This is a simple guitar vibrato effect, used to obtain the variation in pitch. The circuit of is fairly simple. The oscillator is a standard transistor circuit, and has been used for this sort of application many times.
This is a Hybrid tube tremolo circuit. It is interesting that a low frequency oscillator (LFO) in electron tube is not resolved, we do circuit transistor for low frequency. BC547 transistors and used ECC82 or 12AU7, so the circuit operates at low voltage. Tube circuit provides classic, warm sine wave tremolo tones.