What is "J" taper?

[rogerinIowa]

I recently was asked to repair an old Dekley Low Boy volume pedal that had a bad pot. I did a little research on those Interwebs and found that the low boy was used by steel guitarists, and that it uses a "J" taper pot...so naturally I shot over the Geofex to re-read The Secret Life of Pots article, but it makes no mention of this strange taper...


Anyone have a clue?

better yet, has anyone a source for a 500K J taper pot with a 2" shaft (not a D shaft)? I've been everywhere (man) looking for a replacement, with no joy so far. I am stumped....

[alanlan]

Do you know the make of pot?

[rogerinIowa]

Allen Bradley.

I did some research and I understand that Bradley sold out to Clarostat, and that most of the steel guitar cats hate the clarostats for lack of longevity...Clarostat is or was owned by Honeywell. I would use a Dunlop 470K volume pot, but this strange old volume pedal has a string based rotating mechanism that requires a 2" shaft.

[alanlan]

Doesn't look like much info out there.

2 things I would try:

1) Try to contact Clarostat and ask them - I'd be surprised if there wasn't someone who couldn't help you find a replacement.
2) If the pot is just scratchy, you could try to clean it up - failing that you should be able to measure the taper even if it is scratchy and if necessary, you could disassemble and measure the carbon track directly.  One you know the taper, it shouldn't be too much of a problem to find something close enough.

Oh, I did find this:
http://www.oldradioparts.com/2a22fil.txt

[stm]

J taper is a softer audio taper where you have around 30% resistance at noon, as opposed to the 10 to 15% found in more conventional audio taper pots.  You could think of it as an in-between the linear and audio taper.  I've seen it in some Fender amp schematics.

If you are familiar with Duncan's tonestack calculator, you'll see there are two kinds of audio taper in the program, "A" and "B".  The former corresponds to the "J" taper, while the latter corresponds to the sometimes called "A" or log or audio taper.

[rogerinIowa]

Thanks for the info, you guys.

[dhemery]

1) Try to contact Clarostat and ask them - I'd be surprised if there wasn't someone who couldn't help you find a replacement.

I worked at Clarostat from 1981 to 1984. I programmed the laser trimmer that nudged the resistive ink toward the required curve.

Alas, I have no memory of a J taper. Not saying there wasn't one. Only that I don't remember.

[BluffChill]

Thinking laterally...could it be that the taper is literally the shape of a letter J? As in, logarithmic to noon, then linear the rest of the way?

[Phoenix]

If what Sebastian (stm) posted above is accurate (and I have no reason to think it isn't), then most current potentiometer manufacturers would call that an A30 taper. A plain A taper generally refers to A10 (10% at 50% rotation), and if you dig into datasheets and part numbers you'll almost always find they're specified as such in the manufacturers part number, not just listed as "A" taper. Unfortunately, anything other than A10 is generally a bit difficult to come across in stock anywhere, it's usually a special order item with minimum order quantities in the 1,000's, but it should give you somewhere to start at least.