icon Audio MB 25, User Manual

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Page 6
7 Valve Replacement & Bias
Adjustment in MB25 amps
Important: Do not attempt to change the valves in
the MB25s without reading these notes. Failure to
do so could be both dangerous and damaging to
the amplifier.
If you are unsure contact your dealer or
Icon Audio for guidance.
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. Always
switch off and unplug. Beware valves get hot!
Changing or checking valve settings is relatively
straightforward if you follow our notes carefully.
You will get the best performance from your amp.
If you are not technical and just to know enough
‘to do the job’ then you only need to read the bold
type.
Some brands of 300B valves are larger than the
supplied types (120mm from bottom of base) which
will prevent the continued use of the valve cover, you
may need to check this first.
The 300B output Valves in this amplifier use the ‘Fixed
bias’ system. This ‘bias’ voltage needs to be
occasionally checked, and also set up when replacing.
How it works: A negative voltage of about –64volts dc
(varies with brand and age) sets the correct amount of
standing current flowing through the valve. The Higher
this voltage is the less the valve will conduct.
Conversely. If this voltage is to low the valve will
conduct to hard, overheat and could glow red. This
could damage the valve and amplifier. It may also
blow the mains fuse.
1, Tools you will need: The supplied multimeter or
one set to measure aprox 500mv, and a small flat
blade screwdriver. Adjustments are done at zero
volume with speakers connected. Run the amp for
at least 15mins if possible.
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 2000mv or the ‘black
mark’. See pics.
Making the ‘earth’ connection.
3, Checking: You should get a reading of 500mv if
valve is conducting correctly. But bear in mind
that mains voltage fluctuations can affect your
readings up to 5% (i.e. 475 to 525). It is more
important that both valves of a pair read within
about 15mv.
Showing a probe taking a reading
4, Adjusting: If not 500mv, set this by using the
bias adjuster adjacent, then check the other valve
of the pair. The bias adjustment pots are normally
‘mirror image’ in rotation, so some will be anti-
clockwise and some clockwise. They are very
sensitive so adjust very carefully; use tiny turns
of the screw.
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.
If you are not experienced, change and check one
valve at a time. Remove the first old valve and fit
the replacement. Switch on, check & adjust bias
for 500mv, then do the same for the other valve.
If the reading goes above 800mv you should act
quickly to prevent damage. 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 amplifiers are 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 500mv then the valve is
probably faulty or is unsuitable.
Do not attempt to use am amplifier with only one
valve of a pair, this could cause damage. Any drift on
the mains voltage will affect your readings, so double
check when you have finished. 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.
7, DRIVER VALVES to avoid shock & damage to
the amplifier you must use only valves marked
6SL7 (right hand) 6SN7 (left hand).
Make sure they
are new or in good condition and test the amplifier
thoroughly before resuming normal use. There is no
set up procedure for these valves.
icon Audio
351 Aylestone Road Leicester LE2 8TA
www.iconaudio.co.uk
Phone 07787 158791
Fax 0116 244 0593