Thinking deeply about tremolo is a "good news/bad news" proposition. It’s sure to make you sound better, but at the risk of giving you a new guitar-related obsession. Which you need like a hole in the head.
You probably have been told, and rightly, that vibrato is a regular pitch oscillation, whereas tremolo is a volume thing. Check. And musical manufacturers use these words interchangeably, often incorrectly, doing irreparable harm to the English language. Check and Double-check. I say that language is a living thing; and that pickers since the days when Leo was still punching a time clock have been calling a throbbing guitar tone “Tremolo”, and so shall I. Sue me.
DOPPLER, AND WHY IT’S GOOD
When a sound source moves toward you, the sound waves in the air get all bunched up, and the pitch gets higher. When the fire truck passes you and the siren moves farther away, the waves spread out and the note gets lower.
Christian “Puddin’head” Doppler postulated this in 1842.
99 years later, the Leslie organ speaker was introduced.
As a mechanical device, it’s a handful. Tweeters whirling on a turntable atop woofer rotors spinning opposite, with belts, pulleys, and foot pedals involved.
It records great, (John Lennon’s voice was pumped through a Leslie on the psychedelic “Tomorrow Never Knows”, because he asked George Martin to make him sound like a Tibetan monk singing from a mountaintop. To me, it sounds like that Tibetan monk must’ve used a cheap harmonica mic.) and is even cooler to experience live.
BETCHA DIDN’T KNOW
CBS/Fender bought Leslie in 1965.
Pretty soon, the Vibratone came along.
In this, the speaker is stationary, while a spinning styrofoam rotor in front of the speaker squeezes and stretches the air pulses, creating a Doppler effect. Dang clever, I say.
Harrison used it on everything in ‘69/’70, (Badge, It Don’t Come Easy, Let it Be), Stevie Ray Vaughan used it a lot; very prominently on “Cold Shot”.
There are Leslie Pedals which some folks say gives you the sound. I don’t think it gets you but about halfway there. Those B3 guys don’t carry Leslies around for their health.
PEDALS: Too many to list
I favor the Supa-Trem, which features:
Blinking Light... so you can time it to the song...
Big-Ass Knobs... you can control with your boots while playing...
Fast/slow Switch... which doubles or halves the pulsation like a Vibratone.
Hard/soft Switch... for round-wave Fender, or spitty, angry, square-wave Vox
Finally: Tremolo On Guitar Amps ...
True Pitch-Changing Vibrato
Some Magnatone Amps (Buddy Holly "Words of Love", Lonnie Mack, "Wham")
Some Fender Brown Amps
PRO, Vibrasonic
This 6G circuit was complex, not well-received, and therefore short-lived. Sounds WILD.
NON-Pitch-Changing Tremolo
Bias Modulation and Opto "Bug"
This is not the most fun conversation, but I'l go there. Tube amps will likely have "Opto" tremolo, the heart of which is a throbbing light bulb next to a light-sensitive resistor inside a dark box or "bug"
To me, this is the classic clean Fender tremolo, like Buffalo Springfield's "For What its worth". Examples: Deluxe Reverb, Twin Reverb, Super Reverb.
Some amps, usually not very big amps, achieve the pulse by cutting bias power. Not something you'd want to try in a powerful amp. It sounds more like a throbbing car engine. Princetons and Tremoluxes use this.
The Secret...
Set your tremolo on 2 and 2, and leave it there. Try it at home. It reduces headroom, gives body and texture to your tone, and makes the sustaining final chord cool as all get-out.
You’re welcome.