PTB bass- hum on 0, anyone?

Mon Feb 12, 2018 2:14 pm

Howdy folks,
new dude here from Christchurch, New Zealand.
I've had my S-500 for about 27 years now, it's mellowed nicely and I'm still stoked with it after all these years.

I've got a quirk in my bass control I can't find mention of anywhere on the web.
I'm curious to see if anyone else has the same, and if there's a more orthodox fix than the one I used.

The thing: With a fair amount of overdrive, turn ptb bass down to 0 and a whole lotta hum comes out of my rig.
This ain't what you expect when you turn down bass.
Anyone else had this?

I expect it has to do with the pot being so large, 1Meg.
I know with guitar volume knobs as you turn 'em down the impedance mismatch with the amp makes the guitar hum and buzz (on a cranked amp, of course) it's also documented that the effect gets worse on larger pots, probably why we kave 250Ks on our G&L's.

I also surmise we probably don't go smaller values on the bass pot because they would offer less bass cut?
I found a partial fix which eliminates hum (but not any buzz present if at high gain) but also expands the pot to cut low mids (which on my guitar/amp can tend to be a little swampy on overdrive anyhow, so this is actually good).
This was by adding a resistor from ground to the output of the bass pot. The smaller the resistor the more of everything the pot will cut, so you don't want to go overboard... I used a good chunky 85k, the results are good, leaving the overdriven signal still pretty clean when a note isn't played.

Now, if anyone's had this problem before, is there a different way to fix it?