VTL MB-125 User manual
VTL
MB-125 Monoblock Power Amplifier
Owner’sManual
MAKING TUBES USER FRIENDLY
VTL MB-125 Monoblock Power Amplifier
Information in this document is subject to change without notice. No part of this document may be reproduced or transmitted
in any form or by any means, electronic or mechanical, for any purpose, without the express written permission by VTL.
Version History:
Version 2.2: April 23, 2008
Copyright ©2009 by VTL Amplifiers Inc. All rights reserved.
4774 Murrieta Street, Suite 10
Chino, CA 91710, USA
Phone 909.627.5944 • Fax 909.627.6988
http://www.vtl.com/
Part Number OM-0010-201
Table of Contents
CHAPTER 1 .......................................................................................................................................1
INTRODUCTION....................................................................................................1
Symbol Conventions used in this guide..................................................................................1
Electrical Safety Notice...........................................................................................1
Water and Moisture...................................................................................................1
Location and Ventilation........................................................................................2
Servicing...........................................................................................................................2
Operational Warnings.............................................................................................2
CHAPTER 2 .......................................................................................................................................3
GETTING STARTED.........................................................................................3
Unpacking the MB-125 from its box................................................................3
The MB-125 Front Panel.......................................................................................4
Check for tube placement.......................................................................................................4
MB-125 Back Panel...................................................................................................5
The MB-125 Back View....................................................................................................................5
Connecting Your Amplifier to your system..................................................5
Voltage Setting.............................................................................................................6
Power Source for your Amplifier......................................................................6
CHAPTER 3 .......................................................................................................................................7
OPERATING THE AMPLIFIER..............................................................7
Powering your system on.......................................................................................7
Ground Loop Hum.....................................................................................................8
Powering the system off..........................................................................................8
Tetrode-Triode Switching......................................................................................8
Bias Checking and Setting.....................................................................................8
Top view and Bias Measurement points of the MB-125 Amplifier .................................................11
CHAPTER 4 .....................................................................................................................................12
MB-125 Owner’s Manual
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CARE AND MAINTENANCE OF YOUR VTL
AMPLIFIER...............................................................................................................12
BREAK IN PERIOD..........................................................................................12
Power-on Period.......................................................................................................12
Tube Types....................................................................................................................12
Tube Life........................................................................................................................12
Changing Tubes...................................................................................................................13
Changing the AC and B+ Fuses......................................................................14
Cleaning.........................................................................................................................14
Transporting MB-125............................................................................................14
CHAPTER 5 .....................................................................................................................................15
SPECIFICATIONS...............................................................................................15
CHAPTER 6 .....................................................................................................................................16
WARRANTY..............................................................................................................................16
APPENDIX.......................................................................................................................................17
Warranty Registration...........................................................................................17
Service Notes...............................................................................................................17
MB-125 Owner’s Manual
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CHAPTER 1
Introduction
Congratulations on your purchase of the VTL MB-125 Monoblock Power Amplifier.
he MB-125 Monoblock Amplifier, a powerful 125 watts per channel amplifier from the VTL
monoblock amplifier family, represents the entry level monoblock using our pure tube design. This
pair of high performance monoblock amplifiers is designed to deliver a most involving and true-to-live
listening experience to you. Proper care and maintenance of the MB-125’s will assure their lasting
value and maximum performance. Please take a moment to read through this owner’s manual and please take
time to get familiar with the features of this product, so that you can become familiar with the operating and
maintenance procedures needed for the MB-125’s. After you have finished reading this manual please keep it
in a safe place for future reference.
T
The VTL team is proud that you have selected the MB-125 power amplifiers as a new member of your home
audio system. We sincerely hope that the VTL MB-125’s will contribute towards your enjoyment of music and
give you many hours of listening pleasure.
Symbol Conventions used in this guide
Certain symbols are used in this owner’s manual to draw your attention to important points being discussed.
For your own safety and that of your equipment you should note and heed the warnings that follow these
symbols.
The “Warning - Pay particular Attention” symbol used is
and the “Warning - Observe These Precautions For Your Safety” is
Electrical Safety Notice
Electrical voltage from power cables is hazardous. We recommend that the power cord connected to this unit
be used with a properly grounded outlet. To prevent electrical shock do not remove any of the covers of this
amplifier, especially while the unit is powered on.
