Powering the system off
1. When you’re done listening to your system always turn the amplifier 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. Wait at least 20 seconds after the amplifier is powered down before you proceed to turn off the other
components in your system
3. 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 150 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 ST-
150, operating at approximately 80 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.
ÎCaution: you must power off the amplifier first before changing the tetrode-triode setting.
After the amplifier is 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.
Power your amplifiers back on again, and your amplifier 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
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 lifetime 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.
ÎCaution: 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
ST-150 Owner’s Manual
VTL
8
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