Learning More: Secrets Revealed In The Schematic - StewMac ’64 REVERB UNIT Assembly Instructions Manual

Learning more: secrets revealed in the schematic

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 unit creates
the power needed to drive the spring reverb tank . The signal
is affected by the gain, processing, output and power
stages as it passes through the circuit .
Mic level
Gain
Line level
The gain circuit increases the signal
strength to line level (about 1 volt),
by passing it first through a 12AT7
preamp tube .
Inside the airless capsule of a tube,
electrons flow from a heated cathode
to be received by the
anode plate . Between
these two elements is
a grid receiving the
tiny voltage from the
guitar . The guitar's varying musical
signal controls the flow of electrons
to the plate .
This three-part component is called
a triode . The 12AT7 and 12AX7 tubes
are dual-triode tubes; combining the
elements of two tubes in one housing .
This circuit doesn't provide much
gain to the signal from your guitar,
because the focus of this reverb unit
is signal processing .
stewmac.com
Processing
The processing stage shapes the
tone of the signal . This begins with
the dwell pot, which determines
how long the reverb will linger in the
sound .
The 6V6 tube receives the signal from
the 12AT7 tube and drives the signal
plate
grid
through the reverb tank . The 12AX7
cathode
tube then recovers the processed sig-
nal from the reverb tank and passes
it to the tone pot for further pro-
cessing . The tone pot determines the
amount of highs or lows that are bled
to ground . The tone pot then passes
the processed signal to the mix pot
where the processed signal is mixed
back in with the "dry" signal . The dry
signal is a part of the signal that was
split from the input jack and sent to
the mix pot unprocessed .
Power
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 a series of three large electrolytic capacitors which 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 .
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 .
Output
Line level
In a typical amp the signal is passed
to the power tubes for final amplifi-
cation and then sent to the output
transformer to step down the voltage
and step up the current to drive the
speaker .
However, this is not a typical amp and
doesn't have a typical output stage .
This effects unit's output stage is
simply the mixed signal that is sent
from the mix pot to the output jack .
This signal is a mic level signal, just like
the one that comes out of your guitar .
34
Amp
Mic level
© 2018 StewMac