Water and Moisture
The amplifier should not be used close to any water or moisture sources. If liquid enters the amplifier it must
be immediately unplugged and returned to your dealer for servicing. If this occurs you should under no
circumstances try to power the unit on before the unit has been properly serviced by a trained repair technician
- there are hazardous voltages present in this unit that can cause serious injury if they come in contact with you.
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Location and Ventilation
Install this amplifier in a location which is stable and well ventilated. If the amplifier is placed in a
built-in installation, ensure that there is adequate room for a supply of air to flow through the
ventilation openings. Allow at least 9 inches clearance on the top and around the sides of the
amplifier. Be sure the amplifier is at least 10 inches away from your preamplifier to prevent possible
noise introduction into your system. Tiptoes or other isolation accessories may prove useful in reducing
mechanical vibrations. Do not place amplifiers directly on high pile carpets as this will block the
ventilation vents. Do not place the amplifier next to heat sources such as radiators, stoves or other appliances.
Do not place amplifier on the floor where small children can tamper with the equipment. If it is not possible
to place amplifier out of the reach of small children it is recommended that you remove power cables when the
equipment is not in use.
Servicing
Do not attempt to service the amplifier beyond the procedures described in this manual. For all other service
and questions, please contact your authorized VTL dealer or the factory.
Operational Warnings
Do not touch the tubes after the amplifier is turned on. Tubes can get very hot while the amplifiers
are operating. Turn off the amplifier and allow the tubes to cool down before attempting to work with the
tubes.
Do not attempt to disassemble the amplifier chassis or remove any covers from the amplifier.
Always consult with your VTL authorized dealer or the VTL factory support department before
attempting any service work on any VTL unit.
Always connect your loudspeaker to the amplifier before powering the unit on. Operating any VTL power
amplifiers without a speaker load connected can damage the output transformer, and will not be covered
under the warranty. Also, ensure that no speaker cables can become loose during use and that there are no
intermittent faults with the cables.
Tube amplifiers can be heavy and awkward to lift, with the weight unevenly distributed, and you should
not attempt to move the unit without help. Each unit of the MB-125 monoblock weighs approximately 50
lbs (22 Kg).
Do not attempt to make or break any connections to the amplifier while the unit is powered on as this can
damage the amplifier and/or the rest of the system, and this damage is not covered by warranty.
Do not flip the triode tetrode switches while the unit is powered on, as this can damage the switch and is
not covered under warranty.
Do not exceed fuse ratings or attempt to bypass any fuses, as this can cause an extremely hazardous
condition and will void any warrantees. Use only the same type and rating of fuses as specified in the
owners manual and marked on the unit.
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CHAPTER 2
Getting Started
Unpacking the MB-125 from its box
The MB-125 monoblocks are shipped in cartons and wrapped in thick plastic. The plastic is not strong
enough to support the unit, and may tear if you try to lift the unit out of the box with it. Also there are
protruding switches which could break if the unit is not properly handled, and in addition to the awkward,
unbalanced heavy load the unit has a cleaning polish on it which makes it slippery and hard to grasp. We have
found that the best way to remove the amplifier from the box is as follows:
1. With the box on a thick carpeted floor, fold back all the flaps of the top of the carton, leaving all
the packing foam in place. Roll the carton onto one long side on top of the flap.
2. Roll the box once more onto the open top and, making sure that none of the flaps is trapped
under the carton, lift the carton off the unit.
3. Lift the top foam away from the upside down unit and carefully cut away the plastic from around
the amplifier.
4. Lift the amplifier up and away from the packaging either by holding onto the metal handles on
the front of the unit or by rolling the unit onto a short side and then again so that it lands on its
four rubber feet. Then lift away the packaging and set all the packaging in the carton.
5. When lifting the unit be sure to only lift it from the bottom under the transformers. Be careful
not to break any switches. Be sure to only set the amplifier down on its bottom side on the four
rubber feet on a stable surface. Setting it on any other side may damage protruding components.
If the unit is too heavy or too awkward to lift then do not attempt to do this by yourself, but rather
find someone to help you. Save the carton and all packaging for any future shipment of the amplifiers.
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The MB-125 Front Panel
The MB-125 Front Panel showing the Power on switch and the power on LED indicator.
Check for tube placement
Use the following procedures to check whether the tubes standing on the deck of your amplifier are placed
firmly in their sockets, as the tubes can become dislodged and loosen during shipping of the amplifier.
Visually inspect through the vent holes to ensure that all tubes are fully seated before powering the unit on for
the first time. If there are any tubes not fully seated then you should refer to your VTL dealer, and take this
procedure with the amplifier.
