Guitar amplifiers often include some kind of reverberation effect to add space and depth to the generated sound. The reverb itself may be digital, but many major manufacturers, such as Fender and Marshall, use the analog method to preserve the authentic "classic" sound. A reverb tank consists of a light metal spring in a sealed metallic box. The signal is passed over the spring, causing it to vibrate, which creates the familiar "shimmering" sound.
Disconnect the amplifier from the power outlet. Decide on a location for your reverb tank. It's common to mount it on the floor of the cabinet, where the tank will be least affected by the vibration of the speaker. Place the tank inside the cabinet, ensuring that there is adequate clearance. You want as much space as possible between the tank and the rear of the speaker. Placing them too close together can result in feedback.
Remove the tank from the cabinet and place it onto the sheet of cardboard. Draw around the tank and mark the screw holes. Cut out the tank shape from the cardboard and place it under the tank to ensure it fits. This sheet will be fitted under the tank when it is screwed into the cabinet, acting as insulation to prevent feedback.
Identify the inputs and outputs on the tank. They should be marked with labels stamped into its outer casing. Connect the leads from the tank to the reverb send and return sockets on the rear of the amplifier head.
Slide the tank into the covering bag. Many reverb units come with a bag to help reduce the chance of feedback. If yours didn't, you can use any heavy cloth or vinyl bag of appropriate size. Close the ends of the bag by twisting them shut, making sure the leads will still reach the amplifier head.
Place the tank in the chosen spot on the floor of the speaker cabinet. Slide a rubber grommet under each of the screw holes and insert the screws. Tighten them just enough to hold the tank in place. Tightening them too much aids the transfer of vibration to the tank, which can result in additional noise. Connect the amplifier back to the mains and test.