Fender Super Bassman, Owner'S Manual

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Fender Automatic Bias Fender Automatic Bias
Many musicians desire the rich
and full tones produced by a
tube amplifier, but heavy amps
full of glass tubes which must
be maintained and occasionally
serviced can be intimidating.
The goal of Automatic Bias is to
maximize tube life and ensure
peak performance, allowing the
musician to focus on what is important... the music!
BIAS – What is it? Why is it important?
The bias setting on your tube power amplifier determines the idle
current flowing through each tube. In other words, the bias setting
(WARM/NORMAL/COOL), determines how much power is flowing
through each tube at idle (while the amplifier is not being played).
Tubes are complex devices that are difficult to manufacture in a
consistent way. To account for this, tubes are often measured and
then given "grades" to sort them by performance characteristics. All
tubes of the same grade can be set at the same bias, which is why
in traditional tube amplifiers (without Fender Automatic Bias) it is
necessary to replace all output tubes at the same time and with a
"matched set" of the same grade. Furthermore, if the new matched set
is not the same grade as the old matched set the amplifier would need
to be taken to a service center to have the bias adjusted.
Fender Automatic Bias monitors the bias of each tube individually.
If one tube fails, it is not necessary to replace all output tubes with a
matched set. Simply replace the single failed tube with a tube of the
same type and grade. When it is necessary or desired to replace all six
(6550C) output tubes at the same time, a matched set (of six tubes)
of ANY grade may be used. Visit
www.groovetubes.com for more
information about the tube grading system.
The bias setting affects amplifier tone, dynamics, tube life, and power
consumption in the following ways:
WARM: More aggressive (dirtier) tone, faster attack. Shorter tube
life, higher power consumption.
NORMAL: Typical setting. Good balance between tone and tube
life.
COOL: Less aggressive (cleaner) tone, smoother attack. Longer
tube life, lower power consumption.
Adjusting the tube BIAS:
Press the up/down ADJUST buttons for WARM/COOL settings. Each
button may be pressed 5 times from the NORMAL setting for the full
range of bias adjustment. The range of bias (per tube) is 23mA (COOL)
to 28mA (NORMAL) to 33mA (WARM). The amplifier does not need
to be in standby for this adjustment. Allow 10-20 seconds for the
adjustment to be completed. The LEDs will stop blinking when the
adjustment is complete.
NOTE: To preserve the amplifier’s rich tube tone, Fender Automatic Bias
will not make any bias adjustments while the amplifier is being played.
Fender Automatic Bias will always wait until the amplifier is at idle or
being played at very low levels before making any adjustments.
Press and hold both the up and down arrows to restore the bias to
factory settings (NORMAL). Always restore the bias settings to factory
setting after installing an entire set of new tubes with a different grade
than the previous set.
Output Tube Status LED indicators:
Fender Automatic Bias has one LED indicator for each output tube,
corresponding to the location of each output tube from left to right
on the back of the amplifier. These LEDs show the current status of
the output tubes and can aid in service and trouble-shooting. Here is
a definition of each of the LED's display functions:
GREEN LED mov-
ing left to right: The tubes are warming up. You should always wait longer
than one minute before taking the amplifier out of STANDBY.
The SPEAKER OUTPUT switch must be ON or the amplifier will
remain in the warmup state.
NOTE : If the STANDBY switch is set to ON before one minute
(not recommended), the display will not update until after the
one minute warm up period has passed.
LED is solid
green: The tube is OK and operating at the desired bias level (WARM/
NORMAL/COOL).
LED is green and
blinking: The bias for that tube is being adjusted. It can take up to 20
seconds to complete the adjustment.
NOTE : If an LED blinks continuously for longer than 5 minutes
(after power-up or bias adjustments), that tube is worn out and
should be replaced.
LED is orange: The tube is wearing out and should be replaced with a new tube
with the same tube rating.
Within any pair
(1A/1B, 2A/2B, 3A/3B)
one LED is red
and the other LED
is orange:
The tube indicated with the red LED has failed, and should
be replaced (refer to the Tube Replacement section on next
page for instructions). The tube indicated with the orange LED,
has been shut off to allow the amplifier to run safely with the
remaining pair(s) of tubes (green LEDs).
NOTE : In this condition the amplifier will continue to operate
safely (and allow you to finish your gig), but will operate with
only the remaining power tubes at reduced power.
Within any pair
(1A/1B, 2A/2B, 3A/3B)
both LEDs are
red:
Sometimes, if a tube fails under certain high level signal condi-
tions, Fender Automatic Bias cannot determine which tube of
the pair has failed, and displays both in the pair red. It is still
safe to operate the amplifier in this mode, but it will operate at
reduced power (as discussed above).
There are two service options in this situation (refer to the Tube
Replacement section for tube replacement procedure):
1) Replace both tubes that were indicated red, and the appro-
priate fuse {BB}.
-OR-
2) Troubleshoot to replace only the failed tube. This is the best
option if replacing both tubes is not practical (like on a gig),
and spare fuses are available.
a. Replace either of the tubes that were indicated red, with
a new or known good tube. Keep the removed tube
handy, in case it was not actually the failed tube.
b. Replace the appropriate tube fuse {BB} with the same
type and rating.
c. Turn POWER {M} on, keep the amplifier in STANDBY {N} at
least 1 minute.
d. Take the amplifier out of STANDBY, wait 10-15 seconds for
Fender Automatic Bias to determine the tube status and
update the display .
e. If all LEDs are now green, the bad tube was replaced (dis-
pose of the removed tube properly).
f. If the two LEDs remain red, the wrong tube was replaced.
Keep the removed tube and proceed to step g .
g. Remove the other tube in the pair with the red LED
indicator and replace it with the tube that was removed
during step
a . Repeat steps b-e .
h. If there are still two red LEDs, both tubes had failed.
Replace the tube from step g with a brand-new tube (dis-
pose of the removed tube properly). Repeat steps b-e .
All LEDs are red
or orange: Multiple tube failures, the amplifier will be silent. Service each
pair of tubes as described above. This is not likely to occur.
NOTE : If any tubes are missing (not installed), the LED indica-
tor for that tube will display orange. A pair of tubes may also
display orange if the fuse for that pair of tubes is missing and
Fender Automatic Bias did not detect a tube failure.
All LEDs flash at a
slow rate: The amplifier has been placed in STANDBY {N} or the SPEAKER
OUTPUT switch {Q} has been set to MUTE.
WARM
NORMAL
COOL
OUTPUT TUBE STATUS
AUTOMATIC BIAS
PATENT PENDING
1B 1A 2A 2B 3A 3B
ADJUST