Mesa Boogies Backward Bias System

Mesa Boogie amp have long claimed that their amps require no bias adjustments. They further state that their amps are biased in the factory and never require biasing. All you need to do is buy their tubes when the time comes to replace them. This seems contrary the advice of just about every other manufacturer and expert. So, what is going on?

First some background:

Mesa uses the same fixed bias method that most guitar amps have been using since the late 1950's when amp manufacturers largely switched from cathode biasing to fixed bias. In fixed bias negative voltage is applied to the grids of the power tube with cathode grounded. In the very earliest and cheapest vintage amplifiers the user had no easy way to adjust the bias condition of the tubes. Not all power tubes even on the same type would work in the amp. There was no way to properly set the bias condition ideal which is important for safety, tube life and tone. Non-adjustable fixed bias amp required techs to change resistors in the amp to adjust the bias---very inconvenient. Often time player just put new tubes in without adjustment with mixed results.

Soon after moving over to fixed bias Fender and other manufacturers realized what a poor system non-adjustable fixed bias was and added a bias adjustment on all but the lowest end amps. These single adjustment fixed bias systems remain the norm in amp to this day. A single adjustment is also less than ideal since it requires that power tubes be replaced in pairs or quads rather than one at a time. As time has gone on, forward thinking companies have installed bias controls for each power tube allowing for single power tube replacement and balancing the tubes DC ideal condition for the least hum.

So what is Mesa doing differently?

Mesa has elected to eliminate the bias controls in their amps. Yes, they have gone back to the late 50’s. Mesa require the user buys Mesa tubes that are spec so that hopefully the bias condition will be okay without adjustment. While it seems simple it limits the user and since tube supplies shift it is common for the “exact replacements” to be off. Often Mesa sets the bias permanently cold so more of their tubes will work in the amps. Since the user can no longer select the brand or type of power tube some tonal variations are off limits to them.

Why would they buck the convention? It cut out some labor during production and it sells a lot of tubes. Plus make Mesa seem like Mesa have something advanced. It probably does cut back on service problem caused but user error but the there is a cost in tone.

In my view fix bias amps must have bias controls. This gives great flexibility to the player when selecting new tubes and allow the power tube to be biased for the best tone.