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7. Bias Adjustment
Read notes all the way through before commencing.
If you are unsure about any aspect contact your
retailer, Icon Audio or a competent service engineer.
The Stereo 40i uses the 'Fixed bias' method of valve
operation. This has the advantage of higher power, and
cooler running. However occasionally (once or 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 330mv (or 0.33v) in the four sockets next to
each EL34 and adjusting if necessary.
1, Tools you will need: A suitable meter set to measure
approx 350mv DC, and a small flat blade screwdriver.
Adjustments are done at zero volume with speakers
connected. Run the amp for at least 15mins (if it is
working correctly). A suitable meter is available from
Icon Audio on request.
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: You should get a reading of 330mv if valve
is conducting correctly. But bear in mind that your mains
voltage fluctuations can affect your readings up to 10%
(i.e. 300 to 365). It is more important that both valves of
a pair read within about 10-15mv.
Showing a probe reading 1
4, Adjusting: If not 330mv, set this by using the bias
adjuster adjacent, then check the other valve of the pair,
(L to R 1&2, 3&4). The bias adjustment pots from left to
right are normally 'mirror image' in rotation, so some will
be anti-clockwise and some clockwise. They are very
sensitive so adjust very carefully; use tiny turns of the
screw.
5, If one or more valves is showing erratic readings
or you cannot set the 330mv, then that valve is
probably faulty.
Important: Do not attempt to change the EL34s
without reading these notes. Failure to do so
could be both dangerous and damaging to the
amplifier.
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 necessary to remove the bottom cover.
Beware valves get hot in operation!
5, Changing valves: You should if possible check the
bias setting before you attempt to change the
valve(s), in order to familiarise your self with the
procedure.
The safe way is to change and check one or two
valves at a time. Remove the first old valve and fit the
replacement. Switch on and measure bias, you
should be setting the reading for each valve to about
330mv, Do not allow the reading to go above -
600mv. 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.
If all is well there should be no more than a barely
detectable hum from the speakers, and the amplifier
should sound OK When tested.
6, If you cannot set up 330mv then the valve is
probably faulty or is unsuitable.
It is safe to run the amp with either the left or right
pair removed, but do not use one valve of a pair on
its own. Any drift on the mains voltage will affect your
readings, so double check several times when you
have finished. 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.
7, To avoid damage to the amplifier you must use
only valves marked EL34/6CA7, 12AX7/ECC83,
12AT7/ECC81. Or that you know to be direct
equivalents. Use only valves which you know to be
new or good condition and test the amplifier
thoroughly before resuming normal use.
8,
12AT7(ECC81)
12AX7/ECC83; Originals may not be marked but can
be identified as having what looks like a large silver
'u' shaped clip on either side.
The outer pair are 12AT7/ECC81 these are phase
splitter double triodes. Neither of these requires any
set up procedure. It's just 'plug and play' although
care should be taken when removing and inserting
not to bend the pins. If this happens gently bend the
pins back into shape. (These valves are similar with
st
EL34
the same pin connection; accidental wrong insertion
would not cause damage).
Icon Audio are happy to check the valves/amp or
your re-bias your amp free of charge.
Replacing the EL34 Valves
Replacing
the
small
valves:
12AX7(ECC83)
and
The
inner
pair
are
Page 7
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