School me on pot variance/tolerances

[Aceman]

So two volume pots at 500k, should, IMO start at zero, and run to 500k with say an audio taper (or whatever)
Any two pots should sound the same, but besides the actual value, which might be 450 to 550 or so,
Can their be a difference is the curves as well, even if both linear, for example?
So I could have two pots that are theoretically the same, but one gets to a louder volume, and faster than the other?

 

[GuitarStv]

Linear are generally pretty straight, but most 'logarithmic' taper pots aren't actually logarithmic at all. When you measure them you'll see a combination of two or sometimes three slopes to sort of emulate log values. This graphic explains it pretty well:


So log pots will tend to have different characteristics from batch to batch and depending on manufacturer. One will get louder faster, one will get louder slower. It's the wild west.

 

[beaubrummels]

I'm not convinced the middle section of the ramp is that discernible. But the quick ramp at the two ends I think is obvious, like one pot has nothing then a sudden quiet signal while another actually fades in from 0 as you turn it.

 

[Chistopher]

A good way to think of a 500k linear pot is that there is a resistive material of for this example 1" length. That material has a resistance of 50k per 1/10". A 500k audio pot has two materials. The first material goes up to the midpoint and is 10k per 1/10". The midpoint reads 50k, ie 10% of the total pot value. The second half of the pot has a material that reads 90k per 1/10", so that when the pot is on 10 the pot reads 500k.

I don't know if this is exactly going to be a breakthrough in helping you understand pots, but I've never seen them explained this way on a guitar forum, so I figured I'd share for posterity.

 

[Aceman]

No - I meant what I said. I "understand" pots.

Would two different pots, from two different time periods, say a decade apart, but both intended to be 500k, could they be notably different.

GuitarStv's chart on the right answers my question.
- going from 60% to 80% on two of them (red/blue) yields a drastically different response than yellow.
- and of course, top/bottom end could be different as well.

Actually I'm asking about effect pedals. I've been working on making my ~early 90's reissue Rat sound like my 2015 Ikebe Rat 2.

 

[beaubrummels]

A good way to think of a 500k linear pot is that there is a resistive material of for this example 1" length. That material has a resistance of 50k per 1/10". A 500k audio pot has two materials. The first material goes up to the midpoint and is 10k per 1/10". The midpoint reads 50k, ie 10% of the total pot value. The second half of the pot has a material that reads 90k per 1/10", so that when the pot is on 10 the pot reads 500k.

I don't know if this is exactly going to be a breakthrough in helping you understand pots, but I've never seen them explained this way on a guitar forum, so I figured I'd share for posterity.

FYI resistance material printed on a substrate is ohms per square, not per linear inch. If I slice that track with a laser anywhere, it changes the number of 'squares' on the track, changing it's effective resistance. And when a pot is on 10, it reads next to nothing because the wiper is almost off the track.

 

[Chistopher]

Totally fair—yeah, it’s ohms per square, not per inch. I was just going for a simple way to visualize how audio vs linear pots feel in use, not how they work on a spec sheet.

 

[Chistopher]

No - I meant what I said. I "understand" pots.

Would two different pots, from two different time periods, say a decade apart, but both intended to be 500k, could they be notably different.

GuitarStv's chart on the right answers my question.
- going from 60% to 80% on two of them (red/blue) yields a drastically different response than yellow.
- and of course, top/bottom end could be different as well.

Actually I'm asking about effect pedals. I've been working on making my ~early 90's reissue Rat sound like my 2015 Ikebe Rat 2.

What are you trying to fix on your Rat? I've got a way to spread out the useful range of the gain control on a Rat as well as remove the big deadzone on the filter knob if you want me to dig it up.

 

[Aceman]

What are you trying to fix on your Rat? I've got a way to spread out the useful range of the gain control on a Rat as well as remove the big deadzone on the filter knob if you want me to dig it up.

Nothing. There are not broke. The question is do they sound the same. I say yes.
- But not with all the knobs at noon, or anywhere the same.

And they shouldn't. I was just surprised at how much gain adjust it took to get Rat B to be the same as Rat A.

 

[Chistopher]

Did you get both of your Rats new? The 90s reissue might have been given the Ruetz mod, which is alarmingly common given it doesn't require soldering, which would greatly lower the gain.

Outside of that, Rats aren't known for being 100% on spec, especially the MIA ones. If the 47R resistor on one reads 10% low and the other reads 10% high, there could be a very big difference in gain.

