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Hum Problems
If you experience hum, try disconnecting all inputs, if
hum persists this is probably an amplifier fault.
If not, identify which input is causing hum. Connect one
input at a time. A common cause is a 'hum loop' caused
by having to many earths, and may be identified by
unplugging each input source from the mains. One
remedy for this is to use an interconnect which only has
the screen connected at one end. Other causes of low-
level hum can be from adjacent equipment, so
experiment with moving equipment around to see if this
makes the hum better or worse.
Valve Replacement (see also section 7)
Recommended
valves
KT88/6550, 6L6, 5881 and KT66 are also compatible.
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
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 with a suitable meter 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
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 01/2021), or you have one already.
2, Connect: the black probe to the chassis 'earth' by
unscrewing the '0' speaker terminal and tightening the
are
EL34/6CA7/KT77
necessary,
although
upon
country.
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.
or
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
4, Adjusting: If all the valves are high or low, set the
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 of valve 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.
*Triode Operation
If this model was ordered with 100% "Pure Triode"
modification (see page 2) the output will be reduced
to about 19 Watts per Channel making it better suited
to higher efficiency vintage designs. All other
specifications and instructions remain the same.
The ST25 is normally configured for "Ultralinear"
operation (50% Triode 50% Pentode) giving about
30W output. This is a good compromise for power,
distortion and sound quality.
This modification is easily made or reversed by an
engineer. More information available on request.
st
output valve
Page 6
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