Owner`s manual

PAGE 6
reAr PANeL CONTrOLS (Continued)
TUBES:
The power tubes used in the 2:50 are Mesa/Boogie STR-440 type 6L6 GC’s. These tubes have been tested in the ampli-
fier 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/Boogie 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 im-
mediately. (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, care-
ful observation will usually reveal which tube is shorting, even though one or two others may also turn red-hot after a few moments.
By viewing the tubes, try to verify 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 repair-
able.
When replacing tubes, please use only the Mesa/Boogie types specified. Using other tube brands and/or types will invalidate your am-
plifier’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.
Make sure you check out the sections on Power Tube and Pre-Amp Tube maintenance located further back in this manual. Some helpful
tips on prolonging the life of your amplifiers’ tubes makes these maintenance sections worthwhile reading.
DRIVER TUBES: 3 x 12AX7
The three small tubes are type 12AX7A, also known as 7025 or ECC83. These tubes are very long
lasting and trouble free in the 2:50; should they ever need replacement due to a specific malfunction, genuine Mesa replacements are
recommended for best performance.
NOTE: You must remove the top cover of the chassis in order to gain access to the
driver tubes. These tubes can become quite warm. Allow them to cool down before
changing and its always a good idea to power down the unit when changing any of
the tubes.
Also, be aware that some of the circuitry components (namely the large round ca-
pacitors running parallel to the three driver tubes) will store a substantial amount
of electrical charge even after the amplifier is powered down. For safety reasons
be sure to grab hold of the particular pre-amp tube when replacement is required.
Do not allow your fingers or hands to come into contact with any other part (s) of the amplifier’s internal circuitry.
No other user serviceable parts exist inside. Refer service to qualified personnel. Always unplug AC power before removing the top
cover of the amplifier.
F
U
S
E
F
U
S
E
FAN
M
ES
A
12AX7
M
A
S
E
7
X
A
2
1
MESA
12AX7
GENTLY ROCK TUBE BACK AND FORTH WHILE
PULLING UPWARD CAREFULLY