Fender Bassman 300 Service Manual page 5

On the top side of the chassis, at the rear of each pair (total = 3) of output tubes, is an LED and a fuse. The LED glows green
when the associated pair of tubes is operating properly, and glows red if a tube failure has caused the related fuse to blow. If a
fuse blows, replace it only with one of the same type and rating. NEVER use a fuse with a higher current rating, as this could
damage the equipment and present a serious safety hazard. Next, replace the tubes in their respective locations, check and
adjust the bias and balance.
NOTE: it is normal for the LEDs to glow red when the amp is in standby mode.
1. Turn on the 300T and let it warm up for at least two minutes with the STANDBY switch in the ON position. Make sure the
amplifier is connected to a speaker or equivalent load.
2. Remove the bias controls cover box.
With a DVM set to it's most sensitive DC voltage scale, measure the voltage between the BIAS 100mV test points, and
3.
adjust the BIAS control for a reading of .1 VDC (100mVDC)..
Next, connect the DVM to the BALANCE 0V test points, and adjust the BALANCE control for a reading of 0 VDC.
4.
5. Replace the bias controls cover box.
NOTES:
1. For optimum sonic performance, set bias to 100 mVDC first, then set balance.
2. If the output tubes cannot be biased to at least 80 mVDC, replace the 6550C output tubes.
3. If the output tubes cannot be balanced, check the bias setting and/or replace the 6550C output tubes.
4. If the power tube trouble LEDs are glowing green, it does not indicate that the tubes are not worn or weak.
Tube layout:
CIRCUIT DESCRIPTION (CONT)
Output section
Output tube bias adjustment instructions
BASSMAN 300
(This is the model name for warranty claims)