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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 Classic 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 350mv (or 0.35v) in the four sockets next to
each KT66/88 and adjusting if necessary.
1, Tools you will need: The supplied meter or one set to
measure approx 350mv, 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).
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 350mv if valve
is conducting correctly. But bear in mind that your mains
voltage fluctuations can affect your readings up to 10%
(i.e. 315 to 385). It is more important that both valves of
a pair read within about 10-15mv.
Showing a probe reading 1
st
output valve
4, Adjusting: If not 350mv, 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. 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 350mv, 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.
Replacing the KT66/88 Valves
Important: Do not attempt to change the KT66/88
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
350mv, Do not allow the reading to go above -
800mv. 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 350mv 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.
7, To avoid damage to the amplifier and electric
shock hazard you must use only valves marked
KT66, KT88, 12AX7/ECC83. 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, Replacing the small valves:
12AX7/ECC83 (inner pair) and 12AT7/ECC81
valves.
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 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.
