Manley STEREO 75+75 WATT AMPLIFIER Owner's Manual

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Manley STEREO 75+75 WATT AMPLIFIER Owner's Manual Manual pdf 8 page image

BIAS PROCEDURESThe Manley STEREO 75 + 75 watt Amplifiers use a fixed bias system that requires verylittle attention. We recommend checking the bias every 2-3 months or any time you replacean 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 digitalmultimeter 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 jackstowards the front of the amplifier. To measure the current draw of each output tube place thepositive 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 goodground 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 biasadjust pot. On the top view picture on page 6, you can see the numbering sequence for the 4output tubes. The corresponding tip jacks (measurement points) and bias adjustpotentiometers are also shown.3. The first step to setting the bias is to turn on the amplifier and ensure that there iszero signal input, either leave the preamplifier or source signal turned off or unplug theamplifier input. Also, leave your loudspeakers connected as these provide an ideal load onthe output. (DO NOT EVER OPERATE YOUR AMPLIFIER WITHOUT SPEAKERSCONNECTED!) It is recommended that you leave the amplifier on long enough to ensurethat 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 slowlyuntil you measure 275mV to 300 mVDC (0.275V to 0.300VDC). Since you are measuringacross a 10 ohm cathode resistor, this would correspond to a 27 to 30 mA current draw foreach tube by Ohm's law.6. If you are unable to bring the bias voltage down to 300 mV on any tube, you mayalter the setting on the master bias adjust pot which will change the bias for all of the tubes atthe same time. Rebias all the tubes as described above. If after lowering the master biasadjust to its minimum setting you are still unable to bring the voltage down to 300mV on anytube, then we recommend that that tube be replaced.7. Follow step five with the remaining tubes, switching to the next test point andadjusting the next trim pot each time. After you have adjusted all the output tubes, recheckand repeat the procedure as they will drift a little bit during adjustment. Once they are all set,your amplifier should be in perfect operation.