Swapping 12ax7s and impact on profiles

    I am in the fortunate situation that I have some nice NOS or ANOS preamp tubes for my Mesa TA15. I used to run a Philips 1965 i65 (Heerlen) in the Marshall side of the amp and the profiles wit loadbox and IR were around 6. Felt right.


    I swapped the i65 for a 1960 Valvo (Hamburg, i60 type, 45° getter) and it sounds great in the amp. Nice breakup, clear, a little sweeter than the Philips etc, however, the Kemper consistently profiles the amp "wrong". It does not sounds off, has a definition of 10 (does not feel right), despite refining the heck out of it. It sounds nasally and I am puzzled.


    Any ideas what went wrong?

    No, the profiles are through a loadbox and IR. So all things considered the same conditions. If I profile the vox side I get very consistent results with earlier attempts. This side has a completely separate preamp circuit, so I could compare if something else was amiss

    Fairly hot, but not hotter than the vox side (which yields consistenly definition results around 2.5) nor different than with the i65. I guess the Kemper does not deal well with the clipping charactistics of the Valvo 12AX7?

    Quote from gtrbldr

    Fairly hot, but not hotter than the vox side (which yields consistenly definition results around 2.5) nor different than with the i65. I guess the Kemper does not deal well with the clipping charactistics of the Valvo 12AX7?

    I doubt that that is the case.
    so far, running both preamp and poweramp high enough to produce considerable distortion from each made profiling difficult - the solution was to simply turn down the master. A loadbox can also influence the result IIRC, best to try it without.

    Well, figured it out. Wasn't the tube, it was my treble knob. Definition result is very responsive to the setting of the treble knob, which puzzles me. The tone stack is situated after the preamp circuit. So it has no bearing on the gain characteristics, which is what the definition is about. According to the manual. The power tubes cook nicely (el84. :-)), but less than the vox side of the amp and that one is hit with a lot more treble content, but results in lower definition settings.


    Anyway, profiles came out a lot nicer after backing off the treble, which is a plus. The nasty honk is gone, but the amp itself sounds a little better with higher treble settings and has no honk there. Guess I have to compensate with the KPA EQ and the kemper is not perfect (#ducks#stilllovemykemper) ;)