My understanding is you can't patent a circuit.
That's a commonly quoted thing - it is and isn't true. I have circuit patents, so strictly speaking yes, you can patent circuits. The thing is, you have to have something novel - taking a tube screamer circuit and changing a few values won't cut it, unless something very unexpected arises from that change (a different sounding overdrive isn't unexpected)
If you try to patent an overdrive circuit at this point, there is so much prior art that is so similar that it'd be very difficult to do that. Also, the costs (dollars for lawyers, and the time of the circuit designer) to get a patent are prohibitive for most builders, who will sell a few dozen or a few hundred pedals of a type - it just isn't worth it. The stuff I was working on, over time, will sell in the tens to hundreds of thousands - there it is worth it.
So, legally, you can patent a circuit, but in the product space we're talking about, it just isn't practical to do so.
