Since I have both schematics, I can tell you they are both very close, and a few changes can have the B version modded back to A and visa versa, AS FAR AS the overdrive circuit and reverb circuit goes, in the B version both the Lead and Reverb share a 1/2 of V3 and V4 12ax7. 1/2 of V3 is Lead 1st stage and the other 1/2 is reverb 1st stage, V4, 1/2 again for the second stages of each the lead and reverb recovery stages.
The A version, the lead and reverb had dedicated 12ax7 for those stages, 1 12ax7 for each the lead and 1 for the reverb.
There are small caps that smooth out the britness/ harshness on the lead 12ax7 circuit on the B version where the A is very much like the IIC+. They both have the same layout as far as the circuit, but added caps to smooth things out and they also added realays and LDR's and went with a new board design, though nothing really major. Chris at Mesa had told me the A version had some issues in the loop, so in the B version, they made some changes to improve performance.
I have experimented on the B version's lead circuit, getting it back to the A version and it becomes briter or more open in the highs, and more raw sounding very, very close to a IIC+. Also changing the output tranny to a 562003 IIC+ tranny makes a world of difference! This B sings and is sweet and smooth very much like JP's.
If you notice on his IIC+ amps, there is another knob where the footswitch jack under the input jack is. That mod is a separate Volume 1 for the lead circuit, so there is more control from a clean mode setting and a lead mode setting can set for higher gain with this mod, almost like a Mark IV without totally separate tone controls.
The IV B can be set up to sound very much like the IV A lead circuit but with some small cap changes, and not that the A needs anything if its working fine, but there where some improvements on the B.
I have even gone mad and added a bias pot, against Randall Smith's blessings , but to the amps betterment! I can now put in what ever tubes I want as long as they are good quality, matched and set the bias properly, hot or cold... I
f you have a B circuit, the caps can be changed to the A version if you really feel it sounds better, but its minimal to my ears, I feel you can get pretty close with slightly different knob settings. The IIC+ probably had slightly more gain and highs due to just a couple caps and a resistor if I remember correctly, plus the 562003 output tranny had better tone and had more iron. Plus tubes were a different quality as well.
The electrolytic caps on the cathodes for the tone controls (stack), are smaller value on the IV (both A&B), where the IIC+ had a slightly larger value giving it more gain and raw-ness to the tone. For example, when you switch the MID Gain on, you get more gain, right? and that is like 2.2uf cap being added when the Mid Gain is engaged. Harmonics is bypassed. I think Mesa was trying to catch some of the raw flavor that the IIC+ had, but I personally think they should have put it where the IIC+ had it, in front in the tone control stack cathode bypass (Bass shift on the earlier Marks). I think the reason they put the Mid Gain after the tone controls and lead circuit, was to lower some of the flubby bass effect by having the smaller caps in the tone control stack, where you have to turn the bass down to keep the buzzy flubbiness low.
The Bass Shift adds a 15uf cap to the 1st stage 12ax7 increasing bass gain and you will hear more overall gain in the earlier Marks with the Bass Shift pulled out, where the IV does not have the Bass Shift (no 15uf just a .47uf less gain in the bass). So they tried to give you that extra gain by offereing it after the tone and lead circuit to keep things more balanced, but I rarely use the Mid Gain myself.
Also there is no brite switch on Volume 1 in the Lead channel. If you never used a Volume 1 brite switch in an earlier Mark, you probably will never miss it in the IV's, but looking at the old Mark settings list, there were many settings with the Volume 1 brite switch engaged. Not to be confused with the Lead brite switch. I hope the spaces are a little easier on your eyes
Hammer Sound
