Page 6
6 Bias Adjustment
& Valve Change
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.
When checking the bias ensure that any “Active”
pre-
amp or source is “off” with zero volume to
prevent false readings. Passive pre-amps should
have “zero” volume. On a test bench “short circuit”
the input.
The MB 90s use
the “fixed bias’ mode of valve operation.
This has the advantage of higher power, and cooler
running. However occasionally it is advisable to check
the bias reading using the built in meter to ensure best
performance from the amplifier. This is very easy
involving the flick of a switch.
1, Tools you will need:
A small flat blade screwdriver.
Run the amplifier for about 10 mins (if possible) first.
2, Reading the bias:
Check one amplifier at a time.
Move the Bias switch t
o”V1” the pointer in the meter
should be in or near the black section.
Repeat for “V2”.
a. If V1 and V2 are about the same but between 55 and
75, this is normal.
b. If you find the readings over a few days are generally
high or low, adjust for the average.
3. How to adjust:
If one or both valves are high or low, adjust by inserting
a small screwdriver in each adjuster for the correct valve.
4.
The adjustment is very sensitive so adjust very
carefully.
If the reading appears a little unstable this is
normally due to mains fluctuations.
The setting of one valve will affect the other, so repeat
until satisfactory results are obtained.
5. Notes
If one or more valves are showing erratic readings or you
cannot set the pointer in the black section, then that
valve is probably faulty or out of specification. If you are
unable to set the reading high enough this means the
emission of the valve is low.
If there is no reading at all the internal 500ma fuse
individual to each KT150/120 should be checked by a
qualified engineer. Spares should be located inside.
If the reading is high and cannot be reduced this valve is
faulty DO NOT USE!
If the reading is unstable, this usually a valve fault. Swap
with the other valve to confirm, if the fault moves with the
valve, suspect the valve.
If the replacement valve is showing the same fault the
amplifier may have a fault.
7 Valve Replacement
Important!
Do not attempt to change the
KT150/120 output valves without reading these
notes. Failure to do so could be both dangerous to
you and damaging to the amplifier. Keep these notes
handy.
Take care that you orient the valve correctly before inserting. Line
up the centre “Spigot” first. They are easy to break, do not bend
excessively to the side.
The MB90m MK II is specifically designed to use
KT120 or KT150 valves. Use of the lower rated KT88,
6550, KT90 etc is not recommended as the lower
ratingings may be exceeded and will likely result
in early failure.
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!
How do I know when to replace valves?
Generally speaking valve failure may be one of
these:
1, The valve continues to work but the emission
gets low.
In the case of output valves this will result
in not being able to set the bias.
2, The valve gets noisy/microphonic.
Usually
happens to the small valves, can be confirmed by
tapping with a pen.
3, Heater fails.
No glow in centre of valve. Valve is
cold.
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.
4, 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 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.
If the amplifier sounds OK the valves are probably
fine. If the emission drops you will have difficulty
setting the bias for the output valves.
5. Visible Failure:
Not all faults are visible, but the following may be a
sign of failure:
Heater not lighting, (valve stays cold).
Valve assembly glowing red, whole or part.
Blue glow inside the glass.
Arcing or sparks inside.
Glass loose on base
Crack in glass.
Silver “getter” coating inside glass turning white or
being eaten away.
Excessive rattle when shaken.
Visible debris inside (often white flecks) inside glass.
6. Valve life
will depend upon such things as hours
of use and number of on/off cycles, Use of the
“Standby” switch to warm the valves up before use
will extend the life of the valves. It is not good
practice to remove the valves unnecessarily as this
can strain the pins and glass and cause tiny air leaks.
7. Changing valves:
If possible check the bias
setting before you attempt to change the valve(s), in
order to familiarise yourself with the procedure.
If changing all the KT150/120s be ready to adjust the
bias in order not to overload the power supply.
Don’t
worry how low the reading goes this will not cause
damage. 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.
