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PF-20T / PF-50T Bass Guitar Amplifier
12
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. One LED should
be lit green and the other LED off. If not, turn the Bias control until the LED is 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 Power
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 other Bias LED illuminates 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 the Bias control so that only the green LED is illuminated.
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 the Bias to JUST AS the green LED illuminates. For shorter tube life, but better tone, set the
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.
By observing the LEDs as the Bias control is slowly rotated clockwise, a number of tube problems may
be diagnosed by the user as seen in the table below.
NOTE: Depending upon the bias setting, the red bias LED may light or flicker when signal is applied
to the amp input; this is a normal condition.
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 non-functioning 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 Bias Have unit checked by a service technician.
Control or bad tube(s)
Both on all the time Possible bad Bias Control or bad tubes Have unit checked by a service technician.
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