Owner`s manual

PAGE 5
reAr PANeL CONTrOLS (Continued)
GROUND ISOLATION SWITCH: (Continued)
NOTE: If the 2:50 is operated outside of a rack installation, the switch should generally
be set to the Normal position, or else squealing, buzzing and oscillation may occur. Always check this switch before thinking that some-
thing has gone wrong with your amp!
BIAS SWITCH:
Your 2:50 was designed with versatility in mind. The BIAS SWITCH allows you to adapt the power output section of
the 2:50 in order to use the other classic pentode power tubes, the British style EL34. These tubes are largely responsible for the signa-
ture sound of many immediately identifiable and wonderful sounding amps created in Great Britain and used on some of the best record-
ings to date. The nature of their sound is usually brighter in the extremely high frequencies...some
players find this almost thin at first.
However, aficionados of the EL34 sound know that nothing has the lushness of harmonics or
spread like a power amp using EL34’s. In truth EL34’s do focus in on a region of upper harmonics
that 6L6’s reproduce, but not really enhance, in the same way the EL34’s do.
This characteristic is sometimes preferred for sounds that range from a soft clip that would be used for chording or soloing, to an all out
high gain crunch or lead sound. Players that use mostly these types of sounds may prefer the EL34 clip to the 6L6’s that come standard in
the 2:50. If you need a variety of sounds and rely on a clean chording sound much of the time, you will likely prefer the stock compliment
of 6L6’s that the 2:50 comes fitted with. We feel the 6L6 is a more balanced sounding tube that produces plenty of harmonic lushness,
while at the same time delivering the rich lows that are crucial to both a warm clean sound and huge, tight high gain crunch sound.
We recommend the 6L6 for reliability: In our many tests and continued use of the currently available EL34 type power tubes on the market
today, we regret to say that they do not appear to be as rugged in construction as the available 6L6. This is another reason why your 2:50
was shipped with 6L6 power tubes. If you plan to use the EL34’s we suggest that you keep a full set of tubes and extra fuses with you
during all performances in the event of a tube failure occurring when using the currently available EL34’s.
SPEAKERS:
The 2:50 provides one 8 Ohm and two 4 Ohm speaker outputs for each channel. When using a single 8 Ohm speaker or
8 Ohm cabinet on a channel, connect the speaker or cabinet to the 8 Ohm jack. If using two 8 Ohm speakers (assuming they are wired in
parallel) or cabinets on a channel, plug them both into the 4 Ohm jacks. ( The logic here is that two 8 Ohm loads connected in parallel add
up to a 4 Ohm total load.) Mismatching of speaker impedance will not cause damage or severe loss of power. (Extreme mismatches will
decrease tube life, however.) Mesa/Boogie 4x12 and 4x10 cabinets come standard wired to 8 Ohms and are wired in series parallel.
Some Non-Mesa 4x12 cabinets are wired 16 Ohms using four 16 Ohm speakers. By wiring all
four speakers in parallel, you can reduce the cabinet to an impedance load of 4 Ohms (assuming
the speakers are 16 Ohms each.) Further back in this manual we have included a number of pos-
sible wiring schemes - cabs to cabinets, that may be of some use to you when considering how
to configure your rig.
NOTE: If you opt to use only one channel of the 2:50 with speakers connected to that channel’s
output only, you must turn the unused channel’s level control to zero! Otherwise the channel
will be operating without a load, and damage to the tubes
and/or the transformer will result.
4 OHM
8 OHM
INPUTS B
F
U
S
E
F
U
S
E
INPUTS A
GROUND
SWITCH
LIFT
GROUND
BIAS
SWITCH
6L6
EL34
MUST MATCH TUBES SEE OWNERS BOOK
POWER FUSE
4A
SLO
BLO
120 V~
60 Hz
4 A
CHAN. B CHAN. A
8 OHM
4 OHM
SPEAKERS
SPEAKERS
CAUTION: To reduce risk of fire or electric
shock, do not remove cover. No user-
serviceable parts inside. Refer all
servicing to qualified personnel.
WARNING: To reduce risk of fire or electric
shock, replace fuse with same type and
rating only. Do not expose this unit to
rain, moisture or thieves.
4 OHM
8 OHM
INPUTS B
F
U
S
E
F
U
S
E
INPUTS A
GROUND
SWITCH
LIFT
GROUND
BIAS
SWITCH
6L6
EL34
MUST MATCH TUBES SEE OWNERS BOOK
POWER FUSE
4A
SLO
BLO
120 V~
60 Hz
4 A
CHAN. B CHAN. A
8 OHM
4 OHM
SPEAKERS
SPEAKERS
CAUTION: To reduce risk of fire or electric
shock, do not remove cover. No user-
serviceable parts inside. Refer all
servicing to qualified personnel.
WARNING: To reduce risk of fire or electric
shock, replace fuse with same type and
rating only. Do not expose this unit to
rain, moisture or thieves.