The Pandora's box experiment:
The integrated quad, or two different pairs of power tubes. the STR445 yellows remained in the extended class A sockets. I was pleased with the change running the full quad and did try out the other power modes, 45W and 25W. All was good. But how to make the 90W open the gates? Looking at the Mesa description of the STR445 tube was the key to understanding what I am hearing. Copied from Mesa's website online store:
The MESA® STR-445 is a 6L6 that shares some qualities of a 6CA7 in that it has tight, articulate low-end for excellent bass definition along with beautifully balanced mids and a chiming top-end that cuts without harshness. It’s a great choice for our Rectifier Series to tighten low-end, add top-end cut in a perfect “Rock/Claw” frequency, while avoiding “buzz” or “sizzle” in the upper octave.
So that means the character of the 6CA7 in a 6L6 tube, used in the Extended Class A sockets. Step back in time a few decades, the Mark III blue stripe, DRG or Simul Class came stock with the STR416 tubes in the class A sockets. They are were marked with 6CA7/EL34. I may have confused the make as RCA, they may have been made by Sylvania, I assumed they were 6CA7 due to the large bottle size. I tried tubes that were EL34 and they sounded different. So what was in the Class A/B positions, the famed STR415 tubes. Ok, if the STR445 have the characteristics of the 6CA7 what do I have that sounds like the STR415 ( I have these tubes but did not want to remove them from the JP2C). That would leave me with the STR448 (TAD red base). Pulled the outer pair of STR445 (yellow bias color) tubes and replaced them with the STR448 (red bias color). Fired up the Mark VII last night. Double checked to make sure we will not overload or red plate, nope. (trust me on this, due to past experiences with the Mark V90 killing power tubes due to bias issue, I have my reservations but not warranted as the Mark VII is not the Mark V90). Started off with fat clean. WOW that sounded really good. Much more detail than I expected. Bold, crisp, and chime. Ok, still on par with the JP2C loaded with the STR415 or STR448 with the clean channel. I have run both in that amp, the STR448 are very comparable to the STR415 in many ways.
Now for the crunch on CH1. I could not believe my ears. Will have to get back on the Mark VII again to confirm this. Crunch was epic. Nice rolling grind in the low end and much better detail and note definition. Character I liked about the STR445 was still there but overall improved tone. No hint of boxy overtones like I got with the full compliment of the STR448 in the amp. Dropped from 90W to 45W, the characteristic was retained in note definition, sure the bottom grind was different as this is just the STR445. Went back to 90W, the difference in tone was subtle but notable. I can make out the characteristic of the STR448 (reds) when I tried them with the MWDR and 7 string guitar.
Now for VII mode. Holy hanna, Is this the Pandora's Box effect. Yep. Nice deep saturation and grind and retention of the note definition I was getting with the Bad100. Harmonically rich, yeah. This sound would be the money maker for the amp. For a tone stack post gain, then the GEQ has its unique characteristics. I will have to compare this to the BAD100 in crush again. What was missing that was not finally observed was that Simul-Class 3D effect. If it was there, I was not hearing it. Now it is there but not quite apparent like the Mark III or Mark IVb. Time to move the tone stack in the pre-gain and run the traditional Mark modes, staring with the IIC+
Now that is totally different. I believe the Mark VII just surpassed the JP2C pushing the STR415. That tone density was there. I would not say it is compressed but sure, there will be compression. However, the tone density is more of a character that describes the full sound quality. Note definition was still there. The grind, the overall character of what I was hearing was on the edge of the IIC+ abyss if not taking the leap of the ledge. Sort of like a war movie where the plane gets shot up and the engine begins to scream due to the gravity of the orb below. Yeah, we are talking about gravity, this sound will draw you in, time will stop and you will be lost in heaven or hell what ever you want to use to describe it. Not quite there with the Mark IVB integrated quad experiment I did years ago.
Now for the ultimate IV mode I have heard to date: The Sinister gates to hell have been opened again. Now that is the sound I was after for a long time but thought I would never hear it again. The Mark V90 was not capable of producing this sound. 3D content is there but not as accelerated effect like the SED =C= 6L6GC tubes will provide when I ran them in the Mark IVB. Still, the tone density is just amazing. Great note definition, no cloudy blur of sound, I do not believe I would be able to coax this sound out of the JP2C unless I used some external help with a grid slammer or flux drive.
So that is with the STR448 (red) in the class A/B sockets. Wonder what the STR448 (grays) will sound like? I may try the STR445 greens in the center pair and see if things change. As for now, I am loving the STR445 yellows with the STR448 reds in the outer sockets.