There is so much misinformation about what Mesa's Simul-Class is and how it works that I decided to write this Guide in order to set the record straight. To wit: Mark II and most Mark III Simul-Class amps put out 75 watts, not 100 watts. The last version of the Mark III (green stripe) and all Mark IVs are 85 watts. Confusion arises because the switch for the 60/100 watt option (completely different from Simul-Class!) looks similar, and is usually installed in the same far right spot on the rear panel. The Simul-Class switch can also appear on the front panel on long chassis heads. In this case, the switch is often mislabeled "Full/Half Power", and there will often be a "Simul-Class" sticker on the rear of the chassis.

The 60/100 watt option simply turns off two of the four 6L6s. Some amps may have the 60/100 option, some amps Simul-Class, but not both in the same amp. Never.

In Simul-Class, the amp must use two 6L6s in the inner tube sockets for 60 watts Class A/B, and two EL-34s (or 6L6s) in the outer sockets for 15 or 25 watts in Class A. So, on a Mark III Simul-Class amp, when the switch is UP ("Simul-Class"), you get 75/85 watts. This is all four power tubes. Switch DOWN ("Class A"), is 15 watts. Only the outer pair is on.

Finally, remember to always use the stand-by switch. Your power tubes will last much longer. If you found this guide helpful, please click on the "yes" button below.