BIAS PROCEDURES
The Manley 50 Watt Amplfier is a fixed bias system that requires very little attention. If you wish to
adjust or check the bias, complete the following steps. For this procedure you will need a multimeter
to read millivolts (less than 2 volt range) and a small flat screwdriver.
1. Looking at the front section of the top plate of the amplifier, you can see 4 red tip jacks and 1
black tip jack. To measure the current draw of each output tube place the positive probe (red) into
the red tip jack for the tube you wish to measure and the negative probe (black) in the black
(GROUND) tip jack and set the meter to read 'millivolts'.
2. Each of the four (4) output tubes can have its own bias adjusted by altering its own bias
adjust pot. The numbering sequence for the 4 output tubes runs from left to right, as do the
corresponding tip jacks (measurement points). The bias adjust pots are located on the top side of
the amplifier, therefore it will be NOT be necessary to remove the bottom cover to changing the
bias. They also read left to right.
3. The first step to setting the bias is to turn on the amplifier and insure that there is zero signal
input. Also, leave your loadspeakers connected as these provide an ideal load on the output. Never
operate your amplifiers without a suitable load or speakers attatched. It is recommended that you
leave the amplifier on long enough to ensure that the tubes have reached their stable current draw.
5. Set your meter to read VOLTS DC. You will be measuring less than 1 volt so set your meter
for a low range if it is not an autoranging meter. Place the probes in the first red tip jack (negative
probes remains in the black tip jack for whole biasing procedure) and adjust the first bias trim pot
located on the left side top deck of the amplifier until you measure approx. 200 mVDC (0.2VDC).
Since you are measuring across a 10 ohm cathode resistor, this would correspond to a 20 mA bias
current.
6. If you are unable to bring the bias voltage down to 200 mV on any tube then we recommend
that the tube be carefully watched or replaced.
7. Follow step five with the remaining tubes. Once they are all set, your amplifier should be in
perfect operation.
(13 pages)
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