The SurfyVibe circuit is emulating the mono channel of a Magnatone 280.
The vintage Magnatone amps were known for having that amazing wobbly sound, from discreet low intensity effects that makes an instrument mix well into the sound of a band, to extreme effects that can take the sound of guitar out of this world! Depending on settings, it can be deeply bluesy, very modern or spooky sounding.
The SurfyVibe circuit is emulating the mono channel of a Magnatone 280. The "sweeping" mode adds a feedback path to create a unique effect that is a mix of phaser / chorus / tremolo.
The original circuit in the Magnatone 280 used a component called "varistor", a type that has been obsolete for years. This is the reason modern circuits use other techniques to achieve pitch shift.
To get the sound as close to the original as possible, we have found a way to emulate the original circuit. We replace the tubes with jFET transistors and the varistors with special diode arrays.
Is a vintage amplifier true bypass? Changing the tone a little bit is part of the game here and we wanted to keep it so. We did not invent a new effect, our purpose is to allow everybody to enjoy that sound without carrying around a heavy and expensive instrument.