Page 7
7. Bias Checking
It is essential to check the bias at least once a year.
This will confirm the condition of the valves and
maintain the performance of the amplfier.
Read notes all the way through before commencing.
If you are unsure about any aspect contact your
retailer, Icon Audio or a competent service engineer.
The Stereo 300
uses the ‘Fixed bias’ method of valve
operation. This has the advantage of higher power, and
cooler running. However occasionally (once or twice a
year) it is advisable to check the bias reading using the
supplied meter to ensure best performance from the
amplifier. This is a safe procedure which involves
measuring 500mv (or 0.5v) in the four sockets next to
each 300B and adjusting if necessary.
1, Tools you will need: The supplied meter or one set to
measure approx 500mv, and a small flat blade
screwdriver. Adjustments are done at zero volume with
speakers connected. Run the amp for at least 15mins (if
it is working correctly).
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 your mains
voltage fluctuations can affect your readings up to 10%.
It is more important that both valves of a pair read within
about 25mv of each other.
In some areas fluctuating
mains may make measurement difficult, in which
case use best estimate or use an analogue type
“AVO” meter.
Showing a probe reading 1
st
output valve
4, Adjusting: If not 500mv, set this by using the bias
adjuster adjacent, then check the other valve of the pair,
(L to R 1&2, 3&4). The bias adjustment pots from left to
right are n
ormally ‘mirror image’ in rotation, so some will
be anti-clockwise and some clockwise. They are very
sensitive so adjust very carefully; use very small
turns of the screw. If the reading appears a little
unstable this is normally due to mains fluctuations.
5, If one or more valves is showing erratic readings
or you cannot set the 500mv, 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.
Replacing The Valves
Icon Audio are happy to advise on replacement
valves and the performance of your amplifier, we can
also advise on the latest upgrades available. If you
decide to do this yourself please read the following
notes first.
Important: Do not attempt to change the 300B
without reading these notes. Failure to do so
could be both dangerous and damaging to the
amplifier.
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 is to change and check one or two
valves at a time. Remove the first old valve and fit the
replacement. Switch on and measure bias, you
should be setting the reading for each valve to about
500mv (say 400-600), Do not allow the reading to go
above 900mv.
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 (at zero vol), and
the amplifier should sound OK When tested.
6, If you cannot set up 500mv 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.
7, To avoid damage to the amplifier and electric
shock hazard you must use only valves marked
300B, 12AT7/ECC81 6SL7(6188). Or that you
know to be direct equivalents.
Use only valves
which you know to be new or good condition and test
the amplifier thoroughly before resuming normal use.
8, Replacing the small 6SL7/12AT7 valves:
The outer pair are 12AT7/ECC81, the inner pair are
6SL7(6188). Neither of these require any set up
procedure.
It’s just ‘plug and play’ although care
should be taken when removing and inserting not to
bend the pins. If this happens gently bend the pins
back into shape.
Icon Audio are happy to check the performance of
your valves/amplifier and your re-bias it amp free of
charge.
