughly how much would it be?
No it´s not.
It´s a 33k resistor (and a cap in parallell) in series with the mid potentiometer. At least on the MK 2b it is. The mid pot gets lifted from ground. Basically the tone stack allmost stops from working and becomes near flat. It produces a fat an flubby sound, and of course with more gain.

The fat switch might be the pull Fat switch (on the treble knob) that changes the treble cap from 250pF to 1000pF, but I´m not shure..
 

Hofi

Inspired
Don't forget about the place of the tonestack in a boogie. The more bass,mid,treble the more gain.
 

Chronos

Veteran
The fat switch might be the pull Fat switch (on the treble knob) that changes the treble cap from 250pF to 1000pF, but I´m not shure..
Do you mean the pull to shift? Thanks!

So I wonder then just what is the Gain Boost doing to the amp? Anyone read schematics that could figure it out? I'm slightly hesitant to guess it's just a boost to the input trim, but if so it would be real easy to just bump that if need be.
 

stevehollx

Regular
The layout of a real mark series amp had the knobs in order of the amount of effect on the gain (aside from the actual lead drive gain knob): volume, treble, bass, mid. Volume and the the treble knob have a huge impact on the gain structure. You really have to consider that the Volume, treble, bass, mid, presence knobs are all to shape the gain structure, and you use the post-gain graphic EQ to shape EQ in the traditional sense of wanting more low-end thump, less ice-picky highs, etc...

FWIW, I cut my teeth on Mark series amps for 10 years of my playing, and owned a III (red stripe) and IV through most of my playing career until the axefx. I consider those amps very much a part of my playing and sound, but I've never loved the mark series modeling in the Axe and not sure why. It always seems to saturate or feel different than I was used to with the real amps, and struggle to dial in a good tone that I would get with the standard settings of: volume 8, treble 7, bass 3, mid 5, gain 7-8. Wondering if G3'd a IIC+ will pique my interest.

Conversely, I've been LOVING the out-of-the-box tones on the AFX's Splawn and HBE models in 18.x for high gains in both rhythm and lead contexts. Also been liking the Herbert for leads and VH4 stuff for crunch...looking forward to the Lonestar being G3d to compare to the VH4 crunch channels. Haven't found a warm/round dumble-type amp in 18.x yet that has stood out.
 
Last edited:

Chronos

Veteran
since it is changing I would not bother understand what is there but rather preparing for what is to come
Tweaking isn't something I'm afraid of, and isn't a problem. Just trying to learn about some of the real life knobs as they can be confusing. The bright switches for example on the real thing confuse me, as Cliff has mentioned that one pull Bright switch on the left is the same as the bright switch in axe, and there is a separate amp sim model created over the other pull Bright knob. I would've thought it's just the same thing on either the rhythm or lead channel but apparently not if it has it's own model in the axe.

After taking a closer look at the amp Cliff just put through the G3 process I see it doesn't even have the Gain Boost switch :lol so it seems almost trivial for me to keep asking, but I'm just extremely curious to know if it did any tone shaping or just simply more clipping.

Did some IIC+'s come with Deep knobs and others Gain Boosts? Because it's in the same spot as other one's I've seen...unless those weren't really IIC+'s.

Man this amp is just freakin confusing. How did Mesa keep everything straight with them? Guess they didn't? Tried something different with every other model they built?
 

tguitar

Inspired
I have a IIb and these amps have a Gain Boost. Some of the earlier IIcs have a b faceplate so it would show Gain Boost instead of Pull Deep. The Gain Boost on the b however is not just an input trim boost. It gives a gain boost but also, I believe, shifts the mid range frequency to give more bottom end. You also may see some IIcs that may have gone back to add a geq. If Mesa didn't have c plates around, there are stories of them using older b plates instead.
 

Chronos

Veteran
So what I'm kinda gauging from everything is that the + models didn't actually have a "Gain Boost" but it was either Mesa not wanting to throw away left over faceplates from other models and used it on a number of + models even though it was actually the Pull Deep. That or it was a non + model that got upgraded to the + and the faceplate was reflecting what it used to be.

I wonder is there any way to model that frequency shift you mentioned from the Gain Boost in the axe?
 
Do you mean the pull to shift? Thanks!

