I was going to type out a lengthy explanation of exactly what tube bias is and what it does, but them I remembered that Fender has a pretty good explanation in the 100t owners manual, so here is a copy and paste of that section:
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 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.
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Auto-Bias Feature:
Adjusting the tube BIAS:
NOTE: To preserve the amplifiers 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.
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.
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.
Within either pair (1A/1B, 2A/2B) 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 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 (1A/1B, 2A/2B) both LEDs are red:
Sometimes, if a tube fails under certain high level signal con- ditions, 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) Replacebothtubesthatwereindicatedred,andtheappro- 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. Replaceeitherofthetubesthatwereindicatedred,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 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.
