Converting a 9 pin 1/4" input to a 4 pin

[Leo_Gnardo]

I replace those crappy plastic jacks in the Hot Rod series with Switchcraft metal jacks, running short jumpers to the PCB. Switchcraft jacks fit, just barely, and are far more robust.

Now that's more like it! Doesn't hurt to have a 3/8" internal-tooth washer between jack & chassis. Also use authentic Switchcraft jacks, no pretender substitutes. Crank down on that mounting nut, apply a dot of thick cyanoacrylate (Krazy) glue where it joins the threads, and you're good to go for the next couple decades.

[g1]

Unless they want it to be insulated from the chassis. This can often be the case with plastic jacks. Then you end up with a ground loop and extra hum.
In those cases you need to use insulating shoulder washers.

[Enzo]

And those 9-pin jobs often do other functions as well, so you MAY have to rewire some other stuff for it to function correctly.

[drewl]

I've clipped the 9 pin jacks for rush jobs when I was out of 4 pin ones.

And yes, if you decide to go with a metal replacement like Switchcraft, it will need to be isolated from the chassis, otherwise there will be added hum.