Mesa/Boogie 400 manual Power Tubes, Bias

POWER TUBES

The main power tube used in the Strategy 400 Stereo is the MESA STR 415 type 6L6 GC which provides smooth, warm musical power and is very reliable. The 6L6 we use is extremely rugged and has very low 3rd harmonic output when overdriven. All twelve sockets should be fitted with STR 6L6's when the Strategy is being used in any application other than live guitar. We have specially adapted 4 of the sockets-- those along the rear edge-- to accept either 6L6's or STR 416 type 6CA7/EL34. This tube is characterized by more prominent harmonic output when driven hard which produces a brighter, more grinding high-end, usually preferred for guitar. No internal changes are required; the two type of tubes are immediately interchangeable. Thus one model amplifier with interchangeable tubes replaces the two different types (normal and Simul-Class) described in some of our catalogs.

You can expect 6 month 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 only it 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 five cylinders"). Sometimes tubes 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 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 failures as much as is possible and damage -- though it almost never occurs -- would be minor and easy to repair. When replacing tubes, please use only the MESA types specified. Using others makes 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 and bend them in toward the center before plugging in the tube, if need be. Avoid excessive wiggling of the tubes when removing or installing as it can break off the small plastic key which insures correct orientation of the tube in the socket.

BIAS

As with all MESA amplifiers, the bias is permanently set during construction and never needs adjustment. This saves you any technician's fee for readjustment and prevents the bias from "wandering" or being accidently set improperly.

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