9
Changing The Tubes
Tubes wear out in direct proportion to how often and how hard you use the 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 all 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-VR, 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 puching down on its top.
• Replace the perforated metal screen and tighten its screws.
• Power up the amplifier and let it sit for at least 20 minutes. Bias and balance the amplifier as directed in
the section below.
Setting Tube Bias and Balance
Tube Bias:
Allow the amplifier to warm up to proper AC line voltage for at least 20 minuts. With no input signal present,
adjust each bias control until its adjacent LED illuminates.
The Bias 1 control affects the three power tubes on the left (as viewed from the rear). The Bias 2 control affects
the three power tubes on the right (as viewed from the rear).
Balance:
Insert a 40Hz signal into the amplifier. Adjust the Volume control for approximately 25 VRMS output. Slowly
adjust the Balance control until its adjacent LED illuminates.
SVT-VR Bass Amplifier
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