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6 Bias Adjustment
Please read these notes all the way through first.
If you are unsure about any aspect contact your
retailer, Icon Audio or a competent service engineer.
When checking the bias ensure that any "Active"
pre-amp or source is "off" with zero volume to
prevent false readings. Passive pre-amps should
have "zero" volume. On a test bench "short circuit"
the input.
The MB 845s use the "fixed bias' mode of valve
operation. This has the advantage of higher power, and
cooler running. However regular checking (say once a
month), is advisable to check the bias reading using the
built in meter to ensure best performance from the
amplifier.
It is recommended that the bias is checked regularly. But
even if the reading is slightly over or under no action
may be necessary if both readings are the same as this
may be your local 230/115v power variations. If you find
variations at different times of the day it is better to play
safe and be "under" rather than "over". The performance
of the amplifier will be unaffected. Ideally they should
read the same.
1, Tools you will need:
For checking none. For adjusting a small flat blade
screwdriver.
2, Adjustment is done at zero volume with speakers
connected. Run the amplifier for about 10 mins (if
possible) first.
First select V1 with the "Bias" switch the pointer should
be in or close to the black section.
Then select V2 again the pointer should be in or close to
the black section.
If V1 or V2 is more than +10% over. Adjust until the
pointer is in the middle of the black section.
If V1 or V2 is more than -10% under. Adjust until the
pointer is in the middle of the black section.
3, How to adjust:
The adjustment is very sensitive so adjust very
carefully. If the reading appears a little unstable this is
normally due to mains fluctuations. Make sure you have
the BIAS switch set to the correct 845 valve then slowly
turn the screw until the pointer is in the middle of the
black section.
4, If one or more valves are showing erratic readings or
you cannot set bias because the reading is too high:
Switch off and refer to Icon Audio.
If one or more valves cannot have the bias set because
the reading is too low:
The valve may be defective, consider replacement.
If there is no reading at all:
The internal fuse may have blown or the valve is "dead".
If it is still glowing the internal fuse may have blown.
Contact you dealer or Icon Audio.
5. Valve Testing:
If you suspect any of the valves are not working correctly
Icon Audio will test the valves at no charge. Return them
to us well packed in a rigid box with a note detailing the
problem, which valves you suspect and your contact
details. We will then contact you with the test results.
*See engineers notes at rear
Possible valve faults
Not all valve faults are visible but 845s and driver
valves should not be used if:
1. The "silver" getter material has turned white
or is disappearing (air leak).
2. The valve rattles excessively (a slight rattle is
normal).
3. Cracks in the glass.
4. Loose base.
5. Any pins are damaged or loose.
6. Heater not lighting up (this is harder to see
with the 6SL7/6SN7). If one valve stays
"cold" the heater has probably failed.
NOTE!
If there is no reading at all the internal fuse individual
to each 845 should be checked by a qualified
engineer. Spares should be attached to the bottom
inside.
After significant bias adjustment or valve change the
hum adjustment MAY be necessary if you notice
increased background hum. See section 5.4.
Page 8
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