Ground Loop Hum
If you get a ground loop hum from your stereo system after installation and turn on you should immediately
turn off the amplifiers and consult your VTL dealer or the VTL factory. Under no circumstances should you
attempt to lift or defeat any grounds on electrical equipment plugged into the AC, as these grounds are installed
for your and the equipment’s safety, and an ungrounded component can present an extremely hazardous
condition and is illegal under most electrical safety codes. For your own safety please refer all questions of this
nature to a properly trained service technician.
Powering the system off
1. When you’re done listening to your system always turn the power amplifiers off first. Locate the “Power
On” switch on the lower right hand side of the front panel of the amplifier. Turn the switch to the “0”
position. The amplifier should now be in powered-off state, and the “Power-on LED” on the left of the
front panel should be extinguished.
2. Repeat the above step for the second monoblock amplifier in your system.
3. Wait at least 20 seconds after the amplifiers are powered down before you proceed to turn off the other
components in your system
4. Turn the preamplifier and source components off, if you prefer to keep these off when not in use.
Tetrode-Triode Switching
Your VTL power amplifier gives you a choice of listening to your system in tetrode mode or in triode mode. In
tetrode mode, your amplifier is capable of delivering up to 125 watts per channel. This mode of operation is
most suited to music that requires a wide dynamic response and lots of bass punch. Triode mode for the MB-
125, operating at approximately 60 watts per channel, is most suited for vocal and small instrumental
ensembles. In this mode, you will experience the delicacy, detail, and intimacy of the music in its very best
form.
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Caution: you must power off the amplifiers first before changing the tetrode-triode setting.
After the amplifiers are powered off, look for the switch marked “Tetrode Triode” in the back of the amplifier.
Flip the switch in the direction as indicated by the markings.
Make sure both monoblocks have the same “Tetrode” or “Triode” setting. Power your amplifiers back on
again, and your amplifiers should now be operating in the mode you specified.
Bias Checking and Setting
The idle (or quiescent) draw for the output tubes has been factory preset for approximately 27 mA for each
tube, and should be re-set upon initial installation into your system. This setting should only be performed by
someone trained in this procedure, as it requires removal of one of the covers that will expose potentially
hazardous voltages which are present in the unit. Do not attempt to undertake this procedure yourself as you
could be injured. The idle current of the tubes will fluctuate slightly over time at a rate that is dependent on the
MB-125 Owner’s Manual
VTL
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