How Spring Reverb & Tremolo Works
By Rob Robinette, edited 3/31/2017
Have comments or corrections? Email rob at: robinette at comcast dot net
This webpage is available in PDF form: How Spring Reverb & Tremolo Works.pdf
WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. See more tube amplifier safety info here.
Old school spring reverb literally uses the movement of springs to delay and replicate an audio signal. Reverb simulates the reflected sound from a room's interior. Spring reverb was invented by Hammond and first used in their organs to liven up the tone in acoustically flat church interiors. Fender licensed the reverb circuit from Hammond and Surf Guitar was born.
In the AB763 schematic below the guitar audio signal is tapped off to the reverb circuit on the input side of the big 3.3 megaohm Reverb Mix resistor (at upper-center-left). The reverberated wet signal is injected on the other side of the Reverb Mix resistor. The 10pF capacitor in parallel with the Reverb Mix resistor is simply a bright cap. All of the Vibrato channel guitar dry signal must pass through the Reverb Mix resistor or its bright cap.
AB763 Blackface Reverb Circuit
Reverb is at bottom left. Note how the dry signal is tapped off one side of the big 3.3 megaohm Reverb Mix resistor and the wet signal is injected on the other side of the resistor. The 10pF capacitor in parallel with the Reverb Mix resistor is simply a bright cap.
Guitar signal enters at upper left and gets amplified by the Reverb Driver (both stages of a 12AT7 tube in parallel) then flows into the Reverb Transformer which trades high voltage for current. The amplified current is sent to the Reverb Tank's Input Transducer (see tank detail below). The weak 'wet' signal from the tank's Output Transducer is amplified by the Reverb Recovery amp and passed through the Reverb Level control and back to the guitar amplifier.
Reverb Tank Detail
Signal enters tank on left and exits on the right. The Input Transducer's input coil moves the transducer magnet which moves the spring which moves the output transducer magnet which generates the reverb signal voltage in the output coil.
Referring to the 65 Princeton Reverb schematic above, the dry signal enters on the upper left and flows through a 500pF Reverb Filter capacitor which filters out most of the guitar signal's low frequencies. Low frequencies are too long and turn to mud when reverb is applied. The filtered signal then gets boosted by the Reverb Driver amplifier. The Reverb Driver is needed to generate the power to physically move the reverb springs.
After the Reverb Driver the amplified dry guitar signal is then sent through the Reverb Transformer which trades high voltage for current. Amplified current is needed to drive the reverb tank input transducer. The tank's Input Transducer is simply an electromagnet used to move the spring. The amplified audio signal flows through the input transducer's coil which generates a magnetic force. The magnetism generated in the coil is alternately attracted to and repulsed by the transducer's magnet which makes it and the attached springs move.
The transducer movement travels down the Springs and causes movement of the Output Transducer magnet at the other end. The moving magnet's magnetic field cuts through the transducer's output coil which generates the 'wet' reverb signal voltage. In other words, the Input Transducer transforms electrical energy into mechanical movement. The Output Transducer transforms mechanical movement into electrical energy. The weak wet signal generated by the transducer output coil is then amplified by the Recovery Amplifier and flows through the Reverb Level (volume) pot and back to the amplifier.
A reverb tank is simply made up of two transducers connected by two or more springs. The longer the tank the longer the reverb reflections.
The time it takes for the spring movement to travel from input transducer to output is the reverb delay. Multiple springs with slightly different makeup add multiple delays simulating sound reflections from multiple room features. The original spring movement doesn't actually stop at the output transducer. A diminished 'wave' is reflected back along the spring toward the input transducer, bounces off it and returns in weakened form to the output transducer generating multiple diminishing reverb 'reflections.'
Fender 6G15 1960's Standalone Reverb Unit
Long a favorite of surf guitarists the 1960's Fender 6G15 standalone reverb FX box is as large and as complicated as a tube guitar amplifier. There are kits available for this very cool reverb unit.
The guitar plugs in to the Input Jack at far left. Preamp 1 (upper left), Preamp 2 and the 6K6GT all boost the dry signal to make it strong enough to jiggle the springs in the Reverb Pan. The signal looses a lot of energy traveling along the springs so the Reverb Recovery triode on the upper right boosts the weak signal coming off the Reverb Pan Output Transducer. The Mixer Pot mixes the bypassed dry signal (blue) and wet signal (green) for output to the amplifier.
The signal path colors correspond to the colors on the schematic above to help you follow the layout's convoluted signal path.
See the Compact Reverb Add-On Circuit page for more info on reverb circuits.
AB763 blackface style tremolo is the most common tremolo circuit in use. It acts as an automatic wavering volume control which acts directly on the guitar audio signal. This circuit requires two triodes (whole tube), one to generate the oscillation and one to drive a neon light bulb (yes really). This tremolo circuit is also used in the Fender blackface and silverface line of amplifiers. Fender sometimes erroneously calls tremolo "vibrato." Real vibrato waivers the signal pitch, not the volume like tremolo.