Pot Problems

[Silver Blues]

Thought I'd ask.

Ever since I replaced the broken Alpha pot in my bass with a CTS of the same value, the volume control only works through the last 25% of its sweep. (i.e. when off, you can turn it 3/4 of the way up without any output.) It is a log taper pot, where I realize I should have used a linear, but nevertheless, I feel like that behavior is abnormal.

TBCH, I don't mind so much because it's a rare occasion where I don't run my bass full on, but it's kinda starting to irk me. I attached a pic of my control cavity, maybe it's the way I wired it? FWIW, it's wired the same way as its sister and the exact way it was wired prior to the swap. Any help appreciated, thanks.

--Silvie

It's a really bad pic, sorry, but I hope y'all can see where the wires go properly.

[audioglenn]

It's the log taper that is bothering you. The only solution is to replace it with a linear taper pot.

[Silver Blues]

That's what I was thinking. Why's it make such a difference?

--Silvie

[SGD Lutherie]

Even with a log taper, you shouldn't have it turning 3/4 of the way up before you get sound. The sound would come on right away and then you would have a jump at about 8-10 on the rotation.

It sounds like you over heated the pot and damaged it.

[T_Bone_TL]

Linear taper pot - 250K pot, halfway is 125K, 1/4 s 62.5K, etc - each change in the knob makes about the same change in resistance. If you plotted it on a graph, a straight line.

Log (also called "audio" taper sometimes) the amount of resistance change is different depending where you are on the pot - and most of it hapens towards one end. So halfway (turns) might be 50K on a 250K pot, 1/4 might be 20K, 3/4 might be 100K (ballpark.)

Rather than I write it from scratch, try looking at this link and the graph partway down the page:

http://sound.westhost.com/pots.htm

And you thought you'd never have a use for that stuff in math class... ;-)

[SGD Lutherie]

Keep in mind that many audio taper pots are NOT true log taper, but couple of linear tapers composited together (#3 above).

You can see here how the audio taper starts right away, but rises slowly until you get the end of the rotation.

[SGD Lutherie]

Originally Posted by T_Bone_TL

...Log (also called "audio" taper sometimes) the amount of resistance change is different depending where you are on the pot - and most of it hapens towards one end. So halfway (turns) might be 50K on a 250K pot, 1/4 might be 20K, 3/4 might be 100K (ballpark.)

Did you see where he said:

"you can turn it 3/4 of the way up without any output."

That's not normal for a log taper pot to get NO output.

The pot is damaged.

[audioglenn]

Originally Posted by SGD Lutherie

Even with a log taper, you shouldn't have it turning 3/4 of the way up before you get sound. The sound would come on right away and then you would have a jump at about 8-10 on the rotation.

It sounds like you over heated the pot and damaged it.

Good point!

[Silver Blues]

Shite. All right, then... That was my bad. Off to the store...

--Silvie

EDIT: Are they really that sensitive to heat? First time ever I've fried a pot, providing that's what actually happened.

[SGD Lutherie]

Some pots are very sensitive to heat. I've burt out a couple of Bourns pots just by holding the iron on the terminal a few seconds too long. Lots of pots use conductive plastic.

I use my soldering station set to about 650°F, and don't dwell on the joint very long. Melt your solder on and get the iron off as soon as you can.

[Silver Blues]

Thanks, will keep that in mind.

--Silvie

[RitchS]

My problem is that It just makes me giggle for 15 minutes, then want to eat a box of Twinkies and go to sleep. Hardly worth the effort really.

[Southway]

Gettin' in on this late but...

Just the other day I put a bass together, three Epi T-Bird humbuckers, a 500K vol control for each, plus a 500K master vol. No tone pot, used a switch instead. All pots are new CTS. Don'r remember where I got 'em but will have to look it up. I am pretty sure it was Listed as audio taper. The numbers on one: 450S3494 500K 1123

When I plugged her in, I had the same problem as the OP: No (or very very little) output until it was turned almost all the way up.

So, I did a little checking. I took a pot from the same batch, put it on a board and measured the resistance at several points and marked it on the board. I did the same for two Epi T-bird pots, one vol and one tone. Turns out the Epi pots are tapered backwards from each other, and the TONE pot was pretty much the same as the new CTS. So I would say that what the OP had was reversed taper and likely nothing wrong with it.

Then I opened up the bass, reversed the hook-up on one volume pot, and voila! It works like it should, although full volume is now counter-clockwise!

I plan to find out where I got these pots and I'm going to check some others I have as well. Figured I'll post a new thread with the results unless y'all know of a similar thread already here?

Edit: The Epi pots are marked A500K (tone) and B500K (Vol). I know I've seen pots listed with A or B taper, but didn't remember exactly what that meant.

Edit again!: OK, so maybe I'm wrong here about reverse taper or not. I set up a scale at 10 degree intervals and graphed out the resistance of one of the pots like I put in the bass. I measured from the terminal that's usually grounded on a volume control (Left lug with lugs on top looking at the back of pot). The graph looks a lot like SGD Lutherie's #3 above which would be the correct direction, would it not? So why does it seem to work better reversed?

[walterw]

A is audio, B linear. (funny how it's usually the cheapie imports that get it right on basses and use linear volume pots. of course, those same companies tend to then get it wrong and use those same linear volumes on guitars, where they're awful for trying to control the overdive level from the guitar.)

[Borzi_4]

Originally Posted by RitchS

My problem is that It just makes me giggle for 15 minutes, then want to eat a box of Twinkies and go to sleep. Hardly worth the effort really.

HAHA, Beat me to the punch!

[Southway]

Originally Posted by walterw

A is audio, B linear. (funny how it's usually the cheapie imports that get it right on basses and use linear volume pots. of course, those same companies tend to then get it wrong and use those same linear volumes on guitars, where they're awful for trying to control the overdive level from the guitar.)

Ah, OK. I looked again at the numbers I marked down for the "B" pot, and it did look pretty linear. So linear is better for bass?

(Also note my edit I was apparently writing while you posted. )

[Silver Blues]

I had forgotten about this thread

I should mention also (I've been too lazy to replace it) but when I run that bass through my Muff, the pot works pretty much fine.

--Silvie