User Manual

5
SVT-CL Bass Guitar Amplifier
Changing the Tubes
Tube life is directly aected by how often and how hard you play the amplier. Power
tubes should be checked at least once a year—more frequently if you use the amplier
nearly every day. When power tubes wear out, the amplier will begin to grow weak,
lack punch, fade up and down, or lose highs and lows. Power tubes work together in
a push/pull conguration and should all be replaced at the same time with a matched
or balanced tube set. Your dealer can recommend the best replacement tubes for your
amplier.
Preamp tubes aren’t worked as hard as power tubes and typically last longer. When a
preamp tube wears out, the amplier may squeal, get noisy, lose gain and sensitivity, or
just quit working. A service center can determine which tube(s) may need replacing.
To access the power tubes in the SVT-CL, the rear screen must be removed and the tube
retainer(s) must be moved out of the way. Qualied service persons may follow these
steps to change the tubes:
• Turn the amp o, unplug it, and let it cool for at least 5 minutes.
• Remove the screws which hold the perforated metal screen to the rear of the
cabinet.
• Set the perforated metal screen aside.
• Remove the tube retainer(s) by lifting them o the tube(s) and moving them to one
side.
• Grasp the tube at its top and gently work it out of its socket by rocking it slightly
back and forth as you lift upon it.
• When inserting new output tubes, align the tab in the tube’s plastic base with the
slot in the socket and press the tube gently but rmly into place by pushing down
on its top.
• Replace the perforated metal screen and tighten its screws.
• Power up the amplier and let it sit for at least 20 minutes. Bias the amplier as
directed in the section below.
Setting Tube Bias
Allow the unit to warm up at proper AC line voltage for at least 20 minutes. With no
input signal present, adjust each control so that only the associated green LED is lit. The
controls may be slightly interactive. If neither LED is lit, the amp is over-biased. This will
result in some distortion in the power amp and a generally thin sound. If the green and
red LED are lit, the amp is under-biased and too much current is owing to the power
tubes. This will give a big, full sound but will also reduce the life of the power tubes.
Once set, the controls should not have to be changed except as needed for tube
replacement, or to compensate for tube aging. Note that the AC line voltage may vary
from place to place and the LEDs will read slightly dierent. There is no need to ddle
with this every other day. Note that it is normal for the red LEDs to light when there is a
signal present. Bias 1 Control adjusts the three left (as seen from the rear) power tubes.
Bias 2 Control adjusts the three right power tubes. By observing the LEDs as the Bias
Controls are slowly rotated clockwise, a number of tube problems can be diagnosed by
the user, as described in the following table.