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6 Bias & Hum Adjustment
The bias voltage for this amplifier is 180mv DC
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 845is use the "fixed bias' mode of valve
operation. This has the advantage of higher power, and
cooler running. However occasionally (twice a year) it is
advisable to check the bias reading using the supplied
meter to ensure best performance from the amplifier.
This is a safe procedure which involves measuring 0.18v
(180mv) DC in the 2 sockets next to each 845 and
adjusting if necessary.
1, Tools you will need: The supplied meter or one set
to measure approx approx 2v (2000mv), and a small flat
blade screwdriver. Adjustment is done at zero volume
with speakers connected. Run the amplifier for about 10
mins (if possible) first.
2, Connect: the black probe to the chassis 'earth' by
unscrewing the '0' speaker terminal and tightening the
probe in the exposed hole. And the other in the test
socket adjacent to the valve on test. Set the 'Icon' meter
to 2000mv or the 'black mark'. See pics.
Making the 'earth' connection.
3, Checking Bias: You should get a reading of ideally
180mV if each valve is conducting correctly. But this will
vary from about 150mV to 210mV. Bear in mind that
your mains voltage fluctuations can affect your readings
by up to 10%.
If all the valves are reading slightly high or slightly
low, this is usually due to mains fluctuation. In this
case no adjustment is necessary.
TIP: If one or two valves are out of line with the
average, only adjust those valves to the settings of
the others.
Showing a probe in output valve test point
4, Adjusting: If all the valves are high or low, set the
180mv by inserting a small screwdriver in each
adjuster. If one valve is significantly higher or lower,
then it may be faulty or life expired. The adjustment
is very sensitive so adjust very carefully. If the
reading appears a little unstable this is normally due
to mains fluctuations.
5, If one or more valves are showing erratic readings
or you cannot set the 180mv, then that valve is
probably faulty or out of specification. If you are
unable to set the reading high enough this means the
emission of the valve is low.
NOTE!
If there is no reading at all the internal fuse individual
to each 845 valve should be checked by a qualified
engineer. Spares should be located inside.
HUM Adjustment
New amplifiers should not require adjustment, but as
valves age you may notice an increase in hum.
Disconnect the inputs at the rear to be sure that this
hum is only coming from the MB845i amplifiers. You
can either measure by ear or an AC voltmeter set to
measure approx 1000mv. If doing by ear turn the
"Hum Adj" screw near the 845 valves GENTLY for
minimum hum. If using a meter you should set to the
lowest reading typically 1mv AC max on "Low Gain".
A reading much higher than this may indicate an
imbalance in the 845s or a fault elsewhere.
Page 7
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