Indeed there are not many places in the Mk IIC+/III/IV overdrive circuits where you can increase the gain without running the risk of grossly overdriving one of the following stages (by "grossly" I mean that one stage is so overloaded that it goes into blocking distortion). By looking at the schematics I have the following ideas:
You could alter the 270k/68k voltage divider after V3a (first gain stage of the lead drive circuit) and increase the gain by upping the 68k to 82k or 100k and at the same time reducing the 1000pf cap in parallel to the 68k to 500pf. Alternatively you could just drop the 270k to 220k. Both will increase the gain slightly. On paper I would prefer decreasing the series resistor instead of increasing the shunt resistor as that would also reduce the bass a tiny bit which is preferable for higher gain. Increasing the shunt resistor would also increase the bass which is generally not desirable in high gain. I would not do both mods at the same time as it could be too much. When doing these mods I would also consider reducing the 0.022uf coupling cap off V3a with a 0.01uf to reduce bass.
You could also increase the 270k plate resistor on V3b to 330k for more asymmetrical clipping.
You could adjust the 0.22uf lead bright cap to 0.47uf for a bit more gain. This will of course only be effective when the lead bright switch is engaged. The lead bright switch then becomes a lead boost switch (more overdrive, not more volume). This is the only mod of this list I actually tried in an amp and the one I would recommend as it leaves the signal shaping between the stages intact (assuming you like the basic character of the amp).
Finally you could adjust the voltage divider section coming off of V3b (220k II 250pf series to 100k II 500pf to ground in the IIC+ and Red Stripe, 470k II 120p series 470k II 680pf on the Blue and Green Stripes) by making the shunt resistors slightly larger to drive V2b harder (yes, V2b does contribute overdrive in the lead mode, even though it does not in the clean mode unless you crank the volume).
Frankly I don't see the need to do any of these mods in my Mark III as it has more than enough gain in stock form to play any kind of metal, unless your main axe is equipped with something like lipstick pickups which have about half the output of a low output single coil.
And by comparing the various Mark series schematics it is interesting to see which values were never changed from one model to another. They changed the resistor in parallel to the lead drive pot (330k or 470k, which basically affects the bass content when turned up full only), the cap in parallel to the 270k plate resistor (none, 500pf or 1000pf), the cap in parallel to the 68k to ground (500pf or 1000pf) and the voltage divider combination after V3b but they never changed any of the following:
- the 120pf in parallel to the shunt resistor on the lead drive pot
- the 0.022uf coupling cap after V3a
- the 270k/68k voltage divider and
- the 270k plate resistor of V3b
I guess there is a reason for that (which may be as simple as that they liked these values better than anything else they tried).
Still no idea what the ++ mod is.
Cheers Stephan