I did explore the JP2C with preamp tube swaps, typical New Sensor offerings From Russia (Reflektor), you know the group, Mullard, Svetlana, Tung Sol, EH, and such. Was curious if they would work out, hell no, wound up with a motor boating sound that kept getting louder over time. Sure even tried the JAN/Phillips 12AT7, nope did not do much but drop the gain, not bad in the phase inverter but again, reduced the volume. As I suspected, the amp was tightly tuned to the Mesa 12AX7 (JJ ECC83s) tube. I had some new unused ones that tested out good (I use an Orante VT1000 valve tester, it helps to find matched triodes). I could not tune the JP2C with change in preamp tubes. There was some effect but usually made it sound worse. Having a good set of Mesa 12AX7 tubes was key. Matched triodes for the phase inverter and V2. That keeps it all kosher.
The most important tubes seemed to be the power tubes. Mine came stock with the STR440 (gray bias color). Those sounded epic. When I had an issue, it was random static like pops, I had thought it may be a preamp tube. Nope. It was the STR440 tubes. Swapped them out with the STR443 as that was what Mesa moved too, did not like them one bit. They were ok but it took the life out of the tone density. I stopped playing through the JP2C in hopes of getting some Mesa STR415 (Sylvania 6L6GC). Then Sweetwater had a new 6L6 tube I never saw before. STR448 made by PSVane for TAD called the Red Base. Well yeah, those were really good. The first set I ordered without requesting a bias color came in grays. I was overly impressed with them in the JP2C, hence the reason for this post. The extended research and efforts panned out with Mesa as they still have inventory of the STR415 but ordering them is not easy as they are not listed on their website or have an actual part number associated with them. That requires going through customer service by email. I had to wait a few weeks as they were not going to be testing and boxing the STR415 at that time. It can be purchased through setting up a work order through email exchange with Mesa, not the customer service that takes all the calls in Nashville (Gibson) as they are not aware of the process or if they exist. The STR415 arrived one week after I got the STR448. Obviously I was going to try them out. For a total cost of $500 you bet I will. NOS STR415, never used except for the Mesa robo test. To be honest, it took a few heat cycles to open those tubes up. Heat cycle meaning actual use and not just letting the cathodes cook without any current flow (in standby). I did give the tubes some time to heat up for the first run. Now that was worthy of note. So far the STR415 are the best I have yet to experience with the JP2C, worth every penny. The STR448 (in gray bias color) were a close second. Hard to decide which I like better. I also tried the STR445 tubes (green bias color) borrowed out of the Mark VII. Actually swapped them with the STR415 to get an idea how they sound in the Mark VII. The STR445 were almost at the same tone density and note definition as the STR415. The STR448 adds a bit more air to the mix, unique characteristic and very musical in all respects. For now, I will use the STR415 and enjoy them while I can. As for change in preamp tubes in the JP2C, it did not get me to a better place. Only power tubes seemed to make a huge difference.
