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BIAS PROCEDURES
The Manley 120 Watt Amplfier uses a variable "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 2 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 two (2) output tubes can have its own bias adjusted by altering its own bias adjust
pot. The numbering sequence for the 2 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. Mute your preamplifier. Don't play tunes. Also, you MUST 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. 300 mVDC (0.300
VDC). Since you are measuring across a 10 ohm cathode resistor, this would correspond to a 30 mA
bias current. Settings between 275 mV and 300 mV are perfectly OK but try to have each tube the
same.
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 other tube. Then go back and recheck again. Check and set the bias
on your amplifiers every day for the first week or any time you retube. Then check every week, then
every month. A red glowing area on the side of the tube usually indicates it is drawing way too much
current. If you see a red spot on the side of the tube, check and reset your bias immediately.