BIAS PROCEDURES
The Manley STEREO 75 + 75 watt Amplifiers use a fixed bias system that requires very
little attention. We recommend checking the bias every 2-3 months or any time you replace
an output tube. 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) and a small flat screwdriver.
1.
On page 6, which shows the top view of the amplifier, you can see4 black tip jacks
towards the front of the amplifier. To measure the current draw of each output tube place the
positive meter probe into the first black tip jacks which corresponds to tube number one.
Place the negative meter probe into the negative black output binding post to make a good
ground connection.
Set the meter to read 'millivolts' DC (direct current).
2.
Each of the four output tubes can have its own bias adjusted by adjusting its own bias
adjust pot. On the top view picture on page 6, you can see the numbering sequence for the 4
output tubes. The corresponding tip jacks (measurement points) and bias adjust
potentiometers are also shown.
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.
With the meter probe in the first black 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.
6.
If you are unable to bring the bias voltage down to 300 mV on any tube, you may
alter the setting on the master bias adjust pot which will change the bias for all of the tubes at
the same time. Rebias all the tubes as described above. If after lowering the master bias
adjust to its minimum setting you are still unable to bring the voltage down to 300mV on any
tube, then we recommend that that tube be replaced.
7.
Follow step five with the remaining tubes, switching to the next test 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.
