LO POWER SWITCH
In the "LO" position, this switch provides a low (15 watt) power mode while still providing a bright and "searing" top end. As such, it's especially useful for situations (such as recording) where you might want the effect of "pushing" the power tubes into distortion but at moderate output levels.
PRESENCE CONTROLS
Each channel on the Fifty/Fifty has its own "active" Presence control. These controls provide a boost in the upper mid-range zone, which are the frequencies that tend to get obscured in the energy of live performance; use them when you need to cut through a "loud mix."
TUBES
The power tubes used in the Fifty/Fifty are MESA STR-420 type 6L6 GC's. These tubes have been tested in the amplifier during several "burn in periods” before leaving the factory. You can expect 6 months to 2 years (or more) of outstanding performance from your MESA power tubes, depending on use. Tube wear is gradual and usually goes unnoticed until new replacements are installed. Worn power tubes tend to sound flat with reduced punch, clarity and high end. Occasional loss of power or sporadic blowing of fuses is nearly always caused by troublesome power tubes. Often, you can spot the tube at fault and replace it alone, at considerable savings over replacing all the output tubes. A tube that arcs or "flashes over" inside should be replaced immediately. (In an emergency, you can just remove the bad tube and go on with the show...."running on 3 cylinders.") Sometimes a tube will "short out"
intermittently, turning red-hot all over the large metal plate inside. But usually a momentary switching off of the Standby or Power Switch will enable the tube to straighten out its electron flow and return to proper operation. Should this happen repeatedly, careful observation will usually reveal which tube is shorting, even though one or two others may also turn red-hot after a few moments. Try to see which one is reddest or turns red first - that is the bad one and the others are most likely unharmed. Internal circuitry is built to withstand tube failure as much as possible; even if damage were to occur, it would be minor and easily repaired.
When replacing tubes, please use only the MESA types specified. Using other tube brands and/or types will invalidate your amplifier's warranty!To remove tubes, you must first push back the black spring retainers clamped against the tube base. When reinstalling power tubes, check that the spring clamps are making good contact with the tube base; bend them in toward the center before plugging in the tube, if necessary. Avoid excessive wiggling of the tubes when removing or installing them, as it can break off the small plastic key which insures correct orientation of the tube in the socket.
BIAS
As with all MESA/Boogie amplifiers, the bias is permanently set during construction and never needs adjustment. This saves you any technician's fee for readjustment when replacing tubes, and prevents the bias from "wandering" or being accidentally set improperly.