5
BIASING PROCEDURE:
Without an input (bass or signal generator):
1. Turn the unit on with speaker or resistive load connected to appropriate output jack.
2. Put the standby switch in operate position and allow unit to warm up for 30 minutes
3. Using a voltmeter (a digital one is helpful), measure the output tube current draw as
follows:
•
Place common (black) probe of meter on black test terminal located on the rear of
the unit. Place the positive (red) probe on the yellow test terminal. Take a
reading and adjust using the yellow trim-pot to 60mVolt or .060 Volts
•
Keeping the common (black) probe of the meter on black test terminal of the unit,
place the positive (red) probe on the green test terminal and adjust using the green
trim-pot to 60mVolts or .060 Volts.
•
Place the common (black) probe on the yellow test terminal and the positive (red)
probe on the green test terminal. This should read 0 Volts. If not, trim out by
turning the yellow trim-pot until 0 Volts is obtained
Now the amp is properly biased. What we have done here is actually measure how much
current each pair of power tubes is drawing. Each pair should draw 60 milliamperes.
Since there are 1 ohm sensing resistors inside the unit, we can actually measure the
corresponding voltage drop across these resistors that correspond to the current draw.
Normally, the amp should not require re-biasing unless the power tubes have been
replaced. However, if the sound quality seems to have changed as the power tubes
receive more use, then adjusting the bias may be necessary.
