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6 Bias Adjustment
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 th
at 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 MB150Ss use
the “fixed bias’ mode of valve
operation. This has the advantage of higher power, and
cooler running. However occasionally (once 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 4 sockets next to each KT120 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 th
e ‘Icon’ meter
to 2000m
v 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 variations from about 160mV to 200mV will have
little effect. Bear in mind that your mains voltage
fluctuations can affect your readings by up to 10%.
So if all the valves are reading slightly high or slightly
low, this usually means this is due to your mains. In
this case no adjustment is necessary.
TIP:
If one or two valves are out of line with the
average (e.g. 170mv) 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 on any of the KT120s, the
internal 1A (AS) HT fuse may have blown. This
should be checked by a dealer or qualified engineer,
or refer to Icon Audio. Spare fuses are located inside
the amplifier. Replacements are available from Icon
Audio free of charge, on request.
