Manley 175 WATT MONOBLOCK AMPLIFIER - Manual (Page 7)

BIAS PROCEDURES
The Manley Euro 175 Monoblock Amplifiers uses a fixed bias system that requires very
little attention. If you wish to adjust or check the bias, follow the following steps. We
suggest that the bias be checked and reset every 3-4 months.
For this procedure you will need a voltmeter (a hand held autoranging DMM digital
multimeter is the easiest) and a small flat screwdriver.
1. 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. 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.
2. Page 6 shows the top view of the amplifier, you can see 4 black tip jacks and 1 green
tip jack. To measure the current draw of each output tube first set the meter to read
'millivolts' DC (direct current). Place the positive meter probe (red) into the black tip jack for
the tube you wish to measure and the negative meter probe (black) in the green (GROUND)
tip jack.
3. Each of the eight (8) output tubes can have its own current draw adjusted by adjusting
its own bias trim pot. On the diagram, you can see the numbering sequence for the 8 output
tubes, the corresponding tip jacks (measurement points) and bias adjust pots. Everything
reads left to right.
4. Place the red meter positive probe in the first black tip jack (negative probes remains
in the green tip jack for whole biasing procedure) and adjust the first bias trim pot until you
measure 260mV to 275 mV. Since you are measuring across a 10 ohm cathode resistor, this
would correspond to a 26 to 27 mA bias current by Ohm's law.
5. If you are unable to bring the voltage down to 260 to 275 mV on any tube then we
recommend that the tube be carefully watched or replaced.
6. Follow step 4 with the remaining tubes. Once they are all set, your amplifier should
be in perfect operation.
7. Whenever replacing an output tube, or when retubing the unit, use tubes of the same
type and quality as was fit by the factory. Watch and adjust the bias for the specific output
tube or tubes you are replacing as soon as you turn on the amplifier, then check again after a
half hour, and again in a few hours. Should you have any questions, or wish to purchase
approved tubes from us here at Manley Labs, please do not hestitate to call.