This appears to be an impedence mismatch. The guitar pickup is a high impedence device, which when fed into a guitar amp is plugged into a high impedence input. The PA probably has low impedence inputs, as most microphones these days are low impedence. The simple answer is to play through the guitar amp with a microphone placed in front. But if the PA has a HI - LO switch associated with each input, switch it to HI and the problem should disappear.If this is not the case, and you wish to play through the PA, the easy answer is to use a pre-amp - not to increase the gain, but to give an impedence match. I have an MXR micro-amp which has a high impedence input and a low impedence output, which I use to match my guitar to my low impedence mixer inputs. I don't think they're made any more, but you should be able to find something similar second-hand.