The reverb function of a tube/spring reverb can be very sensitive; therefore small tweaks can make big differences in reverb tones. The first things to check are the reverb cables and connectors. Make sure the cables are good and the connectors are making solid contact. Cleaning the RCA jacks and plugs with De-Oxit or some other good electrical spray will help. Next, replace the 12AT7 reverb driver tube. This is the third tube and it’s located next to the small reverb driver transformer. The reverb driver tube runs at very high voltages, so a NOS American tube or a JJ brand tube from Slovakia will work better than a Chinese or Russian tube. Tubes are like guitar strings; they sound best when they are new, they don’t last forever, and sometimes the tone can go away long before it actually breaks. So to really get optimum reverb sound, change the tube.
A great-sounding reverb pan is also essential to having great reverb. Here is a modification to improve the sound of any reverb pan. The reverb pan (also called the “tank”) resides within a vinyl bag on the inside floor of the amp. Remove the bag, open it, and slide out the pan. There’s a cardboard cover on the bottom of the pan that is held by four small screws. With the cardboard removed you’ll see that there are two transducers inside the pan—one drives the springs, and the other is a pickup that transforms the reverberated sound back into a signal voltage. Both transducers consist of a coil of fine wire wound around a small bobbin, with a steel laminated core inside the bobbin. Nearly always, the bobbin is loose on the laminated core. This results in the bobbin moving somewhat instead of all the movement going to the springs. To fix this, you need only to insert a small shim between the steel core and the plastic bobbin. I cut these shims from a bamboo chopstick using a razor utility knife. It looks something like a miniature saxophone reed, and once in place, the shim makes the bobbin tight and secure on the core. Now, all the energy will drive the springs, and none will be wasted on moving the coil. And the same for the pickup transducer—all of the return energy will go into making signal, instead of being wasted by moving the coil.
Want more control over the reverb sound? Here is an inexpensive modification you can do that will transform your oneknob reverb into a three-knob type. You will need a three-foot shielded cable with an RCA male connector on one end and a 1/4" phone plug on the other. Radio Shack sells one, catalogue #33-4054, that works great and sells for $19. Remove the reverb pan from the bag and notice that there’s an input and output jack on the pan. Remove the RCA cable that connects from the output of the pan back into the amp. Now take the new shielded cable and plug the RCA male connector into the Reverb pan’s output jack. Take the 1/4" plug from the other end and insert it into input #1 of the Normal channel on your amp. Now, the pan will return reverb signal to the Normal Channel, thus allowing you to adjust the treble, bass, and volume to suit your taste. For that Pacific Ocean surf reverb, set the normal channel so that the reverb signal has more treble than the dry sound. This will get you that Ventures/Dick Dale type of voicing. But if you are playing blues and want that “juke joint” room sound, try turning the treble down and the bass up a bit on the Normal channel, so that the reverb sound is darker than the dry guitar sound. If you’d like to make this setup permanent, any amp tech can easily change the wiring internally so that the regular reverb return jack is routed directly to the normal channel’s input, thus eliminating the need for the 1/4" to RCA shielded cable while also restoring the reverb footswitch function.