Hi FSX Ive designed and made quite a few PCBs. What circuit are you interested in having made? I maybe able to help you. Are you wanting to make the PCB yourself if you have a design, or do you want someone to make it for you? Do you want to solder the components on the board?
have you any photos of your construction you can show us? What did you build the circuit on - PCB or Perfboard? What is your supply, battery or If you are using a battery is it a good one? What FET did you use and are you sure it is the right way round? What source are you using - a Guitar? Have you tried feeding the input directly into the top of the volume control? Have you adjusted the drain of the FET to 4.5 volts?
Sorry about the Qs but it is necessary to find out exactly what you are doing.
Why would you want to do that? You would have to wire it in reverse and the breakover voltage would be in the hundreds with allowed inverse current very small in microamps. Otherwise you would destroy the diode!
You could have a ground loop. The grounds should all go separately to one point ideally on one of the smoothing capacitors. If you have connected the grounds to one another then that is the problem. Also the input jack ground should be isolated from the chassis and go directly to the input ground connection.