I ran the comparison sans GEQ when I first got the JP-2C. Probably reason why I was not so thrilled with it at first since I did not use the GEQ. It does have a bit more midrange without the GEQ in use. However for the clean channel, I do not use the GEQ as that sounds really good without it. As for the Mark V sans GEQ comparison this was before I considered modifying the amp, it actually predates the 12AT7 saturation involvement. I do not like the Mark V without the GEQ but in comparison the Mark V was a bit brighter and did not have the saturated midrange, I blame that on C39 on V4B circuit.
The main inspiration for making changes to the Mark V was in short the thrill of the JP-2C in its entirety. I loved every aspect of the JP that I had longed for most of my guitar playing decades. That is the tone I wanted but could not get it. Mark III was probably the closest but I had that amp sold a few months before getting the Mark V. I think why I decided to keep the Mark V over the Mark IV was the separation of the three channels. RHY2 was just a let down in the combo format, it did sound much, much better through the 412 cab though.
Onto the mods, I wanted to see if I could replicate the JP-2C character with the Mark V. They both share the same circuit topography in the cascaded gain stages. So why the ice pick in the Mark V and not in the JP-2C. For one, it is the power supply and transformers are not the same. Secondly, there may be some differences in the tone stack and feedback loop in the presence control and last but not least, the design of the GEQ itself. Mark V places the GEQ before the FX loop and is responsible for attenuation of the signal for the send. The V uses NPN transistors both in front stage driver circuit of the GEQ as well as the differential pair to cut or boost frequencies and then drives a PNP transistor to create the output level. Also note that the differential pair is more like a current mirror than it is a differential pair. The topography of the JP-2C dates back to the original IIC+ GEQ and that circuit carried on through the models and was last used in the Mark IV. It is also referenced from a negative voltage vs the +24VDC. I would say they are both similar in concept but different. The send level of the IIC+ all the way up to the Mark IV as well as the JP-2C is created from a tube output. Not a cathode follower circuit like you would find in the Rectifier amps or even the RA and TC series. Sorry for the tangent. The mods in question would be the addition of a resistor jumper to couple the two switched outputs from the GEQ (one for all voices except the IIC+ and then the one for the IIC+ only). That simple mod just made a slight difference and seems to have improved CH1 and CH2 tone as a bonus. It is not a major change but I can tell the difference. Of course the other mod of coupling the two grounds on V6A for the bypass capacitors, again this adds more cathode bypass capacitance to the IIC+ voice and seems to tighten up the other CH3 voices. I originally used a 150 ohm resistor jumper which is one tenth the value of the cathode resistor. I did remove it just to hear a difference. When I went to install it again, I could not find another 150 ohm resistor but used a 15 ohm resistor by mistake. I need to correct it as the 150 ohm resistor did not grossly effect the extreme voice. Still it is an improvement over the stock form. I am just happy the Mark V lost its ice pick. I have the 12AT7 to blame for that as it helped a great deal, the jumper mods aided to tighten up the flubby bass so it is more on par with the JP-2C and even the TC series. The most notable difference with the mods is the response of the amp with a V30 loaded cab. Oh yeah, it almost nails the JP-2C characteristics of CH2 but there are some differences (you are not limited to using the IIC+ to get that sound either as the MKIV and Extreme jump onto that JP-2C beast of a tone too). I would have preferred not to make any mods but since I have done so, I am more pleased with the Mark V. I could not stand the V all that much before except for CH1 and CH2. Before the JP came along, I was chasing the tone bubble with different speakers and other preamp tubes and could not get a character on CH3 that was satisfying for more than 20 minutes. To me, the Mark V sounds more complete after the mods. Such is life. Everyone has a different opinion on tone. I am personally not thrilled with ice pick. Before the mod, I had to set all the controls below 10am, treble at 9am was all I could tolerate and the gain would not result in instant uncontrolled feedback. Having to reduce the settings so low resulted in less than satisfactory sound. Also considering I had to adjust the bias for the 6L6 tubes as the original bias resistor was causing immediate tube failure. Was not interested in making it adjustable, I just wanted to be able to run stock tubes without red plating witin 20 minutes of use, that was getting expensive. Could have sent the amp to Mesa when it was under warranty but did not. I had too much on my mind at the time I was having the issues, copping with life in general was one huge issue in having to deal with a loss that I could not comprehend and it was the Mark V that helped me get through that. It is what it is. As for the JP-2C, it gave me what the Mark V was not. Sort of a blend of the Roadster, Royal Atlantic and the Mark V which is why I am so esthetic over the amp. I can make it bright, extremely bright but yet no ice pick or breaking glass in my eardrums. Awesome. It does have a sinister dark overtone to it and gets more dominant at lower volume settings. Shred switch cures that just as much as the grid slammer. Also, the JP-2C takes on pedals quite well, or at least the Grid slammer and or Flux drive. I could not be happier.
