Fender Rumble Bass Owner's Manual - page 6
RUMBLE BIAS AND BALANCE ADJUSTMENT
WARNING:
Bias and balance are pre-set at the factory. Make adjustments only when necessary. If you are
unsure about setting the bias and balance, take your Rumble Bass to the nearest authorized Fender
service center.
1. Turn on the amplifier and let it warm up for at least three minutes with the STANDBY switch in the
ON position.
2. Remove the front grill to gain access to the bias and balance controls.
3. With a digital voltmeter set to its most sensitive DC voltage scale, insert the Common Probe into
the test point BALANCE. Insert the other probe into the test point BIAS2. Adjust the BIAS2
trim until 90 mVDC is obtained. Remove the Common Probe from the BALANCE test point
and insert it into the BIAS1 test point. Adjust the BIAS1 trim until 0 mVDC is obtained. It is
advised to recheck BIAS2 to confirm that 90 mVDC is still present. If not, repeat procedure.
4. With a speaker (or equivalent) load connected to the MAIN SPEAKER jack, run a 1 kHz sine wave
through the amplifier. Connect an oscilloscope to the positive lead of a 1/4” phone plug so that
you can view the output waveform. Adjust the output signal so that it is just into clipping. Adjust
the BALANCE control so that the viewed waveform clips evenly on both the positive and
negative peaks.
NOTE:
1. For optimum performance, set the bias to 90 mVDC, then set the balance.
2. If the output tubes cannot be balanced, check the bias setting and/or replace the
6550WA output tubes.
RUMBLE REAR PANEL FUNCTIONS
A. LINE CORD - Your Amplifier is equipped with a grounding type supply cord to reduce the pos-
sibility of electrical shock. Be sure to connect it to a grounded AC receptacle. DO NOT ALTER
THE AC PLUG.
B. POWER - This switch turns the AC power on and off. When the switch is off (down) the amplifier
is completely shut down.
C. FUSE - The fuse is in the AC supply of the amplifier and will protect the amplifier and the oper-
ator in the event of an electrical fault.
D. STANDBY - When in the STANDBY position the amplifier is off. However, the tube filaments are
left on as to eliminate warm up time, provided that the POWER switch is on.
SERIAL NUMBER
CAUTION:
CHASSIS SURFACE HOT
WARNING:
TO REDUCE THE RISK OF FIRE OR
ELECTRIC SHOCK, DO NOT EXPOSE THIS EQUIPMENT
TO RAIN OR MOISTURE
AVIS:
RISQUE DE CHOC ELECTRIQUE NE PAS OUVRIR
ATTENTION:
SUPERFICIE DE CHASSIS CHAUDE