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Audio for a periodic service. In case of difficulty
you could email or phone us. You could also
return the valves to Icon UK for testing free of
charge. Carefully remove the valves (the
KT120/KT150s should be held by the base when
removing, to prevent damage) numbering them
with a marker or sticker from left to right from the
front, in order that that may be replaced in the
same position. They should be well packed in
cardboard & foam or similar, (Valves are very
rugged if packed properly).
8 Bias Adjustment
This amplifier uses the technically supe
rior “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 month with the built in meter.
Most of the time no action will be necessary, but this
will give you the reassurance than the valves are in
good condition and working correctly.
When first using the amplifier
Check that pointer is approximately in the black section.
Small variations have no effect. But any valve reading
“high” should be reduced. It will take a few weeks for the
valves to
“burn in” and settle down after which little
adjustment will be needed.
For occasional checking:
The triode switch should be set to UL, unless in constant
triode use.
With the amplifier warmed up (15 mins) and with the
volume at zero, Rotate the bias knob to check V1,2,3,4.
This will measure the bias current and give a visual read
out. The optimum reading is when the pointer is in or
close to the black section of the dial (= approx. 47ma).
It is important to note that if all four valves read
slightly high or slightly low this is normal and may
be your local mains power variation.
If this happens
regularly the bias should be adjusted.
If one or more valves give unstable readings the valve is
probably faulty. This may be proved by swapping the
valves around, or trying replacement valves. In UK Icon
Audio will test your valves free of charge.
As the valves age the reading of each valve will vary.
This may be adjusted by slowly rotating the bias screw
for each valve until the reading is in the black mark.
A small flat blade screwdriver is all you need.
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.
Most dealers and Icon Audio are happy to re-bias
and check you amplifier free of charge.
No reading at all (or 0) may mean a faulty valve or a
blown fuse s
ee “Valve fuses” under section 7.
9 Replacing the 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
KT120/KT150s 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! Remove
power cable when changing valves.
Changing valves:
As new valves will have a different reading it is
recommended that you change one, or one pair at a
time. Remove the first old valve and fit the
replacement. Switch on and measure and adjust
bias. Do not allow the reading to go above 100% as
this could blow the fuse.
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.
If you cannot set the pointer to the black mark, the
valve is probably faulty/worn or is unsuitable.
If the valves are brand new, you will need to check
again after approximately 10 & 100 hours. Regular
checking will ensure the amplifier gives the best
performance.
To avoid damage to the amplifier and electric
shock hazard you must use only valves marked
KT120/KT150 (or direct equivalent e.g. 6550).
KT66s and EL34s are not suitable and will be
overloaded. 6SN7/CV181 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.
Replacing the small 6SN7 valves:
All four front sockets are 6SN7, different types may
be used but preferably in pairs, the inner pair are the
first stage, with the outer pair being 2
nd
stage and
phase splitter. No set up is required. Care should be
taken when removing and inserting not to bend the
pins. If this happens gently bend the pins back into
shape.