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So if I got this right DRG:b Class A: All tubes in Class A (4 x 6L6 or 2 x 6L6 + 2 EL34) Simulclass: Inner pair class A/B, outer pair Class A The above is incorrect: When switch is in class A position ONLY the outer pair of tubes are operating. The Inner pair of Class A/B tubes are...

IMHO, the mark series amps were designed so that along with the EVM speaker there would be no change in character going from very low volumes to very high volumes. You hear people complain all the time about how this or that non master amp "sounds great but it's too loud" and then go down...

The Mark III Blue Stripe lead circuit omitted C30 (The treble bleed cap which tamed the highs) hence the reason the Blue strip is usually referred to as the most "aggressive" of the Mark III's. In the Mark IIC+, Mark III red and the Mark IVa this cap is 500pf. If I recall the C30 was 1000pf...

Mesa part # on both switches is 601073. You can get the exact replacement parts simply by calling mesa. This is correct for the standby switch but the power switch is an on-off-on type. Again a phone call to the service manager can get you exact parts replacements.

I would take the amp to a tech right away. There is a separate low voltage power supply for the switching matrix which has voltage regulators that can overheat and fail. If this is the case the longer you let it go you can get heat damage to the circuit board which is very difficult to...

On the back of the Mark IV is a rocker switch marked Graphic Equalizer with the choices ON--OFF--R2AUTO For the foot controller EQ switch to work this rocker must be set to OFF. Also the mode selector must be set to FT SW--- Not EX SW LOOP SELECT switch must be set...