So I wonder then just what is the Gain Boost doing to the amp? Anyone read schematics that could figure it out? I'm slightly hesitant to guess it's just a boost to the input trim, but if so it would be real easy to just bump that if need be.
Well it´s boost the low mids....
(Or it boost the frequenzys that has been taking away by the tone stack depending on the setting)
 
Tweaking isn't something I'm afraid of, and isn't a problem. Just trying to learn about some of the real life knobs as they can be confusing. The bright switches for example on the real thing confuse me, as Cliff has mentioned that one pull Bright switch on the left is the same as the bright switch in axe, and there is a separate amp sim model created over the other pull Bright knob. I would've thought it's just the same thing on either the rhythm or lead channel but apparently not if it has it's own model in the axe.

After taking a closer look at the amp Cliff just put through the G3 process I see it doesn't even have the Gain Boost switch :lol so it seems almost trivial for me to keep asking, but I'm just extremely curious to know if it did any tone shaping or just simply more clipping.

Did some IIC+'s come with Deep knobs and others Gain Boosts? Because it's in the same spot as other one's I've seen...unless those weren't really IIC+'s.

Man this amp is just freakin confusing. How did Mesa keep everything straight with them? Guess they didn't? Tried something different with every other model they built?
I think that the modeled "bright" amp is with the "pull bright" on the lead master activated.
It connects a cap over the 3.rd or 4:th gain stage´s cathode. It shorts the higher frequenzys to ground giving them a boost. It´s some sort of current feedback high shelf filter.
I can give tighter bass , brighter sound and some more gain on the lead channel.
It only affects the lead channel.
 
Last edited:

FractalAudio

Administrator
Fractal Audio Systems
Moderator
Tweaking isn't something I'm afraid of, and isn't a problem. Just trying to learn about some of the real life knobs as they can be confusing. The bright switches for example on the real thing confuse me, as Cliff has mentioned that one pull Bright switch on the left is the same as the bright switch in axe, and there is a separate amp sim model created over the other pull Bright knob. I would've thought it's just the same thing on either the rhythm or lead channel but apparently not if it has it's own model in the axe.

After taking a closer look at the amp Cliff just put through the G3 process I see it doesn't even have the Gain Boost switch :lol so it seems almost trivial for me to keep asking, but I'm just extremely curious to know if it did any tone shaping or just simply more clipping.

Did some IIC+'s come with Deep knobs and others Gain Boosts? Because it's in the same spot as other one's I've seen...unless those weren't really IIC+'s.

Man this amp is just freakin confusing. How did Mesa keep everything straight with them? Guess they didn't? Tried something different with every other model they built?
The Pull Bright on the Volume knob is the classic "Bright Cap" which engages a capacitor across the potentiometer. This is already modeled in the Axe-Fx via the Bright Switch.

The Pull Bright on the Lead Master knob engages a 0.22uF cap on the cathode of the last triode in the overdrive circuit. As there is no switch for this in the Axe-Fx it requires a separate model. Cathode caps are very common in tube amps. If the value is large the stage has more gain and the response is relatively flat. If the value is small the stage has more gain at higher frequencies. Amp designers use cathode caps to shape the frequency response. Caps in the range of 0.1 to 1.0 uF are commonly used to reduce bass response. A cathode cap works by decreasing the amount of negative feedback through shunting higher frequencies to ground. This reduced negative feedback increases the gain (and reduces linearity). FWIW the Axe-Fx is the only modeler of which I'm aware that actually models triodes using a feedback technique. Other modelers use static waveshapers. The Axe-Fx triode models incorporate feedback so if there is a virtual cathode cap the stage is less linear in addition to having more gain.

The IIC+ does not have a gain boost and doesn't need one. There is plenty of gain. The Pull Deep switch engages a large cathode cap on the final triode stage. With Pull Deep off there is actually a shelving response into the power amp (bass is reduced).
 

mnemonic

Inspired
The IIC+ does not have a gain boost
Ha, I remember seeing a IIC+ with the 'pull gain boost' on the master, and all this time I was wondering why nobody ever talked about it. Turns out it doesn't even have one. Must have been a IIB, or a IIB faceplate on a IIC+.

Learn something new every day!
 
Top Bottom