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BIAS PROCEDURES
The Manley 250/100 Monoblock Amplifiers uses a fixed bias system that requires very little
attention. If you wish to adjust or check the bias, follow the following steps.
For this procedure you will need a voltmeter (a hand held autoranging DMM digital
multimeter is the easiest), a 3/32" allen key, and a small 1/8" flat screwdriver.
1. Remove the four silver hex cap head screws which secure the insert to the faceplate.
Also then remove by hand, the two knurled black decorative nuts on the Triode and Soft-
Start switches. On page 8, which shows the FRONT panel of the amplifier, you can see 8 red
tip jacks plus the ground point. To measure the current draw of each output tube place the
POSITIVE meter probe into the first red tip jack. (If you read a negative reading reverse the
meter probes, no harm will be done.)
Set the meter to read 'millivolts' DC (direct current).
2. Each of the ten output tubes can have its own bias adjusted by adjusting its own bias
adjust pot. Above the measurement tip jacks you can see each tube's bias adjust pot.
3. The first step to setting the bias is to turn on the amplifier and ensure that there is
zero signal input, either leave the preamplifier or source signal turned off or unplug the
amplifier input. Also, leave your loudspeakers connected as these provide an ideal load on
the output. (DO NOT EVER OPERATE YOUR AMPLIFIER WITHOUT SPEAKERS
CONNECTED!) It is recommended that you leave the amplifier on long enough to ensure
that the tubes have reached their stable current draw, at least 1/2 hour is recommended.
5. Place the meter probes in the first red tip jack . Adjust the first bias trim pot slowly
until you measure 275mV to 300 mVDC (0.275V to 0.300VDC). Since you are measuring
across a 10 ohm cathode resistor, this would correspond to a 27 to 30 mA current draw for
each tube by Ohm's law. A reading of 0mV can indicate a failed tube, or an open cathode
resistor. A reading of 0mV on ALL tubes (powered on condition) can mean the B+ fuse has
blown. The cause of this should be investigated before simply putting a new fuse in.
6. If you are unable to bring the bias voltage down to 300 mV on any tube then we
recommend that the tube be carefully watched or replaced.
7. Follow step five with the remaining tubes, switching to the next tube point and
adjusting the next trim pot each time. After you have adjusted all the output tubes, recheck
and repeat the procedure as they will drift a little bit during adjustment. Once they are all set,
your amplifier should be in perfect operation.