If you use Mesa tubes, I believe you can retube without rebiasing. Just call them up and tell them want new power tubes for a 50/50:
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.
