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BIAS PROCEDURES
The Manley DIRECT INPUT 50/50 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) and a small flat screwdriver.
1. On page 8, which shows the FRONT panel of the amplifier, you can see 1 black tip
jack and 4 red tip jacks. To measure the current draw of each output tube place the
POSITIVE meter probe into the first red tip jack and place the negative meter probe into the
BLACK tip jack.
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 and each tube's bias adjust pot. You'll need to remove the top cover by
unscrewing the screws found on the top and sides of the cover. Do not touch any components
inside the amplifier. Do not adjust any other pots except for the ones pictured on page 6.
High voltages are present even after the amplifier has been turned off!
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 muted, source signal turned off, volume
control down, etc. In other words, you want to measure the quiescent current of the
amplifier: NO input signal. 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 red positive meter probe 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.
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 measurement
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.