chipaudette
Well-known member
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
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