5
BLUES JUNIOR™ III
(This is the model name for warranty claims)
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
design areas most likely to become obstacles in
troubleshooting. Information is focused for its
effective use while maintaining the security of
Fender® proprietary information wherever possible.
PRE-AMPLIFIER
J1 is a high-sensitivity input with an input
impedance of about 1M
Ω
. V1B is the first gain
stage. The second gain stage, V1A, is located after
the volume control in the signal path. V1A also
contains the "FAT" switch. The FAT feature is
activated by grounding the node between bypass
capacitor C3 and R9. Via the FAT switch (S1) or
the optional footswitch, the junction of R52 and R54
can be grounded. With zero volts applied to the
gate of Q1, the JFET will essentially ground C3,
activating the FAT feature.
The output of V1B couples to the VOLUME control
(R7). The VOLUME control is actually a preamp
overdrive control that adjusts the level of distortion.
C2 attenuates high frequencies and C1 attenuates
low frequencies at high distortion settings. This
takes the edge off the distortion and prevents the
speaker from flapping. V1A provides a gain of
about 40, and drives the tone control circuitry. After
the tone controls, the signal is fed to V2B, which
drives the MASTER volume control (R41) and the
REVERB drive circuit. From the MASTER volume,
the signal feeds the push pull EL84 power amplifier.
REVERB
U1B makes up a typical solid-state reverb drive
circuit. U1A is the reverb recovery or sense circuit,
which drives the REVERB control (R45). The
reverb signal is summed with the dry signal at
MASTER volume control, which couples to the input
of the Phase Inverter (V3).
POWER AMPLIFIER
The output tubes (6BQ5/EL84) are arranged in a
push-pull configuration that will produce fifteen (15)
watts R.M.S. into eight (8) ohms @ 5% THD, 1kHz.
POWER SUPPLY
The Power Supply is typical of a Fender tube
amplifier. The secondary side of the Power
Transformer consists of 3 windings: high-voltage
winding, low-voltage winding and heater winding.
The high voltage winding is used to generate the
high voltage DC power supplies in the amplifier.
The high voltage DC power supply consists of a
standard bridge rectifier configuration. The
“voltage-drop” resistors R47 thru R49 set the DC
plate voltages used at various tube stages
throughout the design.
The low voltage winding is used to generate the low
voltage +/-15V DC power supply, as well as the
output tube bias supply. The low voltage DC power
supply also consists of a simple rectifier
configuration. 15V zener diodes (D7 and D8) and
“voltage-drop” resistors R50 and R53 are used to
regulate the +/-15VDC power supplies.
The output power tube bias is derived from the -
15Vdc supply. The voltage divider at R51 & R52
determines the bias voltage (-10.7 Vdc). C33 filters
the ripple to 1mV or less.
The secondary heater winding produces a 6.3VAC
power supply that is used for the heater filaments in
the preamp tubes and power output tubes. A simple
rectifier is connected to the secondary heater
winding to produce a supply that is used to
illuminate the LED pilot light.
.
