Pot terminology help?

[StuartV]

I just talked to Peavey about trying to order a replacement pot for a TL-Five. They were able to tell me two part numbers, with specs, but they couldn't tell me which was the part I need. I need to open up the bass and look at the part number of the current part. That seems odd, but okay.... Anyway....

My question is about how to interpret the specs they told me. The pot in question is a stacked knob with Treble boost/cut and Bass boost/cut.

Of the two parts they told me it could be, they said one is a 20KB/100KC pot and the other is a 50KB/50KB pot.

What does KB and KC mean in this context?

Peavey said they have one of the parts in stock, but if I need the other one, I'd have to go to Allparts or somewhere to get it. I looked on Allparts.com and I don't see any pots that say they are anythingKB or KC.

So, based on what Peavey told me, if I have to order a non-OEM part, how do I know what to order? And how do I ensure what I get has center detents on both pots like the stock pots have?

Thanks!

[FunkMetalBass]

They aren't usually represented that way (B at the end?). For any given pot, the standard is {Taper}{Resistance}. The slash differentiates between the two shafts on a stacked pot.

"B" is a linear taper.
"C" is outdated and has been replaced by "A", which is a logarithmic/audio taper.
### is the resistance in ohms. (the K is just a multiplier of 1000 - it's pulled from the metric system)

Specifically, they said you needed a B20K/A100K stacked pot (aka, 20kΩ Linear/100kΩ Logarithmic) and a B50K/B50K stacked pot (aka, 50kΩ Linear/50kΩ Linear).

That second pot is fairly common, but the first one will probably be quite obnoxious to track down or make.

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

[StuartV]

Thanks!

Just to say that I'm not a total doofus, I did Google for the answer first and didn't see anything. But, after I posted, I went back and continued looking and found this:

http://www.kenselectronics.com/lists/pots.htm

Tech Tip about the "taper" of Potentiometers: "Pots" are variable resistors; an A, B, or C is used to indicate the distribution of the resistance.
A control labeled 10KB has 10,000 ohms with a linear distribution of resistance--so the halfway point (center of rotation)--has 5000 ohms from each end terminal to the wiper/center terminal. Linear pots are used for calibration applications--for example--it would be desirable for the speed control on a machine to use the middle of the knob rotation for half of the maximum speed!
AUDIO "A" taper pots are commonly used for VOLUME controls; these have logrithmic distribution of the resistance much like the number spacing on a slide rule. Audio pots have very little resistance in the first portion of the rotation--a 10KA pot has about 2000 ohms from the beginning to the halfway point with 8000 ohms from the halfway point to the end of the rotation.
"C" (reverse logrithmic) potentiometers also exist; these are rarely found in CB radios or consumer electronic entertainment equipment.

I'm guessing from all this that the Treble/Bass boost/cut pot is the 50KB/50KB pot that Peavey mentioned and the 20KB/100KC pot is the one that is Mid boost/cut on top and Mid frequency sweep on bottom.

But, I know nothing about pot resistance values versus EQ for different frequencies. Does it make sense that the resistance on a Treble boost/cut would be the same as for a Bass boost/cut? Or does it make more sense that they would have different values?

Also, is having a center detent implied in any of this? Or would one have to look specifically for a pot with the correct resistance and taper AND further specify having a center detent?

Thanks again for the help!

[line6man]

Originally Posted by FunkMetalBass

"C" is outdated and has been replaced by "A", which is a logarithmic/audio taper.

"C" usually refers to a reverse logarithmic taper.

[FunkMetalBass]

Originally Posted by line6man

"C" usually refers to a reverse logarithmic taper.

This is news to me. My entire outlook on life is now inverted.

I apologize for the misinformation, TS.

[FunkMetalBass]

Originally Posted by StuartV

I'm guessing from all this that the Treble/Bass boost/cut pot is the 50KB/50KB pot that Peavey mentioned and the 20KB/100KC pot is the one that is Mid boost/cut on top and Mid frequency sweep on bottom.

But, I know nothing about pot resistance values versus EQ for different frequencies. Does it make sense that the resistance on a Treble boost/cut would be the same as for a Bass boost/cut? Or does it make more sense that they would have different values?

Also, is having a center detent implied in any of this? Or would one have to look specifically for a pot with the correct resistance and taper AND further specify having a center detent?

Thanks again for the help!

The pot value isn't directly responsible for the frequency at which the boost/cut occurs - it's determined by the circuit hidden inside of the preamp module. In many cases, your EQ pots will all be the same value. Your guess about 50k/50k bass/treble is a good one, and it does seem to be the most likely case.

Center detents are not implied, so you'd have to narrow your search a bit. If it's the B50K/B50K linear, you may be in luck - those are out there somewhere. The other B20K/C100K....your only option may be a custom pot at that point, and that's at least a $100 venture for a custom pot unless you can find the right pots to make your own.

[StuartV]

If it's the 50/50, Peavey has those in stock. Here's hoping.....