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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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