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Output
Gain
Negative feedback
Processing
Power
Mic level
Speaker level
Line level
Line level
You don’t need to read a schematic to build this kit . But it’s
fun to see how the circuit works, and to see the different
subcircuits that interact to shape your sound .
Working with the tiny signal from the guitar, the amp creates
the power needed to drive the speaker . The signal is affected
by the
gain
,
processing
,
output
and
power
stages as it
passes through the circuit .
The gain circuit increases the signal
strength to
line level
(about 1 volt),
by passing it through preamp tubes .
Inside the tube, electrons flow from
a heated
cathode
to be received by
the anode
plate
. Between these is a
grid receiving the tiny voltage from
the guitar . The guitar’s
varying musical signal
controls the flow of
electrons to the plate .
V1 handles the first gain stages of the
normal channel, and V2 handles the
first gain stages of the vibrato chan-
nel . The vibrato channel has an extra
gain stage in the second half of V4 to
make up for signal lost through the
reverb and vibrato circuits .
The negative feedback loop flattens
and extends the amp’s frequency re-
sponse, reducing distortion generat-
ed in the output stage . This makes the
amp’s sound cleaner and more hi-fi .
Some players disable this loop com-
pletely or add a potentiometer in
place of the resistor to create a “grit
control .” We don’t recommend this,
because it places stress on the output
transformer .
The processing stage shapes the
tone
. The AB763 is notorious for its
mid-scooping tone stack .
Both the normal channel and the
vibrato channel have processing con-
trols through their respective treble
and bass controls, and the vibrato
channel has additional processing
through the reverb and vibrato cir-
cuits .
V3 handles the reverb send and the
first half of V4 handles the reverb re-
covery . V5 handles the vibrato circuit .
Both channels are then sent to the
phase inverter which uses all of V6 .
The phase inverter then sends the
signal to the output section .
The power supply stage provides power to the other circuit stages, as well as the
tube heaters and pilot light .
This circuit receives the
AC power
from your wall and passes it through the power
transformer to create higher voltage . The electricity then goes to the rectifier,
which converts it to a pulsing
DC current
.
This then passes through the filter choke and the filter cap board . The filter cap
board contains five capacitors that filter out the pulsing to create a smooth current .
As each cap smoothes a bit more, the current is also passing through resistors
that lower the voltage .
As the signal from the guitar becomes more amplified, the ripple of DC current
becomes less evident and the fully filtered current is sent to the most sensitive
part of the amplifier, the first pre-amp tube in the Gain stage .
The output stage increases the line
level signal to speaker level, which
is typically 8 volts or greater .
Two 6V6 tubes drive the output trans-
former . They receive the split signal,
180 degrees out of phase, amplify
those signals, and send them to the
output transformer to be recombined
into one signal . This transformer
adjusts the impedance to 8 ohms to
drive the amp’s
8Ω
speaker .
This amp has an adjustable fixed bias,
which means you need to check the
bias and possibly make adjustments
when you change power tubes .
cathode
plate
grid
We’ve color-coded these stages on our schematic, to show
how the parts work together . Symbols for components are
in the key at the bottom of the frame .
On the
wiring diagram
we build step-by-step in these
pages, the parts are easier to recognize . But studying these
color-coded stages will help you understand where each
component fits into the creation of your sound .
Learning more: secrets revealed in the schematic