Notes for the servicing technician:
1. Remove the protective wire cage on the top deck of the amplifier. The cage is mounted with
three screws on each side of the amplifier. Loosen the screws and lift the wire cage slowly and
carefully. Do not let the cage touch or hit any of the tubes inside the amplifier. Keep the cage
and the screws in a separate place while you’re working on the amplifier.
2. Make sure that the amplifier is not connected to the power outlet and not turned on. Use your
fingers to hold onto the upper portion of the tube towards its tip. Gently rock the side of the
tube to see if there is any movement in the tube. If there is any movement, that means the tube
has come loose. Press the tube firmly into its socket, using a gentle force and a slight “rocking”
motion.
3. Repeat the above test for the rest of the tubes. There are a total of four output tubes and two
input tubes in each amplifier.
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4. After you’ve examined all the tubes to make sure that they are firmly placed in their sockets, you
can put the protective cage back onto the amplifier unit. Tighten the screws to make sure that
the cage is securely fastened to the unit.
MB-125 Back Panel
The MB-125 Back View
Connecting Your Amplifier to your system
The MB-125 amplifier is a self-contained unit with the amplifier and power supply sections on one
chassis. The two sections are assembled together in the factory and connected to each other via power
cables. The amplifiers are setup to work in this configuration, and under no circumstances should you
attempt to take the amplifier apart.
1. Connect all source components (e.g. CD, Tuner, Tape, DAT, Turntable etc.) to the preamplifier.
Follow the instructions on your preamplifier and source component manuals to connect these
components together.
2. Connect the power amplifier to the output channel of the preamplifier. The interconnect cable
between the amplifier and the preamplifier links the input channel of the power amplifier to the output
channel of the preamplifier. The monoblock amplifier which is going to drive the left channel of your
speaker system should be connected to the left output channel of the preamplifier. The same applies to
the right channel amplifier.
3. Select the input connector type. The amplifier supports only one type of input connector – the RCA
input. Place the interconnect cable firmly into the input jack which matches the type you have, and firmly
insert the interconnect cable into the input jack marked “INPUT”.
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4. Connect the loudspeaker cable to your amplifier. There is a pair of speaker binding posts in the back
of your amplifier for connecting your loudspeaker cables to the amplifier. These binding posts are marked
“+” and “-“ above the posts and “SPEAKER” in between the posts. For example, if you are connecting
the monoblock amplifier to the left channel speaker, you should take the speaker cable connected to the
left speaker and connect it to the amplifier’s “SPEAKER” binding posts. First, loosen the binding posts
on the amplifier. Take the red connector from the speaker cable and connect it to the “SPEAKER”
binding post marked “+”. Take the black connector from the speaker cable and connect it to the
“SPEAKER” binding post marked “-“. Tighten the binding posts by hand, using a nut-driver (3/8”). Be
careful not to over-tighten the posts, as you may break the connection inside the amplifier if you exert too
much force on the posts.
Voltage Setting
Your power amplifier has been set by the factory to the correct voltage for your country where you made your
purchase. The voltage setting should be marked on the Serial Number Badge located on the back panel of
your amplifier. Check to make sure that this complies with your local voltage rating before plugging in and
turning on your amplifier.
Your amplifier is shipped with a power cord that matches the requirements of your country. If you need
a replacement and you are not sure what the appropriate cord should be, consult your authorized VTL
dealer or the VTL factory customer support department.
Power Source for your Amplifier
Your MB-125 Monoblock power amplifier is a high-performance, high power amplifier capable of outputting up
to 125 watts per monoblock. To assure that you can operate your power amplifier under the best conditions
for the optimal results, you should plug the unit directly into a wall AC outlet, preferably one separate outlet for
each unit. Do not plug your amplifier into a light extension cord or into the back of another component. This
will starve the amplifier of current and significantly impact the performance of your system.
See the chapter on Specifications for the power consumption requirements of your amplifier.
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CHAPTER 3
Operating the Amplifier
Powering your system on
After you have properly connected your loudspeakers to your power amplifiers and all your source
components and the power amplifiers to the preamplifier you are ready to power your system on.
1. If you haven’t already done so, visually (by looking through the cage vent holes) ensure that all the tubes
inside the VTL amplifier are properly seated all the way into their sockets. Make sure that the power
amplifier is not powered on at this stage by either disconnecting it from the AC source or turn the power
on switch on the front panel of the amplifier to the “off” or “0” position.
2. Power on the source component(s) you intend to use for your listening session
3. Power on your preamplifier. Make sure that your preamplifier is either in the mute state or turn its volume
knob to the zero position.
