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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 and adjusts the bias of each tube
individually. If one tube fails, it is not necessary to replace all output
tubes with a matched set. If a single tube fails, that single tube can
be replaced with a tube of the same type, but it does not need to
be the same grade as the other tubes in the amplifier. Visit
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 33mA (NORMAL) to 39mA (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.
8
8
Fender Automatic Bias
Fender Automatic Bias
AUTOMATIC BIAS
WARM
ADJUST
NORMAL
2A 1A 1B 2B
COOL
OUTPUT TUBE STATUS
PATENT PENDING
www.
fender.com
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-
The tubes are warming up. You should always wait longer
ing left to right:
than one minute before taking the amplifier out of STANDBY.
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
The tube is OK and operating at the desired bias level (WARM/
green:
NORMAL/COOL).
LED is green and
The bias for that tube is being adjusted. It can take up to 20
blinking:
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.
Within either pair
The tube indicated with the red LED has failed, and should be
(1A/1B,
2A/2B)
replaced (refer to the Tube Replacement section on next page
one LED is red
for instructions). The tube indicated with the orange LED, has
and the other LED
been shut off to allow the amplifier to run safely with the
is orange:
remaining pair of tubes (green LEDs).
NOTE: If a tube fails it is safe to continue operating your
amplifier with the two remaining tubes. However, the ampli-
fier will produce less power and for best performance the
speaker impedance switch {S} should be set to one-half of the
total speaker impedance. Example: If you have a 4Ω speaker
system connected, and a tube fails, set {S} to 2Ω.
Within either pair
Sometimes, if a tube fails under certain high level signal con-
(1A/1B,
2A/2B)
ditions, Fender Automatic Bias cannot determine which tube
both
LEDs
are
of the pair has failed, and displays both in the pair red. It is still
red:
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
(dispose 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
(dispose of the removed tube properly). Repeat steps
b-e.
All LEDs are red
Multiple tube failures, the amplifier will be silent. Service each
or orange:
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.
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