usage of the unit. We recommend that the bias setting should be checked on all the tubes after every 1000
hours of listening (or once per year.)
Using your amplifier with incorrect bias settings will result in poor
performance and a shortened life time for the tubes.
There is a fuse on the back panel of the amplifier that will protect the rest of the circuitry in the event that a
tube draws excessive current. If the fuse blows then you should immediately turn the amplifier off and contact
your authorized VTL dealer or the VTL factory customer support department before attempting to turn the
amplifier back on again. The symptom for this fuse blowing is either a condition where all the tubes appear to
be lit but with no sound coming out of the amplifier, or a tearing sound coming from your speaker if the fuse
blows while the amplifier is playing music. This sound will not damage your loudspeakers as the amplifier is
losing power while the fuse is blowing, and its level is not changed by the volume control position of the
preamplifier.
Caution: Bias checking and setting on all tubes must be performed every time a tube is replaced
in an amplifier.
For checking the bias you should take the unit and this procedure to your authorized VTL dealer and ask
them to check the bias for you.
Notes for the service technician:
Bias measurements are preferably taken with a digital multimeter equipped with the auto-ranging feature. If
you don’t have such a meter you can purchase one at a local electronics supply store such as Radio Shack. The
Radio Shack catalog numbers for usable multimeters are: 22-166, 22-167, 22-163, 22-174 or even 22-179.
Use the following procedures to measure and adjust the bias setting:
1.
Make sure that the amplifier is
powered off
. Remove the protective metal cage of the amplifier,
and make sure that there is sufficient room around the amplifier for you to access the bias
measurement points going across the front of the amplifier. Set the multimeter for resistance
measurement (ohms). (Consult the owner’s manual of the multimeter to make sure that you
have used the correct settings.)
2.
Connect the negative (black, “-“) probe from the multimeter to the negative(“-“) speaker binding
post of the amplifier.
3.
Consult the following figure for the locations of the bias measurement points on the amplifier’s
top deck. Insert the positive (red, “+”) probe of the multimeter into each bias measurement
point. The resistance reading on each point should be 10 ohms, within a 10% range. If the
resistance reading is outside the acceptable range, please consult the VTL factory customer
service department immediately. Repeat the measurement procedure for all 4 bias measurement
points.
4.
Remove the multimeter probes from the amplifier. Check to make sure that the amplifier is
connected to a loudspeaker or a load resistor. Either short the input or put the preamplifier in
the mute state (or turn the volume down) to make sure that no signal is coming from the
preamplifier. Power on the amplifier and let it warm up for about 10-15 minutes so that the
tubes become stabilized.
MB-125 Owner’s Manual
VTL
9
