Fender Vibrasonic AMP Service Manual - Theory of Operation

“CUSTOM”  VIBRASONIC

THEORY OF OPERATION

STEEL CHANNEL

J1 is the high sensitivity input and J2 is the low sensitivity input.  The input is fed to V1A, which proveds a gain of

about 42.  The output from V1A drives the Treble (R7), Mid(R9), and Bass(R7) tone controls, then feeds the Volume

control (R10).  When engaged, the SWEET switch connects C5 to the wiper of the VOLUME control.  This provides a

boost in the high frequencies.  The “Sweet” feature is most noticeable at lower volume settings.  The signal then

feeds to V1B, which provides a gain of about 66.

GUITAR CHANNEL

J3 is the high sensitivity input and J4 is the low sensitivity input.  V2A contains the FAT switch which boosts the gain

mostly at lower and mid frequencies.  V2A drives the tone controls, and the Volume control (R21) feeds V2B.

V1B and V2B share the same cathode resistor (R23) and bypass capacitor (C11).  The output of the Steel channel

(V1B pin 6) sums with the output of the GUITAR channel at V2B pin 6.  From that point the signal follows two paths.

One to the reverb drive stage (V3).  The other through R32 to summing amp V4B.

REVERB

V3 (12AT7) provides a gain of about 33 to drive the reverb transformer.  V4A is the Reverb recovery or sense stage.

The Reverb control (R29) determines the amount of reverb signal that is summed with the dry signal at V4B.  Via the

footswitch, the Reverb can be disabled by grounding the signal at the node between R30 and R74.

VIBRATO

V5A is a low frequency oscillator.  The 

Speed

 control (R56) varies the oscillation frequency.  V5B is the Neon Lamp

driver.  The lamp flashes faster or slower based on the position of the Speed control (R56).  The brightness of the

neon bulb is sensed by the LDR (Light Dependent Resistor).  The LDR connects to the 

Intensity 

control (R64) which

varies the amount of low frequency modulation applied to the input of the Phase Inverter(V6).

POWER AMPLIFIER

The output from V4B feeds the Phase Inverter(V6), which is a classic style split-load type, used without feedback.

The output tubes (5881/6L6WGC) are arranged in a push-pull configuration that will produce 85 watts minimum into a

four(4) 

Ω

 load at 5% THD.

PARTS LIST

PRINTED CIRCUIT BOARD ASSEMBLY

QTY      PART #   DESCRIPTION                                                REFERENCE  DESIGNATION  

1

049266

PCB ASSY CUSTOM VIBRASONIC ALLV

(STUFFED)

3

037805

CABLE RIBBON 6CKT 2-1/4”

1

031069

CAP AE RDL  10uF 100v 20%

C37

5

028465

CAP AE RDL  22uF 25V   20%

C6 C11 C14 C27 C28

1

036954

CAP AE RDL  22uF  63V  20%

C36

3

024819

CAP AE AX    22uF  500v   20%

C32 C33 C34

1

031040

CAP AE RDL  100uF  100V  20%

C35

2

013638

CAP AE AX   200uF  285V    20%

C30 C31

2

025979

CAPCD   50pF 1KV   10%

C5 C10

1

031780

CAP SLVR MICA  150PF  300V

C7

1

007029

CAPCD  220pF 1KV   10%

C38

1

020917

CAPCD   250pF 1KV  10%

C2

1

020842

CAPCD  500pF  1KV   10%

C13

1

047065

CAP PFF RDL  .0015uF 100V

C16

1

037666

CAP PFF   .001 uF  400v  10%

C20

2

024823

CAP MPF RDL  .01uF  400V  10%

C25 C26