SVT-2 PRO Bass Amplifier
Changing the Tubes
Tubes wear out in direct proportion to how often and how hard you play the amplifier. Power tubes should be checked at least once a year – more frequently if you use the amplifier nearly every day. When the power tubes wear out, the ampli- fier 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 rec- ommend 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 tubes in the
•Turn the amp off, unplug it and let it cool for at least 5 minutes.
•Remove the screws which hold the four large plastic feet to the rear of the chassis. Set the feet and their screws aside.
•Taking care not to stretch or disconnect any wiring, gently set the perforated metal screen aside.
•Remove the tube retainer(s) by lifting them off the tube 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. Preamp tubes have a “missing pin” which corresponds with the “missing hole” in the socket – line up the missing pin and hole before pressing the tube into its socket.
•Replace the tube retainer(s) on the tube(s).
•Replace the perforated screen, the four large plastic feet, and tighten their 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 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 overbiased. 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 underbiased and too much current is flowing in 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 different. There is no need to fiddle 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:
Condition | Problem | Solution |
Green comes on, then red | No problem | (The longer the green LED is on before the red |
|
| LED comes on, the better matched the tubes are.) |
Red comes on, then green Tubes not properly matched | Set slightly before green comes on, obtain | |
|
| matched tubes when possible. |
Red comes on, no green | One or more tubes are | Check to make sure tubes are all seated properly; |
|
| if so, find and replace bad tube(s). |
None on | Possibly no high voltage or bad | Have unit checked by a service technician. |
| Bias Control or bad tube(s) |
|
Both on all the time | Possible bad Bias Control or bad tubes | Have unit checked by a service technician. |
If the tubes are bad enough to cause damage to the unit, the Fault Indicator (#16, front panel) will signal and the unit will shut down.
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