Power Amplifier - Fender BASSBREAKER 15 Combo Service Manual

For the MED Gain STRUCTURE, S1-A routes the
V5-A signal to gain stage V4-A. S1-B routes this
medium gain signal to V4-B & then the tone con-
trols. The MED Gain STRUCTURE provides 3
stages of tube gain before the tone controls.
For the HIGH Gain STRUCTURE, the GAIN control
feeds gain stage V5-B directly.
amplified signal is then fed to gain stage V4-A via
rotary switch S1-A. S1-B routes this signal to V4-B
& then the tone controls. The HIGH Gain STRUC-
TURE provides 4 stages of tube gain before the
tone controls.
Following the V4-B stage (common to LOW, MED,
& HIGH Gain STRUCTUREs), AC coupling cap C64
feeds a coax to send the signal to the TRE-
BLE/BASS/MID tone control circuit (R126, R124, &
R125 respectively) found on the CONTROL PCB.
The MASTER VOLUME control (R127) follows the
tone controls.
The MASTER VOLUME control's wiper output is
coaxially coupled to the EFFECTS LOOP's op amp
circuitry (U1-A & -B and U2-A).
LOOP circuitry is found on the REAR PANEL PCB.
The EFFECTS LOOP SEND is J3; the RETURN is
J4. The EFFECTS LOOP circuitry is optimized to
work with low-level stomp boxes.
The EFFECTS LOOP output signal is routed to the
POWER SUPPLY PCB. Op-amps U8-A & -B condi-
tions the signal to the reverb card (attached to P5 -
FMIC p/n 0096569000). This is a digital hall RE-
VERB "daughter card" mounted above the POWER
SUPPLY PCB. The REVERB wet output signal is
then routed to the CONTROL PCB where control
R128 allows the user to set the REVERB's wet out-
put level for its wet/dry mix.
The EFFECTS LOOP output (U2-A) and the RE-
VERB output (U9-B) are sent to summing op-amp
U2-B on the REAR PANEL PCB to create the RE-
VERB's wet/dry mix. The output of U2-B feeds the
POWER AMP input. JFET Q1 is placed at the non-
inverting input of U2-B to kill the signal to the power
amp if the amp is placed in MUTE mode.
The LINE OUT signal remains active for silent re-
cording when the amp is configured in MUTE.
This additionally
The EFFECTS
BASSBREAKER™ 15
(This is the model name for warranty claims)
The reverb for the line out is summed with the dry
signal at op-amp U3-A. An active filter circuit is
configured around op-amp U3-B to provide the
LINE OUT's SPEAKER CABINET EMULATION
equalization when enabled through switch S3.
Switch S4 disconnects the chassis ground connec-
tion to the shell of XLR jack J5 (GROUND LIFT).

POWER AMPLIFIER

The Power Amplifier is a push-pull design using two
EL84s power tubes (V1 & V2) found on the TUBE
PCB. Both V1's & V2's cathode bias is set to ap-
proximately 40mA using 5W power resistors R70 &
R134. Their screen grid bias is set through R77 &
R80.
The power amp's input is a long tail pair phase in-
verter configuration. This differential amplifier uses
both halves of a 12AX7 (V3-A & -B). V3's differen-
tial plate signals are AC coupled to the grids of the
EL84 power tubes.
The EL84 plates drive a custom designed multi-tap
output transformer (T1 – FMIC # 7706195000) to
provide an impedance match for 4, 8, & 16 Ohm
speaker loads. Its primary is connected onto the
TUBE PCB and its secondary is connected to the
IMPEDANCE SWITCH PCB mounted on the rear
panel. The output transformer tap is selected using
S2 (also on the IMPEDANCE SWITCH PCB).
For the COMBO version, the internal speaker is
connected to output transformer secondary thru
terminals P109 & P110.
speaker is disconnected when the MAIN SPEAKER
jack (J1) is connected to a speaker load through the
¼" jack. The EXTENSION SPEAKER (J2) will work
in tandem with either the internal or EXTERNAL
speaker. Obviously, the impedance selector switch
should be set to match the effective speaker load
connected internally & externally to the amplifier.
For the HEAD version, the amplifier can only drive a
speaker load if the MAIN SPEAKER jack (J1) is
connected to a ¼" plug.
SPEAKER (J2) will work in tandem with the MAIN
SPEAKER. Obviously, the impedance selector
switch should be set to match the effective speaker
load connected externally to the amplifier.
7
The COMBO's internal
The EXTENSION