BIAS PROCEDURES
The Manley 500/200 Monoblock Amplifiers uses 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 8, which shows the FRONT panel of the amplifier, you can see 2 black tip
jacks labeled READ BIAS. To measure the current draw of each output tube place the meter
probes into the black tip jacks. (If you read a negative reading reverse the meter probes.)
Set the meter to read 'millivolts' DC (direct current).
2.
Each of the ten (10) 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 10 output tubes. The corresponding reading for the tip jacks (measurement points) is
selected by the 10 position switch under the tip jacks. Start with the switch in the one o'clock
position. This switch reads like a clock where one o'clock is tube #1 and corresponds to the
left-most adjustment pot. Two o'clock is tube #2 and corresponds to the second from the left
adjustment pot, and so on.
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 black tip jacks where they will remain for whole biasing
procedure and switch the select switch to position #1 (one o'clock). 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 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.
