Page 8
7. Bias Adjustment
Please regularly check this important function!
EASY WAY:
With the amplifier warmed up and on zero
volume. Measure the 0.5v DC (500mv) voltage on each
KT88 test point by using the supplied meter on the
indicated range. The black lead connected to the “earth”
or one of the “negative” Speaker terminals as shown.
Note the approximate readings. If they are all similar in
the region of 430-500mv this is fine. If one or two are out
of line, adjust to be similar to the others. Or you could re-
set them all to 490-500mv. This will give the best
performance.
If you are unsure about any aspect contact your
retailer, Icon Audio or a competent service engineer.
You are welcome to speak to one of our engineers
by phone or email.
The Stereo 60 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 500mv (or 0.5v) in the four sockets next to
each KT88 and adjusting if necessary.
1, Tools you will need: The supplied meter or one set to
measure approx 500mv, 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 430-500mv.
But mains voltage fluctuations can affect your readings
up to 10%. It is more important that both valves of a pair
read within about 25mv.
In some areas fluctuating
mains may make measurement difficult, in which
case use best estimate or use an analogue type
“AVO” meter.
4, Adjusting: If not 500mv, 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 very erratic
readings or you cannot set the maximum to 500mv,
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.
No reading (or 0) may also mean a blown fuse
see “Valve fuses” under section 5.
Showing a probe reading 1
st
output valve
Replacing the KT88 Valves
Icon Audio are happy to replace valves and check to
performance of your amplifier, and advise on the
latest upgrades available. If you decide to do this
yourself please read the following notes first.
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 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 500mv, Do not allow the reading to go above -
900mv. 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 500mv 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 (or direct equivalent e.g. 6550). KT66s may
also be used with 50w output power, 6SL7 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.
