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1. BIAS - Used with the bias test points to set the output
tube bias.
2. & 3. BIAS 100mV - Test points used to measure output
tube bias with a digital voltmeter.
3. & 4. BALANCE 0V - Test points used to measure output
tube balance with a digital voltmeter.
5. BALANCE - Used with the balance test points to set the
balance between the power amp tube sections.
Output Tube Bias & Balance Adjustment Instructions:
1. Turn on your 300 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 (2 & 3) and with a screwdriver, turn the the bias
adjustment for a reading of .1 VDC (100 mVDC).
4. Next, connect the digital voltmeter to the balance 0V test
points (3 & 4) and turn the balance for a reading of 0 VDC.
5. Replace the bias controls cover box.
Tube life varies depending upon playing style and
application. Power tubes have a shorter life than pre-amp
tubes which can last for many years.
optimum performance from your amplifier, the tubes should
ONLY be replaced with the same type tubes as specified
above.
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Notes:
•Make bias and balance adjustments ONLY when
necessary. If you are unsure about these settings take your
amp to the nearest authorized Fender® Service Center.
•Always set bias first, then set balance.
•If the output tubes can not be biased to at least 80mVDC,
replace the 6550C output tubes.
•If the output tubes can not be balanced, check the bias
setting and/or replace the 6550C output tubes.
•The tube trouble LEDs are NOT indicators of worn tubes.
If your tubes are worn and sound weak the LEDs will still
glow green. The trouble LEDs turn Red only after tube
failure has already occurred.
FAILURE TO REPLACE TUBES.
Replacement section).
6. TUBE TROUBLE - These LEDs indicate tube failure or
trouble at their corresponding locations.
glows green, the tubes are operating. When the LED glows
red, a tube failure has blown the fuse at the corresponding
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. If the amplifier repeatedly blows fuses, it
should be taken to an authorized Fender® service center.
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IMPORTANT:
changing tubes and remember: THE TUBES GET HOT!
A simple way to check if the tubes are worn is to replace the
set in your amp with your spare set and listen for
improvement. (You carry a spare set of tubes and fuses,
don't you?)
•First, replace the OUTPUT tubes with new tubes; re-bias
and listen (bias/balance instructions are on the next page).
•Next, re-insert the original output tubes and re-bias.
Replace the PRE-AMP tubes with new tubes and listen.
•Finally, try replacing both the output (re-bias) and PRE-
AMP tubes with new tubes and listen.
If there is a marked improvement in the tone of your amp at
any point in this procedure, leave the new tubes in your amp
and purchase another spare set.
The 300's tube compliment consists of:
•One 12AT7A Tube (part #023531), (tube location V4)
•Three 12AX7WA Tubes (part #013341), (tube locations V1
thru V3.
•Six 6550C Tubes (part #048489), (tube locations V5 thru V10).
8
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DO NOT WAIT FOR TUBE
(See Tube Testing &
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ALWAYS unplug the amplifier before
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When the LED
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