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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 20SE
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 900mv DC (or 0.9v) in the 2 sockets next to
each KT88 and adjusting if necessary.
1, Tools you will need: The supplied meter or one set to
measure approx 2 volts 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 900mv if valve
is conducting correctly. But bear in mind that your mains
voltage fluctuations can affect your readings up to 10%
(i.e. 810mV to 990mV). It is more important that both
valves read the same, within about 10-15mv.
Showing a probe reading 1
st
output valve
4, Adjusting: If not 900mv, set this by using the bias
adjuster adjacent, then check the other valve, (L to R
1&2). 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 900mv, 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 KT88 Valves
Important: Do not attempt to change the KT88
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 yourself 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
900mv, Do not allow the reading to go above -
990mv.
Don’t worry how low the reading goes this
will not cause damage. Continue in the same way
and fit all two 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 900mv 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
KT88, 6SN7. 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:
6SN7 valves.
Neither of these requires any set up
procedure.
It’s just ‘plug and play’ although care
should be taken when removing and inserting.
Icon Audio are happy to check the valves/amp or
your re-bias your amp free of charge.
