Watch the Tremolo online guitar lesson by Jeff McErlain from Guitar Effects Survival Guide
There are two basic tremolo circuits found in classic amps; power tube tremolo and photocell tremolo. They produce basically the same effect, a fluctuation in volume. For the best definitions I have come across I’ll borrow from the Strymon website: "Power Tube Tremolo utilized the LFO signal to directly influence the power tube bias of the amplifier's push-pull output stage. The power tubes are biased into lower and higher idle currents, creating the fluctuating gain that produces the tremolo effect. The effects of crossover distortion at low tremolo volumes, increased power tube harmonic distortion at maximum tremolo volumes, as well as the influence of power-supply sag, all add up to the boggy and dirty nature of this tremolo circuit."
"Photocell Tremolo is found in mid-1960s American amplifiers. Those classic circuits used a light-dependent resistor to attenuate the input signal, coupled with a miniature neon bulb that is connected to the LFO. As the LFO oscillates, the bulb gets brighter and dimmer which in turn varies the resistance of the LDR. The varying resistance works with other circuit impedances to change the signal level, which produces a characteristically 'hard' sounding tremolo that moves between two levels, reminiscent of a square wave." Got that? Well it is tricky and they do sound somewhat different but essentially they produce the same effect. The pedal I am using here, the Fulltone Supa-Trem uses a photocell to produce the sounds found in the classic Fender amps and most common tremolo circuit.