Missing Ground on Cigar Box Bass??

ok, I have this cigar box bass essentially complete, wired a volume control, .2 micro-farad cap to knock down the "growl" a little, and a toggle switch to put the "growl" back in, just in case....."overdrive switch" on a cigar box??

Anyway, it's all working as expected, except I have a pronounced "hum"......not feedback, not distortion......just hum. It diminishes A LOT if I press my finger to the ground on the jack.

Does this mean that I need to ground one side of the pickups?? It has radio shack piezo elements under the bridge for pickups, nylon strings, wood top nut, wood bridge. So there is no place to ground anything to ?!?!? All the wires are short, about 3" from pickups to a small terminal strip, about 2" from there to the volume and jack, so I didn't use shielded wire. Maybe that's the problem??

Is it possible to use a piece of copper plate or something similar to act as a "ground-plane" of sorts? or would it have to be so big that it wouldn't do any good if it were small enough to fit in the body?

Help! and thanks in advance.

Thanks for the help.

I think I might need a new volume pot. Rad Shack was out of the 500k audio taper pots, so I picked up a "500k volume control". The body is all plastic, and I've had a terrible time finding any place on it that will act like a ground. If someone knows the major difference between a "500k audio taper pot" and a "500k volume control" I'd love to know what is is.

I'll try a different pot, I did notice that the hum gets worse when I switch the caps into the circuit, and there are actually 2 hums at different frequencies. They both go away it get really good contact and touch the ground side of the jack.

If the new pot doesn't do it, I suppose I could get one of those "static control" grounding clamps for working on a computer, and clamp it to my......um......yeah.

Thanks again.

Replies

Ginwheel Johnny August 4, 2010 at 1:03am

Okay this won t help at all, its just for laughs....anyone remember the opening scene of back to the future, the first one, when Marty plugs in and cranks everything on the Amp....smiles...just thought i d pass that along...seeing as it was a hummin too!!
Outhouse Instruments August 3, 2010 at 5:22pm

Yeah, I think the pot is the problem. Replace it, making sure to ground the case, pot, and the jack together.

Matt
pi_r_squared August 3, 2010 at 9:43am

Matt,

Here's the circuit. I'm sure in needs to be grounded, and as soon as I get a 500k pot with a real case to ground to I'll try it.

In the mean time I do have a 100k pot left over from another project. It doesn't attenuate the volume enough, but I may wire it in and ground to it just to see if it kills the hum.
cbbcircuit01.JPG
Outhouse Instruments August 2, 2010 at 8:07pm

Without seeing the circuit or the actual wiring .... I'd still say that grounding the pot/jack/whole she-bang is the issue.
I was misunderstanding the pot issue tho'. Replace that, you troubles might be over. But even in replacing that I would still use the diagram I linked.

Matt
pi_r_squared August 2, 2010 at 7:52pm

Matt,

The volume control I was using is an all-plastic body. Th 2 Rad Shacks near me are out of "traditional 500k pots" and 250k as well, which is why I bought this 500k plastic one. I need to get a pot that I can ground to.

Also, the hum is MUCH worse when the caps are switched into the circuit. Does this still jive with what you were thinking?

Matt Towe said:
So .... did you get rid the hum?
Did you try the ground wire?

Matt
Outhouse Instruments August 2, 2010 at 2:38pm

So .... did you get rid the hum?
Did you try the ground wire?

Matt
petey twofinger August 2, 2010 at 1:56pm

my last two visits to the shack they didnt have any 500k pots , not even a bin for them ! (cell phone hut) .

just that goofy "volume control" thing , luckily for me there is a frys near by , but man i am getting sick of shelling out $$$$ for little stuff , like 5 bucks for a switch ! the 3 scrap yards by us w oob , i used to do an all day raid there and return with a bag full of goodys . :(
there is a resale store one block away , they really only sell clothes , so i hit their dumpster weekly and get stuff , but , , ,
pi_r_squared August 1, 2010 at 11:40am

It seems that, since a pot can have linear and non-linear (audio) taper, if they call it a "volume control" I would hope it's audio taper. When I adjust it, it acts like it's audio taper. I still have no idea why they have "500k audio taper pots". "500k linear taper pots", and "500k volume controls", all side by side :S
Skeesix July 31, 2010 at 12:28pm

The volume pot might possibly be a linear taper, vs. an audio taper.
pi_r_squared July 31, 2010 at 9:47am

Thanks for the help.

I think I might need a new volume pot. Rad Shack was out of the 500k audio taper pots, so I picked up a "500k volume control". The body is all plastic, and I've had a terrible time finding any place on it that will act like a ground. If someone knows the major difference between a "500k audio taper pot" and a "500k volume control" I'd love to know what is is.

I'll try a different pot, I did notice that the hum gets worse when I switch the caps into the circuit, and there are actually 2 hums at different frequencies. They both go away it get really good contact and touch the ground side of the jack.

If the new pot doesn't do it, I suppose I could get one of those "static control" grounding clamps for working on a computer, and clamp it to my......um......yeah.

Thanks again.

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