Q At what current level should I drive the reverb’s input coil?
AThat’s an important question. Too little current and the output will be noisy, too much current and the core will saturate producing a muddy sound. A maximum current of 30 amp-turns is just about right (see Tech Note)

Input Impedance Code

Number of Turns

Max current (milliamps)



















Q There’s a lot of hum on the output of my unit. How can I reduce it?
A Hum is often the thorniest problem to solve when designing with spring reverbs. The reverb unit’s coils, especially the output coil, are very susceptible to picking up hum from any power transformer. You can test to see if this is your problem by unscrewing the reverb unit and moving it around. You will almost certainly note that the hum gets loudest when the reverb is closest to the power transformer. This problem is very difficult to complete eliminate, but you can usually get it down to acceptable levels by the following:

Q When I turn up the reverb control I get feedback. How can I stop it?
A The reverb unit can act like a microphone–especially when it is mounted in the same box as the speaker. Vibrations via the air or through the panel to which the unit is mounted can cause the reverb’s springs to vibrate, thereby setting up feedback. The first thing to try is to mount the reverb resiliently with rubber grommets or the like. This tactic will help isolate it from panel vibrations. It may also help to mount it on its own little subpanel to further isolate it from vibrations. If feedback via the air is the problem, you can purchase a special bag in which the reverb unit can be mounted. They are available from many of the places the sell the reverbs themselves.

Q I have an existing amp that doesn't have reverb. Should I try to add reverb to it?
A It depends. Do you have enough space for both the tank and the circuitry? If space is tight, will you be forced to mount the tank too close to the power transformer, causing excess hum? Also, does your amp's power supply have the necessary voltages and current available? If you have the space and the voltages, you could give it a try.