 

[Aceman]

Did you get both of your Rats new? The 90s reissue might have been given the Ruetz mod, which is alarmingly common given it doesn't require soldering, which would greatly lower the gain.

Outside of that, Rats aren't known for being 100% on spec, especially the MIA ones. If the 47R resistor on one reads 10% low and the other reads 10% high, there could be a very big difference in gain.

Doesn't appear modded, or no indications/mention, Just old.
- Vintage on 5 = New on 4

Nothing ultra stunning. More than I would have thought, but not like WTF levels. Of course one is LM308 and one OP07. And the "quality" of the tine is the same/extremely similar (as in the chip doesn't mean crap)

I am huge believe in pot variance, but never looked super close like this - never cared. Like I said, I can make them sound the same. It also seems that the pots on the newer one have a more subtle slope. Easier to make fine adjustments to that one, than the other way around. But again, 30+ year old Rat is 30+ years old. Did Mention a little rust on the case? LOL...

 

[Artie]

Almost all pots you buy from normal guitar parts sources will be +/- 20%. That means a 500k pot could be anywhere from 400k to 600k. To make matters worse, even the "good" manufacturers sometimes miss the mark. I have a CTS 500k pot right here that reads 384k.

So I could install two CTS 500k pots, and one would be 384k, and one would be 600k. They will definitely sound different. Although, a subtle difference.

Plus, suppose you wanted to experiment with the difference between a 250k pot and a 300k pot. Your 250k could be 300k and your 300k could be 240k.

Unless of course, you buy in bulk, and match them yourself.

 

[GuitarStv]

I wouldn't call it subtle between 384 and 600k. If you pop that in the volume, the 600 will be noticeably brighter and sharper, and the 384 will sound a little dull.

 

[Aceman]

I wouldn't call it subtle between 384 and 600k. If you pop that in the volume, the 600 will be noticeably brighter and sharper, and the 384 will sound a little dull.

I'm talking about distortion here.

 

[GuitarStv]

I'm talking about distortion here.

Yeah, changes are going to be circuit dependent. I have no idea how much difference pot values would make in a Rat. They're quite noticeable in a guitar volume pot. I had a guitar with singles that I played with, and a 333K pot was painfully bright compared to a 250k one.

 

[Chistopher]

The gain pot on a Rat operates fairly simple. The lower the resistance the more the gain the circuit gets. Turning the knob right lowers resistance. If you have a pot that reads too low, it will be harder to get low gain sounds. If you have a pot that reads high it will be easier to get low gain sounds. Granted due to the logarithmic nature of the pots, the difference will be minimal.

The 47R resistor tied to the gain control is more likely to alter the gain range of the pedal if out of spec.

 

[Artie]

I wouldn't call it subtle between 384 and 600k. If you pop that in the volume, the 600 will be noticeably brighter and sharper, and the 384 will sound a little dull.

I was trying to be kind to CTS. I should send that pot to their quality control department.

 

[Hank-]

No - I meant what I said. I "understand" pots.

Would two different pots, from two different time periods, say a decade apart, but both intended to be 500k, could they be notably different.

GuitarStv's chart on the right answers my question.
- going from 60% to 80% on two of them (red/blue) yields a drastically different response than yellow.
- and of course, top/bottom end could be different as well.

Actually I'm asking about effect pedals. I've been working on making my ~early 90's reissue Rat sound like my 2015 Ikebe Rat 2.

The log taper comes in ratios like 20/80 which is why the graph has different colors. That in turn will affect how it behaves(resistance value) at a specific rotational range. You could measure the resistance using a dmm at 10%,20%,30% etc. of the rotation of the pots and check if the two pots have nearly same or different ratios.

 

[hamerfan]

Linear are generally pretty straight, but most 'logarithmic' taper pots aren't actually logarithmic at all. When you measure them you'll see a combination of two or sometimes three slopes to sort of emulate log values. This graphic explains it pretty well:


So log pots will tend to have different characteristics from batch to batch and depending on manufacturer. One will get louder faster, one will get louder slower. It's the wild west.

No it's not only the batch. Logarithmic Pots come in different flavours, which are characterized by the point of 50% rotation. In your left curve you see a 10% log pot, in your right pic you see a 25%log pot (yellow) and 20% (red and blue). The difference between red and blue might be batch, manufacturer or material of the resistance board.

 

[Chistopher]

Two more pieces of info:

A pot labeled A or Audio isn't necessarily a 10% pot
Lower quality 10% pots can easily fall between 7% and 20% at the center without being out of spec.