Mark IV Rev A SimulClass Crackling

[LocoPatrick]

Hello, Everyone,

My Mesa Boogie Mark IV Rev A is crackling at higher volumes (higher being greater than 1) when in SimulClass mode. The crackling can be heard in the video below. Does anyone have any idea what the cause of this is?

Analysis:
1. The crackling almost sounds like static.
2. The problem occurs only when I am playing. If I am not playing, the amp is quiet. When I start playing, the louder I play, the more pronounced the crackling is. As I turn the volume up, all I hear is the crackling.
3. The problem only happens in SimulClass mode. If I switch to Class A mode, I don't have this problem.
4. The problem occurs whether I am in Pentode or Triode mode, Full power or Tweed power.
5. The problem still occurs after I swap the inner and outer power amp tubes.
6. The tubes are brand new. The problem occurs with the old tubes and the new tubes. The old tubes are 6L6 GC STR 420. The new tubes are 6L6 GC STR 448.
7. The large electrolytic capacitors are brand new. The problem occurred with the old capacitors, and occurs with the new capacitors.
8. I cleaned the contacts for the power tubes, and am confident that I'm getting a good contact.

In the video below, I play the same riff three times:
First time: Class A, turned up (sounds fine).
Second time: SimulClass, turned up (crackling is apparent).
Third time: SimulClass, very low volume (sounds fine).

 

[rarebitusa]

A longshot but you could try one other simple test. Change the speaker impedance to see if anything in the current scenario sounds different. If it's a combo and the speaker jack is in the 8ohm out try switching it to the 4 ohm out. There sometimes can be issues with load matching and the power out stage that causes clipping and unwanted dist. Won't solve the problem but could help narrow the issue.