Ampeg Svt-Cl - Manual (Page 9)

SVT-CL Bass Guitar Amplifier
9
Changing the Tubes
Tubes wear out in direct proportion to how often and how hard you play your amplifier. Power tubes
should be checked at least once a year - more frequently if you use the amplifier nearly every day. When
power tubes wear out, the amplifier will begin to grow weak, lack punch, fade up and down, or lose highs
and lows. Power tubes work together in a push/pull configuration and should be replaced at the same
time with matched or balanced tubes. Your dealer can recommend the best replacement tubes for your
amplifier.
Preamp tubes aren’t worked as hard as power tubes and typically last longer. When a preamp tube wears
out, the amplifier may squeal, get noisy, lose gain and sensitivity, or just quit working. A service center
can determine which tube(s) may need replacing.
To get to 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.
Qualified service persons may follow these steps to change the tubes:
• Turn the amp off, 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 off 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 up on 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 firmly into place by pushing down on its top.
• Replace the perforated metal screen and screws.
• Power up the amplifier and let it sit for at least 20 minutes. Bias the amplifier as directed in the
section below.
Setting Tube Bias
Turn the power on and allow the unit to sit in ‘Standby’ for 3-5 minutes (after following all normal setup
requirements). Next, take the unit out of ‘Standby’ and do a quick check of the Bias LEDs on each control.
Both LEDs should be lit green. If not, turn the Bias controls until the LEDs are lit green. If this seems
impossible, please refer to the chart on the next page for possible fault conditions. Now is a good time to
check for any unusual sounds and possible glowing from the Power Tubes (see ‘Changing the Tubes’
section listed above).
At this time, play your bass for at least 20 minutes to allow the unit to warm up at proper AC line voltage.
You may notice that the Bias LEDs illuminate red while playing. This is normal.
Next, turn down all controls on your bass and set it aside, leaving all amp controls alone. With no input
signal present, adjust each Bias control so that only the associated green LED is illuminated. The controls
may be slightly interactive, as they do affect each other.
So where does one set the Bias? If neither LED is lit, the amp is over-biased (counter-clockwise). This will
result in some distortion in the power amp and a generally thin sound. If the green and red LEDs are lit,
the amp is under-biased (clockwise) and too much current is flowing to the power tubes. This will result
in a big, full sound, but will also reduce the life of the power tubes. For the longest tube life, but poorer
tone, set each Bias to JUST AS the green LED illuminates. For shorter tube life, but better tone, set each
Bias to JUST BEFORE the red LED illuminates.
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
vary slightly. This is normal.