Page 6
If you have connected both to a known good input and a
known good speaker to a particular channel without
success the fault is probably within the amplifier.
Strange noises coming from speakers
Turn volume to minimum on unused input, if the noise
disappears, the fault is with the source or the connection.
If noise persists, the problem is with amplifier.
If a whole output valve glows red (other than the heater),
often accompanied by a hum through the speakers,
switch off immediately, and refer to Icon Audio or a
service engineer, as this could be valve failure.
A valve that is lit up is not a guarantee that it is working
properly; conversely a valve that is not lit up will not be
working.
Valve Replacement
(see also section 7)
Recommended valves are EL34/6CA7 or KT88/6550
(you can also use 6L6, KT66 KT77 at slightly reduced
power).
You will need a QUAD MATCHED SET for best results,
otherwise the bias voltage may vary too much.
If replacing with the same type and brand no adjustment
is normally necessary, although checking is advisable to
confirm the amplifier is working correctly (8v dc on each
valve, see below).
If changing to a different type you must check the bias
voltage as damage and poor performance may result.
Service:
Should you suspect a problem, you could
return the unit to Icon Audio for a periodic service or
return the valves for testing
free of charge
. You should
carefully remove the valves (the output valves should be
held by the base when removing, to prevent damage)
numbering them with a marker from left to right as you
do so in order that that may be replaced in the same
position. They should be well packed in cardboard &
foam or similar, and returned to Icon Audio for testing.
(Valves are very rugged if packed properly).
Mains Supply
This amplifier is either hard wired to work on 230/240v or
115vac
50/60Hz
depending
upon
country.
The
transformer may be re-configured for either voltage by a
qualified engineer. Contact for more information.
6 Bias Adjustment and setting
Read these notes all the way through first.
If you are unsure about any aspect contact your
retailer, Icon Audio or a competent service engineer.
The Stereo 25 uses the Icon
“semi fixed bias’ mode of
valve operation. There is only one adjustment, this
affects all valves.
This will only rarely need adjusting
if at all
as the amplifier can compensate for some valve
deterioration. This has the advantage of higher power,
and cooler running. However occasionally (say once a
year, or if you suspect a problem) it is advisable to check
the bias reading using the optional meter to ensure best
performance from the amplifier. This is a safe procedure
which involves measuring
8v DC
in the four sockets next
to each output valve and adjusting if necessary.
1, Tools you will need: The optional meter or one set to
measure approx approx 20v (DC), and a small flat blade
screwdriver. Adjustment is done at zero volume with
speakers connected. Check and adjust (if necessary) as
soon as the amplifier starts working and again 30 mins
later. Icon Audio can supply a suitable meter for £15 +
P&P at 01/2016), or you can buy one from your local
Maplins or Radioshack.
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 20v or the ‘black mark’. See pics.
Making the ‘earth’ connection.
3, Checking Bias: You should get a reading of ideally
8v if each valve is conducting correctly. But this will
vary from about 6v to 9v. About -20% should be the
maximum variation. Bear in mind that your mains
voltage fluctuations can affect your readings up to
about 10%. There is no provision to adjust individual
valves, which is not necessary on the ST25.
Showing a probe reading 1
st
output valve
4, Adjusting: If all the valves are high or low, set the
8v by using the single adjusting screw. If one valve is
significantly higher or lower, then it may be faulty or
life expired. The adjustment is very sensitive so
adjust very carefully. If the reading appears a little
unstable this is normally due to mains fluctuations.
The Stereo 25 uses semi-fixed bias, where the output
valve is allowed to find its own individual fine bias
level. Only occasional checking should be necessary.
Bear in mind that valves with an output of up to 25%
below the others are unlikely to have much of an
audible effect of the sound at normal listening levels.
If changing to a different type turn the voltage down
on the old valves first to a low level to avoid
overloading the power supply with the new valves.
Allow new valves to warm up for 5 minutes then set
to correct level.
