There are three parts to a reverb circuit: the
reverb drive, the reverb pan (or tank), and the
reverb recovery. Dry guitar signal is amplified
to four watts by the drive circuit to drive the
reverb pan. Inside the pan, a transducer, which
is coupled to a pair of metal springs of different
lengths, converts the electric signal into an
acoustic signal. Since the springs are different
lengths, the sound traveling down them arrives
at the other end at slightly different times. A
second transducer changes the acoustic vibration
of the springs back into an electric signal,
which is then sent to a recovery stage that
amplifies the reverberated signal and mixes it
with the dry signal from the preamp.
REVERB PAN AND CABLES
The majority of all reverb malfunctions occur with
the pan or connecting cables, so your troubleshooting
should begin here. Cables may look connected,
yet may not be connected well, so try rotating the
RCA connectors on the back of the amp and also
where they plug into the pan, making certain there
is a good metal-on-metal connection on both ends
of each cable. You can also try substituting a new
set of RCA cables. Make sure the jack marked
Reverb Input is connected to the input of the pan,
and that the jack labeled Reverb Output is connected
to the output of the pan. If you’re still not
getting reverb sound, remove the pan from the
bag, remove the cardboard bottom from the pan,
and inspect the springs. If either spring is broken,
or if the connection to the transducer is broken,
the reverb pan will need to be replaced.
REVERB DRIVE
The reverb drive is basically a four-watt output
amplifier consisting of a 12AT7 tube and a reverb
driver transformer. Remove the 6V6 or 6L6 output
tubes to disable sound coming from the
main speaker. Unplug the reverb pan, and connect
a speaker to the reverb jack labeled Reverb
Input. Fender labeled it “input” because it connects
to the input jack on the pan, but it is really
the output of the reverb driver. This jack is an
RCA type, so you will need an adaptor to connect
it to a speaker. At this point, you should be
able to play your guitar and hear it coming out
of the speaker attached to the Reverb Input jack.
If not, try replacing the 12AT7 tube. Inspect all
the electronic components attached to the 12AT7
socket. If any of the components look burned,
replace them too. Check for voltage between
pin 1 of the 12AT7 and ground. If there is no
voltage, the transformer needs to be replaced.
RECOVERY CIRCUIT
To check the return circuit, set the reverb control
about halfway up, bump the pan, and listen
for a reverb crash. If you don’t hear anything, try
replacing the 12AX7 located next to the reverb
transformer. And while you’re at it, inspect the
components connected to the 12AX7 socket,
replacing anything that looks burned. g
—Gerald Weber, Kendrick Amplifiers