I built (more or less) this opamp. I know normally the gain would be defined by the ratio of the feedback resistor to the input resistor but here there is no feedback resistor so how do I tweak the gain?
Of course there is a feedback resistor...several of them in fact, along with some feedback capacitors. The RC network is used to change the frequency response (bass and treble) of the amplifier. You haven't shown enough of the the circuit to analyze the gain, but it can be done.