Conrad-Johnson LP260m mono-
block power amplifier Owner’s Manual
7
Adjusting Output Tube Bias
Biasing the amplifier is a simple procedure that can easily be done
by the owner. Only a plastic screwdriver is required (one has been
included with your amplifier for this purpose). After replacing the
tubes, the amplifier should be connected to a load (your
loudspeakers will work fine) and there should be no signal applied
to the amp. It should be connected to your preamplifier with the
volume control turned fully down. Turn the amplifier on. After about
one minute, turn each of the eight screwdriver adjustable controls
(located near the output tube sockets) counterclockwise until the
associated indicator LED just goes off. Clockwise rotation may be
necessary first to turn the LED on; do not turn the adjusting screw
any further in the clockwise direction once the LED has come on.
After the amplifier has warmed up thirty minutes, the procedure just
described should be repeated. NOTE: It is normal for these LEDs
to flash or flicker when the amplifier is in use. This biasing
procedure should be performed whenever the amplifier's output
tubes are replaced and after each six months of average use.
PLATE FUSE: Your LP260m mono-block power amplifier has been
fitted with a plate supply fuse (5x20mm 2Amp fast-blow ceramic)
that is installed to protect the amplifier circuitry in the event of
output tube failure. The fuse holder is physically located inside the
amplifier in fuse clips on the power supply board.
What happens when the fuse blows? A blown plate fuse usually
indicates a marginal output tube and an attempt should be made to
identify and replace it, by either (a) replacing the fuse and turning
on the amp while watching the output tubes carefully; look for a
bright white or blue flash inside the tube. If nothing happens, then
(b) lightly tap the output tubes one by one with the plastic handle of
a screwdriver while watching for flashes inside the tube. If the
tubes are over two years old, or have more than 1,500 hours on
them, a complete replacement tube set is probably in order.
The sonic effects of a blown fuse are very low, very distorted
sounds from the speaker being driven by that channel.
TO AVOID SHOCK, ALWAYS UNPLUG THE AMPLIFIER AND
LET IT SIT FOR 30 MINUTES BEFORE REMOVING BOTTOM
COVER.
Replacement fuses are available from conrad-johnson dealers.
