Owner`s manual

BIAS PROCEDURE
The Manley 250/100 Monoblock Amplifier uses a fixed bias system that requires very little
attention. The term "bias" as used here refers to an externally adjustable voltage applied to
each of the output tubes. This voltage setting optimizes the operation of the tubes; proper
adjustment ensures the best sonic performance and longest tube life. We recommend that this
adjustment be performed upon initial receipt of the amps, and every 2-3 months thereafter.
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 10
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 in triode mode 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 285 mVDC (0.275V to 0.285VDC). Since you are measuring
across a 10 ohm cathode resistor, this would correspond to a 27 to 28 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 an individual tube cannot be adjusted to at least 250mv, or it cannot be adjusted
below 300mv, then replacement is probably indicated (also see "Troubleshooting").
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. ( BIAS IN TETRODE MODE SHOULD
READ 270mV TO 310mV. THERE IS A MASTER BIAS ADJUSTMENT TRIMPOT
INSIDE THE UNIT, THIS TRIMPOT IS FACTORY SET ).
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