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PostPosted: Sun Oct 25, 2009 8:12 pm 
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I recently got my Mark IIC upgraded to a IIC+ by MikeB at Mesa. After playing it continuously for about a week, I've now taken the time to open her back up to take a look at what he did. I'm not quite finished (I've still got the power amp to look at) but I've found some differences between my upgrade IIC(+) and the best schematic that I can find for a real IIC+ (here)...

Bright Cap: My bright cap on my IIC was 250p. On my IIC(+) it's been changed to 180p. The IIC+ schematic shows it as 100p. I guess that means that the corner frequency when my "bright" is pulled will extend quite a bit lower. I wonder why he did this? I find that when my Vol1 is at 6 or lower (for clean playing), the bright is too bright. Maybe I'll lower the cap to 100p and find that I'm in a zone that works for me better. For any of you circuit hounds out there, what is your bright cap value?

100k Between Treble and Vol1: My IIC has a 100k resistor between the Treble pot and the Vol1 Pot. MikeB left it there. The regular IIC+ schematic doesn't show it being there. Anyone have a sense of how (or if) this affects the sound?

1K Cathode on V2B: V2B is the make-up stage after the Reverb and Effects Loop and before it goes off to the Master Volume, GEQ, and Power Amp. On my IIC(+), V2A and V2B are physically swapped, but the schematic still flows correctly, so the V2A<->V2B swap isn't relevant. What might be relevant is that the cathode resistor on this make-up stage is only 1K instead of the traditional 1.5K. My IIC had a 2.2K in there, so MikeB purposefully changed it to 1K and not 1.5K. I wonder why he did this. It should change the bias point...and I think it will push the bias away from center and more towards one of the extremes (shut-off or saturation). I wonder why he did this? Any thoughts as to how it will affect the sound?

So those are the three big differences that I've noticed. There are a few other small differences...0.1u coupling cap after V1B instead of 0.047u, 91K to ground after the coupling cap on V1B instead of 100K, 15K above the Master Volume instead of either nothing or 22K, 22u cathode bypass caps instead of 15u....but these shouldn't matter too much.

The big three that are listed above are very intriguing to me. Any thoughts that you might have (or any first hand knowledge about their presence or absence from other amps) would be greatly appreciated.

Chip


Last edited by chipaudette on Sun Oct 25, 2009 9:27 pm, edited 2 times in total.

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PostPosted: Sun Oct 25, 2009 9:24 pm 
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Well, on the issue of the 1K vs 1.5K Cathode resistor, I just looked at another Mark IIC+ schematic (this one) and saw that it shows a 1K cathode resistor and not a 1.5K.

So, perhaps my new 1K resistor on my IIC(+) is consistent with a standard IIC+. The value still seems kinda low to me, though. I wonder why they did it that way?

Well, I've still got the two other unusual finds listed in the original post. Any thoughts?!?

Thanks,

Chip


Last edited by chipaudette on Sun Oct 25, 2009 9:27 pm, edited 1 time in total.

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PostPosted: Sun Oct 25, 2009 9:27 pm 
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BB was telling me once that if you put a 100k between the treble and the gain pot's ground. it will change the load on the Eq. this along with a 10uf cap off the cathode of V2B will turn your C+ into quote " A fire breathing Dragon!" I tired it and i could hear more gain but i didn't feel i needed it.

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Mesa Mk IIc+ 60w, Rev, Export, combo EVM-12
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mesa MK III blue stripe (sold)
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PostPosted: Sun Oct 25, 2009 9:34 pm 
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The 100K between the Treble and the Vol1 ground terminal? That makes no sense to me at all. I'm not saying it won't work...it's just that I'll have to think about that...and I'm sceptical.

Or, did you mean between the Treble and the first terminal (ie, the high side) of Vol1?

Mine is between the Treble and the first terminal of Vol1. I'm having a hard time seeing how it would appreciably change the loading on the tone stack. I guess I'll have to go off to PSpice and simulate it.

Thanks for the thoughts. Did you ever try the 100K mod without the cathode bypass cap mod?

Chip


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PostPosted: Sun Oct 25, 2009 9:48 pm 
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So I just did a PSpice simulation of the first tube stage, the tone stack, Vol1, and the following tube stage.

I simulated it with and without the 100K resistor (from wiper of the Treble pot to the top of Vol1). I simulated a range of different T-M-B settings permuted with a range of Vol1 settings. I did the simulation with and without the Bright switched in.

In all cases, introducing the 100K resistor actually drops the gain by about 1 dB. Based on the simulation it does not increase gain.

Of course, I am the first to admit that simulations do not tell the whole story...only tests of real hardware can be convincing. Real people's experience will trump my number crunching. I believe that boogiebabies has also suggested introducing the 100K. I forget if it was for gain boost (or cut) or for tone shaping. I'm thinking that I'm going to have to try snipping mine out of my circuit and getting my own first hand experience.

