How can I run my Mesa 2:90 in class A?

[badguitarist]

Guys, I've heard some people pulling their tubes from their 2:90 to run it in class A. I wanna do the same to my 2:90 (for both channels) to reduce its power but I don't know which tubes can be pulled and how the tubes are arranged in both channels. The manual doesn't talk about that. Could anyone help me with this? Any helps would be appreciated!

 

[badguitarist]

I pulled out the 4 central tubes and success using just 1 channel in class A. But the 2 tubes in the unused channel (volume all the way down, presence maxed already) were glowing red while the tubes in the used channel were blue. Is that normal? I didn't dare to run the amp anymore since I found that.

 

[Octavarius]

That isn't normal, and I wouldn't run the amp that way anymore. I strongly recommend to use all of the tubes for safety reasons.

 

[disassembled]

That isn't normal, and I wouldn't run the amp that way anymore. I strongly recommend to use all of the tubes for safety reasons.

What he is doing is the same as switching the Class A button on Mark Series amp so unless there's a problem with doing that in the combo the power amp should be fine. The 290 is a direct copy of a Mark IVA power amp minus the simul/class A switch and Triode/Pentode switch.


Greg