Page 7
7. Valve Replacement & Bias Adjustment
This amplifier uses the technically superior “Fixed
Bias” method of setting the output valve idle current.
For optimum performance and to avoid damage to
the amplifier please check this important function at
least once a year or if you suspect a problem.
EASY WAY:
With the amplifier warmed up and with the
volume at zero. Measure the 0.3v DC (330mv) voltage
on each KT88/120 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 360-300mv 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 330mv. This will give the
best performance. The triode switch should be set to UL.
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.
Icon Audio are always happy to re-bias and check
you amplifier free of charge.
The Stereo 60 MKIII 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.
1, Tools you will need: The supplied meter or one set to
measure 1000mv (1.0v), and a small flat blade
screwdriver. Adjustments are done at zero volume with
speakers connected. Run the amp for at least 15mins (if
possible).
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 approx 330mv.
But mains voltage fluctuations can affect your readings
up to 10%. It is more important that both valves of a pair
read within about 30mv.
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 correct, 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 330mv,
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 at all (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/KT120s
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/KT120s 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 -
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 330mv 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
KT120/KT88 (or direct equivalent e.g. 6550).
KT66s and EL34s will be overloaded. 6SL7 6SN7
or that you know to be direct equivalents.
Use