Any other thoughts from anyone? Anyone else have the 100K resistor? How about that 180p bright cap? Anyone looked at theirs?

Thanks all.

Chip


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PostPosted: Mon Oct 26, 2009 10:38 am 
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sorry didn't mean ground. you replace the buss wire going from the right taglet of the volume knob that goes to the treb pot with a 100k resistor. Ed wrote it down somewhere.ou should be able to find it.

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Mesa MK IIc+ Simlu,Rev,Eq Cream with matching 1x12 EVM Cab
Mesa Mk IIc+ 60w, Rev, Export, combo EVM-12
Mesa EVM-12L 1x12 Theile Cab
Mesa trad 4x12 cab
mesa MK III blue stripe (sold)
Mesa dual recto 3 channel ( sold )


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PostPosted: Mon Oct 26, 2009 11:14 am 
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Hi,

Yeah, replacing that buss bar puts the resistor between the wiper of the Treble pot and the top of the Vol1 pot. That's the case that I simulated in Pspice. As I said, that analysis actually shows a slight drop in gain.

Tonight, I'm going to jumper around my 100K resistor to see if I hear any difference. If it really is only a 1dB change, I doubt I'll hear it. That's only a smidgen of a nudge of a skosh of a turn on the Vol1 pot to compensate.

But, maybe the simulation is wrong. That would be cool. We'll see...

Chip


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PostPosted: Mon Oct 26, 2009 12:09 pm 
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The 100K is a left over from the IIC. It can be left in or replaced by buss wire. It changes the load from the treble circuit to the bass and mid caps.
What you hear is close to the normal circuit with a very slight reduction in the treble. For all reasonable purposes, it should decrease the gain to the tone stack
a small amount, but in adding a cathode bypass to V2A it causes about 8+db of boost instead of 6db. It's very strange behavior, but documented. It's easy
to try it both ways, but I would not expect to hear a drastic difference. The bright cap has been 180pf on every C+ I have seen. It does add a good bit of
brightness below 8 and is more perceptible in the clean channel. Most people claim it does not effect the tone above 8 on Vol. 1 in Lead mode, but you will easily hear
a bit more slice when it's on, so I always have it on. I'm sure the 91K to ground on the V1B is close enough, but the value should be 100K. The Master Volume is probably not
a Taiwanese CTS pot, so it may not have needed the 22K series resistor to the Master/Preamp out. Looking at the RP10, I just don't know where it would be put. On the early C+ this signal was taken off the output of the V2B .047 cap. Later, Mesa placed both wires in an unused pair of pads just above the V2B .047 cap and connected the Master Volume/Preamp out to the cap via the 22K series resistor to assist with the wild tapers of the new Taiwanese pots. The coupling cap off V1B is 0.1u in the actual circuit.
So much for an accurate schematic from Mesa.....


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PostPosted: Mon Oct 26, 2009 9:28 pm 
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Wow. Perhaps I'm misunderstanding...are you saying that the 100K resistor between the Treb and Vol1 pots interacts somehow with the cathode bypass cap introduced onto V2A to produce more gain?!?

Wow, I would not have expected that.

Sadly, I was not able to open up the amp tonight to try it out. Maybe I'll get a chance tomorrow.

Fun!

Chip


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PostPosted: Tue Oct 27, 2009 6:47 am 
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Hey chip could you mail me the pspice file? i would like to have a look at it in the program. would that be ok?

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Mesa MK IIc+ Simlu,Rev,Eq Cream with matching 1x12 EVM Cab
Mesa Mk IIc+ 60w, Rev, Export, combo EVM-12
Mesa EVM-12L 1x12 Theile Cab
Mesa trad 4x12 cab
mesa MK III blue stripe (sold)
Mesa dual recto 3 channel ( sold )


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PostPosted: Tue Oct 27, 2009 7:59 am 
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Hi Shep,

I don't know which spice you use, but I use 5spice, which is a fairly easy-to-use windows front end for spice: http://www.5spice.com/. The hardest part is getting a circuit model for the tubes.

Given that it's a graphical interface, I don't have an analysis file per-se. I have a schematic file (which I can send), and an analysis config file (which isn't too helpful). Then, I just manually make graph-after-graph-after-graph of frequency response while tweaking circuit parameters.

It's not the most efficient process. A traditional spice implementation where a routine can be setup to roll through all the circuit permutations would be faster...but I like the graphical interface.

Are you still interested?

Chip


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PostPosted: Tue Oct 27, 2009 8:34 pm 
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Thanks chip! i was using a different program anyway. yes i am still intrested. i just got the program.

Thanks alot!!

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Mesa MK IIc+ Simlu,Rev,Eq Cream with matching 1x12 EVM Cab
Mesa Mk IIc+ 60w, Rev, Export, combo EVM-12
Mesa EVM-12L 1x12 Theile Cab
Mesa trad 4x12 cab
mesa MK III blue stripe (sold)
Mesa dual recto 3 channel ( sold )


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