4. Using the power cord which comes with your amplifier, connect the one end of the power cord to the
“AC POWER” connector on the bottom angle of the back of the amplifier. Connect the other end of the
power cord to an electrical outlet directly into a wall socket.
5. Turn on the power amplifier in your system one monoblock at a time so as not to trip a circuit breaker or
blow a fuse. The power-on switch is located on the right hand corner of the amplifier’s front panel. When
the unit is powered off, the switch points in the “0” position. Clicking the switch once turns on the unit
and the switch is in the “1”. There is a green power-on LED to the left of the front panel. The LED light
should be on when the system is properly powered on.
1. Repeat the above step for the other power amplifier in your system. Do not turn on both amplifiers at
the same time. This might cause the circuit breaker in your house to trip or blow a fuse in your
fuse box.
Caution: When you turn on your amplifier for the first time, watch the tubes while they warm up
and observe if any of them goes into an exceptionally bright red glow on the plate of any of the
tubes. Turn the system off immediately and refer to your authorized VTL dealer or the VTL factory
service dept. if this happens. This glow may indicate a defective tube which needs to be
replaced.
Caution: If powering on the system using the above steps results in tripping a circuit breaker in
your house, or blowing a fuse on the amplifier, then turn off the power amplifier and the rest of the
system immediately. Find the fuse box and turn the circuit on again to restore power to your
house. Contact your VTL authorized dealer or the VTL factory customer support department
immediately so that they can help you resolve this problem.
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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.
ΖCaution: 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
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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.
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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.
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Top view and Bias Measurement points of the MB-125 Amplifier
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CHAPTER 4
Care and Maintenance of your VTL
Amplifier
Break In Period
Your VTL amplifier is a pure tube product designed to give you the continued optimum performance over a
long time period. Even though the amplifier has undergone a burn-in cycle in the VTL factory, the tubes and
circuits will require further usage and burn-in to reach maximum performance. During the first 100 hours of
usage the amplifier will undergo noticeable improvements in sound.
Power-on Period
We recommend that your amplifier should be powered-off when you are done with your listening session.
Leaving your amplifiers running idle when you are not listening to music will be a waste of electricity as well as
unnecessary usage of the tube life.
Tube Types
The VTL MB-125 Monoblock amplifier uses a total four EL34 tubes for the output stage and two 12AT7
tubes for the input and driver stages.
Tube Life
Your VTL amplifier has been designed to ensure long tube life. Tube replacement need not be considered
until after approximately 3000 hours of use (roughly 3 to 4 years depending on your listening requirements.)
As the tubes age beyond their peak performance they begin to lose their ability to provide full power. This
usually results in some loss of control in the bass and a general softening of the sound in the upper frequencies.
We recommend a complete replacement of all tubes in your amplifier at that time, which will restore it to a
“like new” sound quality. Your VTL dealer or VTL factory service department will be happy to assist you with
the re-tubing process.
Note: use only tube types and tube brands that are recommended by VTL. VTL specified
replacement tubes are available from your authorized VTL dealer or the VTL factory
service department. Any damage incurred to units which use non-VTL approved tubes will
not be covered under the warranty.
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Changing Tubes
Do not attempt to change tubes yourself. Contact your authorized VTL dealer or the VTL factory service
department so that this service can be performed by a trained technician. You will need to take the unit and
this procedure to your dealer for them to perform this procedure and the corresponding bias procedure
properly.
Notes to the Service technician:
Be sure to only touch the tubes themselves, and do not allow any part of your body or hanging jewelry to come
into contact with any part of the circuit inside the unit.
1. Diagnose the tubes: If you are not undergoing a complete retube for the entire amplifier, then
you must first decide how many tubes and which tubes should be changed. Use the bias
checking procedure as documented in the Getting Started section of this manual to find out how
to measure the bias setting of the tubes and to determine whether they are within the acceptable
range. For example if you find out from the bias measurements that tube #1 and #3 are
fluctuating frequently and they are above or below the acceptable range this may indicate that a
replacement of these two tubes is necessary. A full power test of the amplifier may need to be
undertaken to ensure that all power tubes are conducting equally.
2. Power off the amplifier: To change tubes in this amplifier you should first power the unit down,
unplug the unit from the wall outlet and disconnect the amplifier from the rest of your system.
Let the amplifier stay power-off for at least 15 minutes before you start working with it. Make
sure the tubes are cool before you touch them. Using a Philips #2 point screw driver remove
the protective cage from the unit by loosening the two screws from the side of the amplifier.
