9
PF-20T / PF-50T Bass Guitar Amplifier
Changing the Tubes
Tube life is directly aected by how often and how hard you play the amplier. Power
tubes should be checked at least once a year—more frequently if you use the amplier
nearly every day. When power tubes wear out, the amplier will begin to grow weak,
lack punch, fade up and down, or lose highs and lows. Power tubes work together in a
“push/pull” conguration and should all be replaced at the same time with a matched
or balanced tube set. Your dealer can recommend the best replacement tubes for your
amplier.
Preamp tubes aren’t worked as hard as power tubes and typically last longer. When a
preamp tube wears out, the amplier may squeal, get noisy, lose gain and sensitivity, or
just quit working. A service center can determine which tube(s) may need replacing.
To access the tubes in the PF-20T / PF-50T, the rear screen must be removed.
Qualified
service persons
may use the following steps to change the tubes.
Changing Power Tubes
• Turn the amp o, unplug it, and let it cool for at least 5 minutes.
• Remove the screws that hold the perforated metal screen to the rear of the cabinet
(Figure A).
• Set the perforated metal screen aside.
• Remove the tube retainer(s) by lifting them o the tube(s) and moving them to one
side.
• 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 power tube, but it is removed / attached similarly
to the preamp tubes (see “Changing Preamp Tubes” on page 10).
• Using a soft cloth, carefully replace the power tubes by handling them as closely to
the base as possible and rmly pressing down into the tube sockets.
Power
Tubes
Preamp
Tubes
Figure A Figure B (PF-20T) Figure B (PF-50T)
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