I hope you are keeping your Mark III amp. Even if you need to do a capacitor replacement job sometime down the road, it is the closest thing to a IIC+ you will get.
My Mark V90 was originally a head. That was the plan but had some issues with it. After I bought the Roadster and Royal Atlantic I was running those amps more and set aside the Mark V90 for future fixing. We are not talking about just ice pick tones, this is ear drilling broken glass sound. That was with the treble and gain at 9am on CH3. Mark IV and Extreme were the worse. IIC+ was ok but weak on the gain and tone density. Edge and tweed were also ice pick modes I could not stand, The clean and fat had this low frequency hum that was louder than the guitar signal. All I could use was Crunch.
Just when the saturation mod was posted by APEMAN. is about the time I was looking at the bias voltage and found it only matched the print when in Variac power. That is what resulted in power tube failures. 90W was much brigther in tone and Variac was less bright. From that point onward, running at 90W was the only way to ensure was not going to rep plate the power tubes. That did not last very long, Wanted to try the Mesa STR440 tubes again, poof, did not last more than 30 seconds. Bias was way off. At least the SED=C= 6L6GC held up. I eventually did change the bias to increase the negative voltage a small amount. In reality, the bias needs to be measured at the tube and not at the place where it is created. I may go back and address that again but for now the amp is working well.
It was when I tried to slave the V90 into the Roadster is when it became apparent where the ice pick was coming from, the preamp. I had made assumptions it was due to bias or something power tube related. Not. The Mark III would be much easier to work on than any of the more modern amps. Here is a gut shot of Mark V90 vs Mark VII. Neither of them look accommodating for repair.
The Mark V90 chassis:
View attachment 2280
The Mark VII chassis:
View attachment 2281
Considering the simplicity of the JP2C, that too is a tight package.
JP2C chassis:
View attachment 2282
Not an easy fixer-upper.
Trick with the Mark V90 is more or less preamp tube selection. It can be tuned more so than the other amps. Also I may end up with a tube roll video exploring the potential of preamp tube selection, what does the Saturation mod do, And what happens when C39 is removed. I have restored that cap but want to hear it without it in the circuit again. V1 and V3 are the key to some of the ice pick. That was proven in the video comparison between V90 and VII. I actually like edge mode and tweed but that was after going to some old tubes Mesa used to use in the 1990s. Beijing 6N4-J square foil getter tubes. Tightens up the low end so not as flubby, also fixes the ice pick. V1 being a lower gain 12AX7 (Svetlana which is basically a slight change to the Mullard CV4004 or Tung Sol 12AX7 which are made by the same company brokered by New Sensor, Aka Sovtek or Reflektor. ) then to shore up the final stage, a Mullard reissue long plate 12AX7 which is a better version of the Sovtek LPS. So with that on board, I could easily regain some of the top airy end the Mark VII has by removal of the C39 cap, 120pF (connects the grid to cathode on V4B). The Mark IVB also had a boxy sound to it on the lead channel. That amp has a 250pf cab on the grid to cathode of the same triode in the design but different tube location. I would not go into cutting out parts. I will see for myself first. As for the Mark VII, have no clue what is used in that one. No schematic available to review. JP2C, no way is that there. If it is a recreation of the IIC+, they did not use that trick on the last gain stage of the lead drive circuit. However, it is unclear what other mods Mesa did to the IIC+ or IIC++ some had different parts if Simul-Class or Class A/B.
