5.
Change the meter setting to measure DC voltage. Insert the negative probe (black) from the
multimeter into the negative speaker binding post of the amplifier fitting it into the post where
the speaker cable is connected. Insert the positive probe into the bias measurement point of
each tube going from the #1 point to #4. The reading on the multimeter should stabilize after a
second or two and indicate a reading between 275 to 300 millivolts DC (0.275 to 0.300 Volts.)
This reading may fluctuate a little due to variations in the AC lines. It is not unusual for a 120V
rated AC line to vary between 115V and 122 V.
Caution: If the voltage reading is 50 millivolts or more below or above the acceptable
range then adjust the trimpot in the direction required and ensure that the reading is
changing. If the reading does not respond or continues to climb rapidly, or fluctuates with
large swings back and forth then you should turn off the amplifier immediately and
contact the VTL factory service department for assistance.
6.
In general a 10 - 15% variation around the 275-300 millivolts range is an acceptable bias
measurement. When the measurement is outside this range, you will need to adjust the bias
setting by using the following procedures:
1.
Keep the positive probe from the millimeter in the same measurement point where you
found the out of range bias reading. Locate the bias adjustment point whose number
corresponds to the bias measurement point. See Figure 2 for the locations of the
measurement points.
2.
Insert a 1/8” flat tipped screwdriver (with a plastic shaft or a properly insulated handle) into
the bias adjustment trimpot which corresponds to the out of range measurement point.
Rotate the screwdriver slowly to see the changes on the multimeter reading. Try rotating the
screw in both directions to see the effect of the change. Adjust the setting of the bias to the
proper range, i.e. 275 to 300 millivolts DC.
3.
After you’ve completed the bias adjustment changes, it is a good idea to go back and check
the bias measurements for all the other tubes since changes made to one tube will slightly
affect the setting of the others.
4.
Place the amplifier’s protective cage firmly back onto the amplifier after you are satisfied that
the measurements are stable.
Caution: If your observations do not conform to the above instructions, or if you do not feel
confident about performing the bias procedures yourself, please consult with the VTL factory
service department. Improper setting of the bias will affect the current draw of the output
tubes and affect the performance and lifetime of the tubes.
MB-125 Owner’s Manual
VTL
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