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6 Bias Adjustment
Why is bias so important?
With pure Class A single valve operation the valve must
be at 50% of its current. This enables it to increase to
100% and down to 0% at full power as the music
waveform demands. Too low and the full power will not
be achieved without distortion, too high and the valve will
be operating at a higher power, at a higher temperature
and shortening its life.
Why isn't it done automatically?
Although possible it would require a transistor circuit to
continuously monitor the KT120. Apart from the
complexity and reduced reliability (transistors don't like
heat and high voltage spikes), the circuit may
misinterpret "music" as a need to adjust the bias, which
will have consequences for the listening quality. Overall
we feel the "fixed bias" method sounds better.
Our simple Bias check method will help you keep the
MB30 at its optimum performance:
How to check and adjust Bias:
1. Run the amplifier for at least 10 minutes. If changing
valves check initially, and again after ten minutes.
2. You will need a small flat blade screwdriver to fit the
adjustment pots.
3. Switch off the pre-amp or source unit to prevent false
readings.
4. Push the bias switch up. The pointer should be in the
black section of the dial. Maximum reading of 75.
Minimum reading of 50. See Notes below.
5. If adjustment is necessary turn the adjacent "pot"
slightly until the pointer reaches the desired position.
Notes
If the BOTH amplifier readings are slightly low or high,
this could be due to local power supply fluctuations. In
which case leave and check at another time. Indeed you
may even notice small variations whilst you are
checking, this is normal. If you find significant variations
at different times, the best compromise is to set a little on
the low side in order that a reading of 75 is not
exceeded.
If the reading cannot be controlled or is erratic, this is
normally the sign of a faulty valve. No reading at all
usually means the valve is faulty.
Swapping valves with the other MB30 may enable you to
eliminate whether a valve is faulty or not.
Valve Testing:
If you suspect any of the valves are not working correctly
or you are concerned about their condition, Icon Audio
will test the valves free of charge. Return them to us well
packed in a rigid box with a note detailing the problem,
which valves you suspect and your contact details. We
will then contact you with the test results.
*See engineers notes at rear
Possible visible valve faults
Not all valve faults are visible but KT120s 6SN7s and
GZ34s valves should not be used if:
1. The "silver" getter material has turned white
or is disappearing (air leak).
2. The valve rattles excessively (a slight rattle is
normal).
3. There are any cracks in the glass.
4. The base is loose. (sometimes repairable)
5. Any pins are damaged or loose.
6. The heater does not light up (this is harder to
see with the 6SN7). If a valve stays "cold" the
heater has probably failed.
Generally speaking valve failure may be one of
these:
7. The valve continues to work but the
emission gets low. In the case of KT120
output valves this will result in not being able
to set the bias properly.
8. The valve gets noisy/microphonic. Usually
happens to the small valves, can be
confirmed by tapping gently with a plastic
pen.
9. Heater fails. No glow in centre of valve.
Valve is cold.
10. 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 at all.
11. Dramatic Failure. Occasionally the demise
of a power valve may be obvious with
internal sparks and noise through the
speakers. In that case, switch off and do not
use until a replacement is available. Before
re-commencing use see chapter 6 "Bias
Adjustment". If there was any associated
burning smell etc, we would recommend a
qualified person examines the inside of the
amplifier first.
12. The whole internal metal structure glows red.
This is usually an internal short, and may be
checked by replacement valve.
7 Valve Replacement
Important!
Before attempting any valve change
ISOLATE each amplifier by removing the IEC
cable from the socket on the rear, for 10 minutes
to allow high voltages to discharge.
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!
Valve replacement procedure:
1. Read the notes on "Bias adjustment" first.
2. Remove valves by holding the base and gently but
firmly with a slight rocking motion. (be careful too
much movement will break the central spigot).
4. Replace the valve with the same or equivalent
type.
5. Prepare to adjust the bias of the KT120 according
to section "6" so have a small screwdriver ready.
6. Switch and adjust bias as necessary. Check again
after 30 minutes.
Page 7
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