Hum increases when guitar volume pot is turned down

[greg]

I have an amp (a homemade Marshall 20 watter) which is nice and quiet with the guitar's volume pot run wide open, but hums noticeably when I back the guitar's volume pot down.

This is the case with both guitars I use with it.

Any ideas?

Thanks,

Greg

[R.G.]

Is the wiring to the input jack reversed, signal for ground?

[fred46]

line reversal for input

a really nifty trick is to reverse line input with a dpst switch.
see heath ta-16 scematic for ideas of employing same.