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.

 

[Jamme1961]

Could a bad screen resistor be doing this? Its on the two inside tube socket's only - in class A which is the two outside tube sockets on alone there’s no issue.

 

[LocoPatrick]

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.

I did some additional testing, including the conditions you suggested. No difference--the crackle is still there:
1. Different input cable
2. Different speaker cable
3. Different speaker cabinet
4. Different speaker output

 

[LocoPatrick]

Could a bad screen resistor be doing this? Its on the two inside tube socket's only - in class A which is the two outside tube sockets on alone there’s no issue.

Very possible. I found a Mark IV schematic, and their really isn't much to the tube circuitry that is isolated to the two inner-most power tubes:
2.2kΩ control grip resistors
220kΩ bias resistors
470Ω screen grid resistors

I'll see if I can get in there with an ohmmeter and verify the resistor values and connections.

 

[Jamme1961]

Go to page 23 has the MKIVA and then the B schematics- also there’s notes from Mesa about stabilization update - ( page 60 ) hope this helps

 

[LocoPatrick]

Go to page 23 has the MKIVA and then the B schematics- also there’s notes from Mesa about stabilization update - ( page 60 ) hope this helps

That's really helpful! The schematic I found already showed the 2.2kΩ installed. The power amp schematic on page 41 in the document you provided does not have the control grid resistors. The input signal connects directly to the control grid. This might just be what I need. I'll take a look later today to see if these resistors are installed on my amp.

I'm guessing that the connection to the control grid is relatively high impedance. Do you think a 1/2W resistor will be adequate?