I guess I was under the assumption that the STR450 NOS Siemens EL34, aka RFT EL34 tubes had limits on the screen voltage of 425V. Perhaps this is just a myth or rumor that was not correct. May just be a Mullard/Phillips issue and not associated with RFT/Siemens/Telefunken/Haltron or Trigon.
The only experience with the STR450 tubes that left me believing the myth is after I killed the TC50 while using those tubes. Amp was dead after replacing the fuse and swapping the STR447 tubes back in. Strobe mute circuit took a hit, so it had to go to Petaluma for repair. TC50 runs a plate voltage of 450VDC. At least the RA100 is lower at 400VDC. I basically have the full quad for use in that amp once I run out of the STR442 (SED=C= EL34/6CA7) tubes. I will not try the STR442 tubes in the Mark VII. They were too saturated in the TC amp, I already red plated a pair in the Badlander 100 (they were at EOL so it was bound to happen).
Yeah, I experimented with the STR440 (Shuguang), STR441 (TungSol GE copies), STR448 (TAD red-base, PSVANE), STR415 (NOS Sylvania made for Mesa) and a quad of the STR454 tubes (SED =C= 6L6GC). I do not remember trying out the STR443 tubes though. I have them but never tried them. Sure, even combined the STR445 tubes with the STR448 keeping the STR445 in the center pair which seems to be the defining tube spots and the outer pair is just for support at 90W.
What sounded boxy in the Mark VII: STR440, STR448, STR415. If you want that Mark V90 sound use any of these and you will gain that boxy tone if that is your thing.
What sounded actually good: STR441, STR454 Not boxy, a bit more headroom. I liked the STR454 the most.
Then the stocker tube STR445, tried different bias colors and preferred the yellows over the greens which were the original tubes in the amp.
Mixed quad with the STR445 in the center pair, with the STR448 red bias color or yellow bias color sort of helped with the detail at gig level. I am back to running the full quad of the STR445 yellows.
However, I thought the STR447 EL34 really sounded good. Similar tonal pallet of the STR445 tube but more note details. I did not seek out using other variants of the EL34 tubes as most of what I have in inventory are not Mesa branded tubes except for the STR450 or the STR446. Could have run the STR446 (TAD red-base EL34 made by PSVANE). Generally when the manual claims "known to be unreliable" as the boiler plate stuff, makes me skeptical on using them. If this is true, the STR447 costs far less than the STR450 or even the STR442 if you can find them.
No variac or spongy power mode to reduce plate voltage like the dual Rectifier amp, or Mark V90. How are they getting buy with the EL34 in the Badlander and TC amps? No mention of "keep a stock of fuses and lots of EL34 tubes on hand as they have been found to be unreliable for use". No schematics either. Makes having a bias probe something to consider. I do have some amps that have an adjustable bias (EVH and Marshall) so it would make the job easier to get a proper balance and bias since they both have a left and right bias adjustment.