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Replacing the KT88 Valves
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.
When replacing valves, it is recommended that you
use a "matched quad", or two matched pairs for best
performance.
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 to change especially if they are a different
type is to change and re-bias one pair at a time.
Remove the first pair of old valves and fit the
replacement pair. Switch on and measure and adjust the
bias.
Do not allow the reading to go above 95. 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 the bias 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.
NOTE: If you are changing to a different type of
valve, be ready to change the bias quickly, as the
difference may be quite large, in which case change
one valve at a time.
If the meter should go over 100% reading, switch off and
allow to cool down, on switching on the valve will warm
up slowly allowing time to make adjustment.
7, To avoid damage to the amplifier and electric
shock hazard only use valves marked KT88 (or
6550), 6SN7/CV181 Or that you know to be direct
equivalents. EL34 valves will work but be at their
maximum ratings so KT88s only are recommended. Use
only valves which you know to be new or good condition
and test the amplifier thoroughly before resuming normal
use. The new high power KT120/KT150 are not suitable
as they will exceed the maximum heater power and
power transformer burn-out may result.
8, Replacing the smaller 6SN7/CV181 valves:
These do not require any set up procedure. It's just 'plug
and play', although care should be taken when removing
and inserting not to break the centre spigot. If using two
pairs of different brands use the outer sockets for one
pair and the inner sockets for the other pair..
Icon Audio are happy to check the valves/amp or your
re-bias your amp free of charge.
Explanation:
Meter Function:
This is set to give optimum bias at 65% reading. When reading power 100%
= both channels 2x 50W or one channel 70%. In triode mode 100% = both
channels 2x 30W. All continuous at 1kH.
*RMS watts Based upon the RMS voltage output (V²/R). Indicates the
maximum continuous undistorted output power.
*Push Pull is a very elegant way of 2 valves in opposition, virtually
cancelling out non linear distortion, noise and hum. Whilst dramatically
increasing the power and damping factor. In Class AB about 1/4 of the
output is pure class A. Unlike transistor designs there is no crossover
distortion at any power of the Stereo 40.
*Ultralinear (or Distributed Load). This is a true "win-win" output stage
design. Having virtually the all the characteristics and low distortion of pure
Triode valves, whilst keeping 90% of the power of pure Pentodes types. But
still inferior to pure triode.
7 Specification & Features
(Typical conditions @ 235v 50Hz)
• KT88 or 6550 output valves
• 6SN7 double triodes for first stage
• 6SN7 double triodes phase-splitter
• Low or medium feedback used (L and H)
• Icon in-house designed and wound output
transformers with tertiary winding.
• Hand wired point to point components
• Ceramic valve bases for minimum noise/leakage
• 50w RMS per channel Ultralinear at clipping
• 30w RMS per channel Triode at clipping.
• Signal to noise level -85db
• Feedback zero dB, -5.5dB, -14dB
• Damping Factor 4.7, 9.16, 30.6
• Frequency response 20Hz to 20kHz 8 watts
• Medium Feedback -0dB, -0.25dB
• Low Feedback
-0.25dB, -0.5dB
• 0 Feedback
• -3dB points Med F.B.75kHz. Low 45kHz
• Typically 0.15% THD at 8 watts, low feedback
• Channel balance typically less than 0.2dB
• 4 and 8 ohms output taps
• Choke regulated power supply
• Supplied with attractive valve cover
• Audiophile quality metal film resistors
• Audiophile quality polypropylene audio caps
• Japanese ALPS volume pot.
• Internal wiring using PTFE silver plated cable
• Valves carefully matched for best performance
• Gold plated Input & speaker terminals
• Inputs for CD, Tape, Tuner, Aux
• Record loop with monitor switch
• 122mV sensitivity for 40w output "0" F.B. UL
• 280mV sensitivity for 40w output low F.B. UL
• 825mV sensitivity for 40w output Med FB UL
• 220-240volts, or 117v for north America
• 76w standby, 190w 0 Signal, 240watts max.
• 1.6 amp (3amp USA) AS rear fuse (with spare)
• 390mm W, 210H, 410D amplifier overall, 25kg
• Carton= 38x54x52cm 28kg packed
• IEC mains lead, (5amp fused)
• Conforms to CE ROHS and WEEE where applicable
Specification subject to change without notice.
-2dB, -3dB
Page 7
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