Mesa Mark IV problem

[OverDriven]

Hi...I'm having a problem with my Mark IV and I'm just looking for suggestions on what the problem could be. The amp sounds fantastic on all channels until you get the master to around 4 and the channel volumes to around 5. Anyone who owns a Mark series knows that this is INCREDIBLY loud...louder than most amps on 4. When the amp is played at that volume, I get this intense sound that can only be described as "farting out". I'm fairly certain that the sound isn't coming from the cab (a Mesa 2x12 with V30s), but is generated within the amp. However, I have not tried it with another cab so I'm not 100% sure. I can tell you that everything in the cab seems tight. The tubes are somewhat old, but not what I would call "going bad" yet. Any ideas what might cause this sound on all channels at high volume? Thank you in advance for any and all help.

Joe

[Gtr_tech]

Could be a tube thats been rattled to near death, or a flaky connection thats vibration sensitive.

[Amp Kat]

Sounds like it's overbiased to me and going into crossover distortion. Did you switch to maybe a hotter tube and didn't adjust the bias ? It is possible that it could be something else like a bad power tube but which it also sounds like or maybe a combination of the two. Better have it checked out.