10
P. TUBE TROUBLE - This LED indicates tube failure
or trouble at the tube’s respective location. When the
LED glows green, the tubes are operating properly.
When the LED glows red, this indicates a tube failure
which has blown the fuse at the respective fuse
location on the top surface of the chassis. NOTE: It is
normal for the LEDs to glow RED when the amp is in
STANDBY mode. 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. If
the amplifier repeatedly blows fuses, it should be taken
to an authorized Fender service center.
Q. BIAS - This trim adjustment is used in conjunction
with the BALANCE and the bias test points to set the
output tube bias.
R. BIAS 100mV & BALANCE 0V - These are bias
test points used to measure output tube bias and
balance with a digital voltmeter.
S. BALANCE - This trim adjustment is used in
conjunction with the bias test points and the BIAS
adjustment to set the bias balance between the
power amp tube sections of your 300T.
Output Tube Bias Adjustment Instructions
1. Turn on your 300T and let it warm up for at least two
minutes with the STANDBY switch in the ON position. Make
sure the amplifier is properly connected to a speaker.
2. Remove the bias controls cover box.
3. With a digital voltmeter set to its most sensitive DC
voltage scale, measure the voltage between the BIAS
100mV test points (test points 1&2) and adjust the BIAS
control for a reading of .1 VDC (100 mVDC).
4. Next, connect the digital voltmeter to the BALANCE 0V
test points (test points 2&3) and adjust the BALANCE control
for a reading of 0 VDC.
5. Replace the bias controls cover box.
Notes
1. Make bias and balance adjustments ONLY when
necessary. If you are unsure about setting bias and
balance, take your amp to the nearest authorized Fender
Service Center.
2. For optimum sonic performance, set bias to 100mVDC
then set balance.
3. If the output tubes can not be biased to at least
80mVDC, replace the 6550C output tubes.
4. If the output tubes can not be balanced, check the bias
setting and/or replace the 6550C output tubes.
5. The power tube trouble LEDs are NOT indicators of worn
tubes. If your tubes are worn and sound weak the LEDs
may still glow green. The trouble LEDs turn RED only after
tube failure has already occurred. DO NOT WAIT FOR
TUBE FAILURE TO REPLACE TUBES. (See Tube
Replacement Section, page 11).
300T OUTPUT TUBE BIAS INDICATORS AND CONTROLS
Q
R
S
1
2
3
replace with
T100mA fuse only
replace with
T100mA fuse only
550C 6550C 6550C 6550C 6550C 6550
P
P
P
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