Troubleshooting Last Nights Gig. Shrill Sound, Tone Fluctuations, Sound Loss.....

  • Ok, first the gear:


    KPA un-powered toaster (Latest 5.3 beta firmware)
    Atomic 112-18w powered cabinet (2 EL84 power tubes and 12AX7 phase inverter)
    Behringer FCB1010 Midi Footcontroller.


    The problem - Last night at our bands big 10th anniversary gig, about 4-5 songs in, I start getting tone fluctuations. I would describe it as if I was rolling the tone knob on my guitar, to zero, then back up to 10, randomly. This lasted through one song, then seemed to clear up. The next 4-5 songs were uneventful. Then, right in the middle of a slow blues guitar solo (I did dial up the volume here), the sound started cutting in and out randomly, until finally, all I had were the pick attacks, with zero sustain. Sounded like little blips each time I hit a string. Fortunately, I had a 2nd combo amp setup beside my main rig, running parallel as a backup, and it continued working. Eventually, the sound from my KPA returned, and I finished the show with my main rig.


    My first thought is that the tubes in the Atomic 112-18 farted out. (BTW, does anyone know if this 18w Atomic 112 cab can bias adjusted? I can only find instructions for the 50w cab). Then I remembered the funky tone stuff early in the show, and thought that maybe it is some kind of midi problem with the new firmware?


    BTW, my tone for the whole show was really shrill and high-endy. No amount of tone control adjustment seemed to help. This was definitely not normal.


    Any thoughts?

  • First, give yourself a slap on the hand for using a new beta in an important gig :/^^
    Then congratulate yourself for having the forsight to have a backup handy and getting through the gig... :thumbup:


    I used to have one of Atomic Reactor cabs and didn't like it too much. Never tried to bias it though.


    I would have thought exactly the same as you....tubes or a MIDI problem, but then why/how would it correct itself.


    The obvious thing would be to try and reproduce it and then start removing/changing things.

  • I've used the latest beta 5.3 firmware in several rehearsals, and at least 2 previous shows, without incident. Certainly, nothing like mentioned above.


    I probably have the last 10 backups on the USB stick that stays with my Kemper. Restoring a backup in the middle of a gig though, would be a serious last resort. Come to think of it, I've owned my KPA since 2012, and I've never needed to roll back a firmware upgrade. I think I've restored backups, maybe twice in that time.


    I'm leaning more towards the Atomic as the problem. I pulled the EL84 power tubes last night, and noticed what appeared to be black soot on the inside of the tubes glass. If it were one of my regular tube amps, my first thought would be that the amp was over biased and overheating. I had in a pair of my favorite Mesa Boogie EL84 6BQ5 tubes, but they might have been too much for the Atomic. I swapped in a set of Mullard EL84's last night, and played for an hour or so, without problems.


    Searching the web, I've found one person who mentioned that the Atomic 122-18 cabinet doesn't need biasing. Every time biasing is mentioned, it is in reference to the 50w version of the Atomic cab.

  • Come to think of it, I've owned my KPA since 2012, and I've never needed to roll back a firmware upgrade. I think I've restored backups, maybe twice in t

    Same here. Of all the on stage issues I've had, it's never been the KPA.
    Looks like you found the problem.