Regarding the III to IIC+ mod, after buying a IIC+ to compare and double check my schematics, I've just finished off the III+ mod for lions on a Green Stripe. A few things I'd like to revisit after getting a IIC+ finally and comparing the two and tracing that properly as there are some mistakes in the IIC+ schematic I drew up before which I have now corrected (links to google drive are the same):
1. The 47K and 0.047uF on the V2A anode for the III can be swapped around (R113/C505). This is how it's done in the RP10A IIC+ boards. I think this changed for the RP11A board though. It shouldn't make much difference in sound though.
2. Adding in the extra voltage rail with an extra 30uF filter cap will work but there is a 4V drop in the IIC+ between D and D+. For all intense and purposes replacing R140 with a 5.6K resistor and making the power rail for the Preamp/Lead/Reverb 12AX7s as D will do exactly the same job. It's better that way and more stable than running a load of wires and putting in an extra cap. You're talking about 4V plate voltage difference by doing all that effort. NOT worth it IMO. The extra cap a hangover from the IIC where there was a significant voltage drop between each of the rails. Then they stuck a 1K resistor in instead of a 6.8K for the IIC+ and it does basically nothing. In the later IIC+s the voltage was increased even more by replacing the 5.6K resistors with 1K, meaning there was virtually no voltage drop which is the reason they ended up using 3x30uF caps in the III.
3. R205 should be put between R239 and R225/R122/C516 and where R205 was before make this go to ground with bus wire. This makes a BIG difference to the sound. Be careful in doing this as you'll need to lift components off the ground and form a triangle pyramid of components, similar to when you add the extra 100K and 10uF/63V in for the EQ section.
4. R238 resistor that goes to the presence pot is 1.5K on the IIC+. NOT 3.3K. I have never seen this as 3.3K in any pictures I can see of a IIC+.
5. Do not try to wire the III+ pots or grounding exactly like the IIC+. Leave everything as is. If you try to change it you'll get weird grounding and hum issues. I have this now on my III+ which I went overboard on and now need to change back. It'll all look a lot neater too. It's wired that way for a reason because of the PCB style. If you want exact wiring of a IIC+ you need to build a clone board of everything and basically build a DIY IIC+ clone in a III chassis. That's way overboard for a 'mod'.
6. The extra treble shift LDR in the Red Stripes onwards and the extra diode/resistor can be removed without any impact (well perhaps - see No. 7)
7. I can't seem to engage R2 when the Lead is engaged on the Green Stripe. I can on the Black Stripe. I'll look into why this is done (guessing Mesa changed the LDR layout slightly to prevent this) but without changing will prevent the ++ being switchable on the Lead channel which kind of defeats the point. I will check the LDR rails and fix this though. Is this how the Blues and Greens are supposed to behave regarding R2? I can't tell since I've already done the mod...
Side by side the III+ Green Stripe and IIC+ sound very similar. Each has a slightly different characteristic. However, they sound and feel the same and I can play each for hours. There are also some component difference values which would count towards this but no more than how two C+s will sound different side to side. The EQ sliders have a wider range on my IIC+ (measured with a multimeter) so if you set the controls the same they sound different, but you can make them sound the same by trial and error. Same with the pots due to manufacturing at the time.
I'd still pick my IIC+ over the III but just because of the more high quality hand built feel and separate PCB boards/twisted wires everywhere rather than the all in one PCB. Overall there is less hum in the IIC+ because of this and it's quieter overall.
I'll give you an update once resolved the R2 LDR for lead mode issue.
Cheers,
Jon
