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6 Bias Adjustment
Read these notes all the way through first.
If you are changing either the 805 or the
2A3 you must readjust the Bias and
probably the hum adjustment.
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 be
set at "zero" volume. On a test bench "short circuit"
the input.
The MB 805s use the "fixed bias' mode of valve
operation which is controlled by the 2A3. This has the
advantage of higher power, and cooler running. However
occasionally (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
sockets next to each 805 and adjusting if necessary.
See P3 diagram.
1, Tools you will need: The supplied meter or one set
to measure approx 2v DC (2000mv), and a small flat
blade screwdriver. Adjustment is done at zero volume
with speakers connected. Run the amplifier for about 10
mins (if possible) first.
2, Connect: the black probe to the chassis 'ground' by
unscrewing the '0' speaker terminal and tightening the
probe in the exposed hole. And the red probe in the test
socket adjacent to the valve on test. Set the 'Icon' meter
to 2000mv or the 'black mark'. See pics.
Making the 'ground' connection.
*See engineers notes at rear
0.4v (400mv) DC
in the
3, Checking Bias: You should get a reading of
ideally 400mV if each valve is conducting correctly.
But this can vary from about 350mV to 420mV max.
Bear in mind that your mains voltage fluctuations can
affect your readings by up to 10%.
So if both the 805 valves are reading slightly high or
slightly low, this usually means this is due to your
mains. In this case no adjustment is necessary.
Showing a probe in output valve test point
4, Adjusting: If all the valves are high or low, set the
400mv by inserting a small screwdriver in each
adjuster. The adjustment is very sensitive so
adjust very carefully. If the reading appears a little
unstable this is normally due to mains fluctuations.
5, If one or more valves are showing erratic readings
or you cannot set the 400mv, 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. Icon Audio can test the
valve for you, or ideally a matched pair or single
valve be substituted*.
NOTE!
If there is no reading at all the internal fuse individual
to each 805 should be checked by a qualified
engineer. Spares should be located inside.
Page 7
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