TBP-1
®
™
(This is the model name for warranty claims)
5
CIRCUIT DESCRIPTION
This section provides concise information about
new or unusual circuitry designs incorporated into
this amplifier model. The purpose is to aid the
service technician by providing insight into the de-
sign areas most likely to become obstacles in
troubleshooting. Information is focused for its ef-
fective use while maintaining the security of
Fender® proprietary information wherever possible.
PRE-AMPLIFIER
The Instrument signal flows from the input jack J1
on the rear panel (or J2 on the front that will over-
ride J1) directly to the first tube stage V1-B which
presents a high-impedance to the instrument.
Switch S1 allows for a -6dB cut in the first gain
stage without affecting the load presented to the in-
strument. V1-B plate current is sensed by resistor
R5 and amplified by U1-A in order to provide a Pre-
EQ signal (for the Balanced Line Output) and Tuner
Output without loading down the tube output. V1-B
also drives a classic Fender tone stack EQ circuit,
made up of potentiometers R19-21 and passive
components C10, C11, C13 and R16. The switch
section of potentiometer R19 connects C12 and
R17 to change the tone stack response and provide
additional bass boost when the Bass knob is pulled
out. Pulling out the Treble knob activates a bright
boost at mid Volume settings by connecting ca-
pacitor C14 which provides a high-frequency
bypass path around Volume control R22. Tube
stage V1-A provides necessary buffering and re-
covery gain following the Volume and “cut only”
tone controls.
The tone stack signal is also presented to the Tube
Overdrive section (V2 and associated components)
via C17, R36 and Gain control R37. Relay K1
shunts this signal to ground when Overdrive is dis-
abled. Front panel Gain switch S7 (or the Gain
footswitch) controls Field Effect Transistors Q1-3 to
select the desired signal path, ‘clean’ only (Q1 on)
or a blend of clean and tube overdrive (Q2-3 on).
Clamping is provided by diodes D5-6, D8-9 and
D11-12 to assure proper operating levels for Q1-3
and Op-Amps U2 and U3. The clamp voltages are
set by Zener diodes D67 and D68. U2-A provides
the buffering needed to connect the clean path
high-impedance tube circuitry to subsequent lower
impedance solid-state circuitry. Likewise, U3-A
does the same for the tube overdrive path, but with
an added low-frequency boost (due to C24-25 and
R49-51) to make up for the high-pass frequency re-
sponse of the overdrive circuit. U2-B and U3-B are
set up differentially in order to cancel any hum
caused by ground potential difference between the
front panel (where the input jacks are referenced),
and the rest of the circuitry. Note that U3-B pro-
vides a signal phase reversal to allow coherent
blending of the clean and overdrive signals.
The clean and overdrive signal paths are summed
by U4-A and presented to a tone shaping circuit
(U4-B). U5, U6 and U7 make up Fender’s unique
Vari-Q
TM
circuit, which is a state-variable parametric
equalizer where the Q and Level parameters are
controlled simultaneously by one knob (R79). Front
panel Vari-Q
TM
switch S6 (or the Vari-Q
TM
footswitch)
controls FETs Q4 and Q5 to select or bypass the
Vari-Q
TM
equalizer circuit. U1-B provides a buffered
output for the Effects Loop Send (J4), while U9-A
buffers the Effects Loop Return (J5). Again differ-
ential circuits are used to isolate grounds for hum
cancellation. S2 sets the nominal operating level
for the Effects Loop (J4-5), while Trim control R115
(along with U9-B) provides a +/- 6dB range, used to
compensate for an external effect’s volume loss or
gain. The FX Loop footswitch controls FETs Q6
and Q7 to select or bypass the FX Loop circuitry.
The output of U11-B provides a Post-EQ (and Post-
FX Loop) signal, and feeds the Room Balance cir-
cuit (U12-A), Master Volume circuit (U12-B) and
finally, the Main Output (ground isolated and buff-
ered by U13-A). For bi-amping, the Main Output is
sent to a 3-pole Butterworth-response state-variable
filter (U13-B, U14, U15-A). The Active Crossover
Frequency is set by R143, a triple-ganged potenti-
ometer (rear panel thumbwheel knob). U15-B and
U16-B provide recovery gain for the Crossover Bal-
ance control (R155) and buffer the HF and LF Bi-
amp Outputs (J7-8)
The Post-EQ signal is further shaped by a speaker
cabinet simulation circuit (U11-A). U10-A provides
phase reversal to put the Post-EQ signal in phase
with the U1-A Pre-EQ signal, both of which are pre-
