SVT CLASSIC
Changing the Tubes
The performance characteristics of tubes are degraded in direct proportion to how often and under what conditions
the amplifier is used. 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 typically last longer than power tubes. 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 gain access to the power tubes in the SVT CLASSIC, the rear screen must be removed and the tube retainer(s)
must be moved out of the way. Tube replacement should only be performed by a qualified service person.
• 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 tube retainers.
• Replace the perforated metal screen and tighten its 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
Allow the unit to warm up at the 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 illuminated. The controls may be slightly interactive. If neither
LED is illuminated, 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 illuminated, the amp is under biased and too much current is flowing 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 causing the LEDs to react slightly different. There is no need to readjust the bias
as these slightly different conditions occur. Note that it is normal for the red LEDs to illuminate 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:
Condition Problem Solution
Green illuminates, then red No problem The longer the green LED is illuminated before the
red LED illuminates – the better matched the
tubes are.
Red illuminates, then green Tubes not properly matched Set to just before the green LED is illuminated.
Obtain matched tubes whenever possible.
Red illuminates, no green One or more tubes are non-functioning Check to make sure tubes are all seated
properly. If so, find and replace the bad tube(s).
None illuminated Possibly no high voltage or bad Have unit checked by a qualified service technician.
Bias Control or bad tube(s)
Both illuminated all the time Possible bad Bias Control or bad tubes Have unit checked by a qualified service technician.
If tube wear is bad enough to cause damage to the unit, the Fault Indicator (#11, Front Panel) will flash between red
and green and the unit will shut down.
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Power
Tone Control Range
Weight and Dimensions
