The VCO of Ul, a phased-locked loop. produces a low-frequency square-wave signal from 5 Hz to 2500Hz, which is controlled by varying the voltage to pin 9 via R2. The frequency threshold is set by Rl and Cl. Increasing the value of Rl increases the frequency, and decreasing the value of Cl decreases the threshold frequency. Transistor Ql is operated as an amplifier with the Q point at 4 V, using a transistor with a voltage gain (beta) of 100.
Tremolo - schematic

The gain without tremolo is about 2 V. With bypass capacitor C4 in the circuit, the gain is set at 33 dB. The instrument drive cannot be greater than 30 mV with C4 in the circuit (no tremolo), to avoid distortion. Without C4, the drive can be 1.5 V maximum with no distortion. Ul"s output is coupled to the collector of transistor Ql via depth control R4. The square-wave signal pulls the audio at a frequency determined by R2. The oscillator is activated by foot switch Sl.

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