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PF-20T / PF-50T Bass Guitar Amplifier
11
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 tubes in the PF-20T / PF-50T, the top screen must be removed first.
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 tube cover to the top of the chassis [Figure A].
• Set the perforated tube cover aside.
• Using a soft cloth, grasp the POWER tube as closely to the base as possible and gently work
it out of its socket by rocking it slightly back and forth as you lift up on it [Figure B].
NOTE: The retainers at the base may be spread apart to help remove the tube.
NOTE: The PF-50T has a 12AU7 Phase Splitter Tube in the powe section, but it is removed /
attached similarly to the Preamp Tubes (instructions below).
• Using a soft cloth, carefully replace the POWER tubes by handling them as closely to the base
as possible and firmly pressing down into the tube sockets.
• Grasp the PREAMP tube retainer at its top, push down and twist left [counter-clockwise] until the
tabs line up with the notches and lift up on it to remove [Figure B].
• Using a soft cloth, grasp the PREAMP tube as closely to the base as possible and gently work
it out of its socket by rocking it slightly back and forth as you lift up on it.
• Using a soft cloth, carefully replace the PREAMP tube by handling it as closely to the base
as possible. Line up the connector pins with the socket, then press down into the tube socket
to lock the tube into place.
• Replace the perforated tube cover and screws.
• Power up the amplifier and let it sit for at least 20 minutes. Bias the amplifier as directed
on the next page.
Figure B [PF-50T]
Preamp Tubes
Power Tubes
Figure B [PF-20T]Figure A
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