50/50 STEREO TUBE POWER AMPLIFIER
Operating Manual
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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.
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DRIVER TUBES
The two small tubes are type 12AX7A, also known as 7025 or ECC83.These tubes are very
long-lasting and trouble-free in the Fifty/Fifty; should they ever need replacement due to a
specific malfunction, genuine MESA replacements are recommended for best performance.
LINE FUSE
The Fifty/Fifty is protected by a 3 amp fuse when operating on 100 - 120 V. AC. Should the
fuse blow, replace it with a Slo-Blo fuse of the same amperage.
GROUND ISOLATION SWITCH
This mini-rocker switch is extremely useful in reducing the hum and buzz caused by ground
loops which occur in rack installations. Ground loops occur when there is more than one path
for grounds between electronic units. There are three main type of ground loops; each one
contributes its own trademark component of buzz or hum. The three types of ground loops
are:
1) Grounds being connected via the ground lugs of the power cords on the units. The rule-of-
thumb here is that one and only one unit in a rack should have a connection to the AC power
ground; all the other units in the rack should have their grounds lifted by using 3-to-2
adaptors on their AC plugs.
2) Ground loops occurring because of two or more audio cables connecting two components.
The most common solution is to disconnect the shield from the sleeve connection at one end
of some of the inter-connecting cables, so that only one cable between each device has an
intact ground shield. Some experimentation is required to find which cable is best for
conducting the ground, and which cables are best suited to have the special "clipped
ground." We recommend trying the intact-ground cables for the signal path which flows, say,
from an Effects Send jack to an effects unit; and trying the clipped-ground method on the
cable which connects the effects output back into an Effect Return.
3) Ground loops created by the physical contact between the metal chassis of the
components and the steel rack rails. It is for this third type of ground loop that the Ground
Isolation Switch was provided. The switch effectively lifts the electronic circuit ground from the
metal chassis ground (which otherwise must be done manually by disconnecting a wire
inside of the amp). Here again, the rule is that one device in your system - and usually only
one - needs its circuit ground connected to its metal chassis ground; all others should be
lifted (in other words, disconnected) for quietest operation. (Some effects units provide similar
switches; some are constructed such that the input ground and the output ground are
permanently separated; in other cases, the method used to isolate the grounds is unclear).
Again, some experimentation is vital to discovering which combination of "lifted" and
"grounded" works best in your rack .... and our Ground Isolation Switch makes it much easier.
NOTE: If the Fifty/Fifty 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 something has gone wrong with your amp!!
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Now that you've had this brief explanation of its functions and features, it's probably time to
plug up! We think you'll be pleased with the sound of your Fifty/Fifty - so enjoy it and enjoy
your music!
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