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6. Bias Checking & Adjustment
If you are unsure about any aspect of bias contact
your retailer, Icon Audio or a competent service
engineer.
The Stereo 40 MK IV uses the 'Fixed bias' method of
valve operation. This has the advantage of higher power,
lower feedback and cooler running. However you should
regularly check the bias reading using the built in meter
to ensure best performance from the amplifier.
very easy.
1, Tools you will need:
A small flat blade screwdriver ( normally supplied).
2, How to read the meter.
If possible warm up the amplifier for 5 minutes, volume
at "zero". Rotate the bias knob through V1-V4. This
corresponds with the rear output valves left to right
(viewed from the front). Each valve should have the
black pointer in or near the black section. (50-75).
About 10% percent difference ON EACH PAIR (1,2 and
3,4) will make little difference in performance.
Greater than this requires adjustment, especially if the
reading is 80 or more it should be reduced as this valve
is drawing too much current. Lower than 50 will cause no
harm but the performance will be reduced.
NOTE
3. The readings are affected by your local mains voltage.
So if they are all slightly high or low this is probably OK,
and check again later. If one valve is giving a high or low
reading adjust that one valve in line with the others.
4. How to adjust the bias:
Make sure you are reading the valve to adjust, e.g. read
V1 to adjust V1 screw. If the reading is incorrect, set this
by using the screw very slowly up or down until the
correct reading is obtained. They are very sensitive so
adjust very carefully. If the reading appears a little
unstable this is normally due to mains fluctuations.
You may need to repeat this a couple of times as the
adjustment of one valve may affect the other readings
slightly.
5, If one or more valves are showing erratic readings or
you cannot set the correct voltage, 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 too low. Normally adjustment intervals are
2-3 times a year or if you suspect a problem.
6. Possible problems: If the reading is unstable, 100%,
or "zero %" this is probably a valve failure, in which case
refer to your dealer or Icon Audio. Also the internal HT
fuse may have blown for 1&2 or 3&4 (spares inside).
When not reading bias, the meter on the front panel
gives an approximate power indication. (Full power both
channels in mono = 100%).
Valve Replacement
How long do the valves last?
In normal use about 5 years (5000 hrs) when they will
start to wear out. The quality and power will slowly
deteriorate. Sometimes the heater will fail and the valve
will remain cold to the touch.
Occasionally a 6L6/KT66 may suffer catastrophic failure
which may cause a noise through that speaker and
possibly over heating which may blow a fuse. In the UK
Icon Audio offer free valve testing if the valves or
complete amplifier is returned to us. The decision to
replace one valve or all valves will depend upon the
age of the remaining valves.
You are welcome to email us for advice or
replacement valves.
Replacing the 6L6/KT66 Valves
Important:
6L6/KT66 without reading these notes. Failure to
This is
do so could be both dangerous and damaging to
the amplifier.
When replacing valves, it is recommended that
you use a "matched quad", or two matched pairs
for best performance.
Health & safety; High voltages are present inside
the amplifier and on exposed valve sockets when
valves are removed, so take suitable care. It is
not normally necessary to remove the bottom
cover. Beware valves get hot in operation!
7, Changing valves: You should if possible check the
bias setting before you attempt to change the
valve(s), in order to familiarise yourself with the
procedure.
The safe way to change especially if they are a
different type is to change and re-bias one pair at a
time.
Remove the first pair of old valves and fit the
replacement pair. Switch on and measure and adjust
the bias.
The bias adjusters are "clockwise" to increase "anti-
clockwise" to decrease.
Make sure the selector is set to the correct valve!
Do not allow the reading to go above 95. Don't worry
how low the reading goes this will not cause damage.
Continue in the same way and fit all four valves. Do
final adjustment when the amplifier is fully warmed
up.
8, If you cannot set up the bias then the valve is
probably faulty or is unsuitable.
If the valves are brand new, you will need to check
again after approximately 10 & 100 hours, after that
only occasionally or if you suspect a problem.
If the meter should go over 100% reading, switch off
and allow to cool down, on switching on the valve will
warm up slowly allowing time to make adjustment.
9, Use only valves marked 6L6/KT66/6P3 or that
you know to be direct equivalents. And 6SN7/6N8
for the smaller valves (V5,6,7,8).
Use only valves which you know to be new or good
condition and test the amplifier thoroughly before
resuming normal use.
EL34 and KT88 valves are not recommended
The new high power KT120/KT150/KT170 are not
suitable.
10,
Replacing
valves:
These do not require any set up procedure. It's just
'plug and play', although care should be taken when
removing and inserting not to break the centre spigot.
If using two pairs of different brands use the outer
sockets for one pair and the inner sockets for the
other pair..
Do
not
attempt
to
the
smaller
6SN7/CV181/6N8
change
the
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