Carefully remove the cover and the screws and store them in a safe place.
3. Removing Tubes: Locate the tube you need to remove and hold onto the upper portion of the
tube towards its tip. Gently rotate and rock the tube with your fingers to loosen it from its socket
until its pins are completely disengaged from the socket. Take the tube out of the unit.
1. Plugging in a new tube: Hold onto the upper portion of the tube towards its tip. Lower the
tube onto the socket, making sure that the pins from the
tube matches the holes in the socket. There are either two
pins on the tube which are spaced at a wider distance from
each other than the rest of the pins (9 pin miniature tubes),
or there is a locating keyway in the middle of the tube (8 pin
octal tubes). Make sure that these locators go into the side
of the socket which matches. Press the tube firmly into its
socket, using a gentle force and a slight “rocking” motion.
When the tube is properly and completely inserted into the
socket it should be firmly implanted and does not give in to
any movement at all when you try to rock it.
2. Replacing the protective cage: Lower the amplifier’s
cover chassis slowly back into the opened amplifier box,
making sure that the cover is put on in the correct front and
back position. Locate the screws which were removed from
the cover and put them back in the correct holes. Tighten
MB-125 Owner’s Manual
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all screws to make sure that the cover is securely fastened to the unit.
Changing the AC and B+ Fuses
The B+ fuse for your amplifier is rated at 1.0A fast acting ceramic. There is also one primary mains fuse rated
at 6A Slo Blo (100/120Volt AC) or 3A Slo Blo (220/240 Volt AC) ceramic. The B+ fuse is housed in a holder
located on the back panel of the amplifier, and the mains fuse is housed in a separate holder also located on the
back panel of the unit. To change either fuse make sure the unit is powered off, and disconnect the AC power
cord from the unit. Move the amplifier so that you can work with the panel in the back. The type of fitting
used for both fuses is called a bayonet fit, which describes the action needed to remove the fuse cap. Locate
the appropriate fuse holder and to loosen it turn the screwhead approximately one quarter of a turn in a
counter-clockwise direction while pressing inwards. The fuse cap should spring out from its housing with the
fuse held inside the cap. Take the fuse out and replace it with a new one, and insert the fuse cap back into the
holder, pushing it downward while turning it in the clockwise direction to lock it in place.
WARNING: For continued protection against fire hazard only replace the fuse with the same type and
rating as was originally specified for the amplifier. If you have problems locating the correct fuse
contact your VTL dealer or the VTL factory service department.
Cleaning
Your VTL amplifier should be dusted occasionally with a damp non-abrasive cloth. You should not use any
solvents for cleaning the front panel, as this can damage the lettering and the finish. It is recommended that
you use a contact cleaner such as Pro-Gold on the input and output connectors regularly to ensure optimum
sound and proper signal conducting capability.
Transporting MB-125
If you should ever need to ship your amplifier and need help doing so then ask your VTL authorized dealer to
give you assistance in packing the unit into the cartons for the amplifiers. Your dealer should also be able to
arrange for new cartons to be sent to you and help you with the packing should you require it.
MB-125 Owner’s Manual
VTL 14
CHAPTER 5
Specifications
MB-125 Monoblock power amplifier
Vacuum Tube Complement 4 x EL34, 2 x 12AT7
Output Power
20 Hz – 20kHz, into 5 ohm load
Tetrode = 100 Watts
Triode = 50 Watts
T.H.D. 20 Hz – 20 kHz @ 125W
T.H.D. 20 Hz – 20 kHz @ 60W
Tetrode: < 3%
Triode: < 3%
Input Sensitivity @ 125 W 700mV
Input Impedance 145,000 Ohms
Optimum Load Range 2 - 8 ohms
S/N Ratio @ 125W, tetrode
S/N Ratio @ 60W, triode
-95dB, 120 Hz
-95dB, 120 Hz
Power Consumption Idle = 100 W
Full Power = 300 W
Primary Mains Fuse Rating 100/120V = 6A Ceramic Slo Blo
220/ 240V = 3A Ceramic Slo Blo
B+ Fuse Rating 0.75A fast acting ceramic
Dimensions W = 16 inches (40cm), D = 12 inches
(30cm), H =7 inches (18 cm)
Weight 42 lbs (22Kg) per monoblock {total 45 lbs
(25 Kg) packed}
MB-125 Owner’s Manual
VTL 15
Table of contents
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