Fender Blues Junior Service Manual - Specifications

BLUES JUNIOR

SPECIFICATIONS

Type

PR 295

Input impedance:

1M ohm

Power output:

15 Watts R.M.S. @ 5% T.H.D

Rated load impedance:

8 Ohms

Speaker compliment

One 8 ohm 12" Eminence,

replica of Jenson P-10

Dimensions:

Height:

16

inches

(41 cm)

Width:

18

inches

(46 cm)

Depth:

9-13/16 inches

(25 cm)

Weight:

31 lbs.

(14 kg)

THEORY OF OPERATION

J1 is a high-sensitivity input with an input impedance of about 1M

Ω. V1B is the first gain stage

which 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).  From the MASTER volume, the signal splits to feed the power

amplifier and the Reverb.

U1A makes up a typical solid-state reverb drive circuit.  U1B is the reverb recovery or sense

circuit, which drives the REVERB control (R51).  The reverb signal is summed with the dry

signal at C6, which couples to the input of the Phase Inverter (V3).

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.

The output bias is derived from the -15Vdc supply.  The voltage divider at R31 & R37

determines the bias voltage (-10.7 Vdc).  C14 filters the ripple to 1mV or less.