Rotary speaker sounds are complex to recreate but there are several stompboxes dedicated to the task on the market.
A cool alternative to chorus or phaser pedals, they are generally a source of lush modulation such as the Fender Vibratone tones as heard on SRV’s Cold Shot. Most, though, offer stereo operation, have large footprints and are a lot more than this Big Spender.
What you’re getting here is a compact mono pedal that could slip into a tight pedalboard space yet still offers a second footswitch for transitioning from slow to fast speeds. The Speed knob sets your chosen fastest speed and hitting the Ramp footswitch takes you from there to the slowest speed or visa versa.
There’s plenty of output in the volume knob to add a bit of grit. The Big Spender may not be quite as slavishly authentic as some rotary pedals out there but it’s certainly a decent shout for the asking price.