Reverb Troubleshooting

I WOULD LIKE TO LEARN HOW TO TROUBLESHOOT the reverb circuit in my Fender amps. Can you please explain how to do this? —Thom Bailey, Lancaster County, PA
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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.


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.


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.


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