It seems to be saying you can increase amplitude with... increased voltage? so, maybe, try attaching a 9 volt battery? I warn you, i don't really know what I'm talking about so you may fry your circuit 🍞
Thanks @warren and @roberts_ecofarm! This information is really helpful, and gives a way forward.
I will dig through my boxes of extra parts and find a potentiometer, maybe also a battery case for a 9v battery, then remake this circuit. I'll write in with how it goes.
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Also if you have a resistor you could test the idea bc a potentiometer is an adjustable resistor, if that's helpful.