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 Post subject: potentiometer differences
 Post Posted: Wed Dec 10, 2008 2:01 am 
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Location: Spring Valley Ca.
I have noticed that bass pots [volume and tone] have different values that vary between basses. Could anyone help me out here?


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 Post subject: Re: potentiometer differences
 Post Posted: Wed Dec 10, 2008 3:24 am 
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A pot is a variable resistor, for one thing. A resistor has a certain role in which frequencies pass through it. An audio taper 250K pot and a 1M pot have different variable resistances, mostly involving upper limit and rate of change.

The higher the value of the pot (250K is the standard low end for passive basses, 1M or 1000K is about as high as you see them go), the less treble frequency is bled off or attenuated. So a higher value resistance generally means brighter tone will result. The resistance of your pickups should affect your choice of pot--i.e. how you balance them.

RIC uses 330K, which is a little brighter than the standard 250K, perhaps to match the slightly higher output of their pickups.


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 Post subject: Re: potentiometer differences
 Post Posted: Wed Dec 10, 2008 4:38 am 
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Thank you very much cassius, It's great to be able to get answers to all of my hair brained questions.Thanks again, Mark


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 Post subject: Re: potentiometer differences
 Post Posted: Wed Dec 10, 2008 7:17 am 
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It is hard to find decent audio taper pots today that roll off volume or tone the gradual way you would like. RS Guitarworks has some good ones, but they're spendy. Aside from them, CTS and Alphas often taper off the volume too quick and don't taper off the tone quick enough. There are 10% and 30% audio taper (log) pots, and the 10% are the best for these applications, but it is hard to find a true 10% taper pot these days.

I'm not sure who makes the ones RIC uses, but they're pretty good, as are all things RIC. :)

Greg


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 Post subject: Re: potentiometer differences
 Post Posted: Wed Dec 10, 2008 8:52 am 
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Thanks Greg, I appreciate your help. This is all new for me. I have always just plugged and played, and not really had much of a clue Thanks again, Mark


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 Post subject: Re: potentiometer differences
 Post Posted: Wed Dec 10, 2008 10:18 pm 
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Quote:
It is hard to find decent audio taper pots today that roll off volume or tone the gradual way you would like. RS Guitarworks has some good ones, but they're spendy. Aside from them, CTS and Alphas often taper off the volume too quick and don't taper off the tone quick enough. There are 10% and 30% audio taper (log) pots, and the 10% are the best for these applications, but it is hard to find a true 10% taper pot these days.

I'm not sure who makes the ones RIC uses, but they're pretty good, as are all things RIC. :)
Mr. Hall may correct me on this, but I believe they're custom-ordered CTS - they are on the Rickenbackers I've worked on, although that does not include the very most recent production.

I agree about the generally terrible quality of pots these days - even from formerly respected companies like CTS. As far as I know they have moved a lot of the production offshore (at least to Mexico) and the quality just isn't there any more, at least not in the standard 'commercial' range. I've had problems with the ones you can easily get via the normal parts resellers - with the tapers, the physical strength of the casings (which now often fall apart when pushing on Strat-type knobs), and the feel of the turning motion which is rough and imprecise. I'm actually considering using RS Guitarworks pots for repair work even with the much higher price - at least I can pass it on, and customers will know they're getting the best job I can do. I've had several customers complaining about exactly these kinds of problems even on top-quality instruments recently.

And if you think normal pots are bad, most mini-pots are worse - which means almost all push-pulls (sadly even including the one RIC uses on the 4003), since they are based on these... the last available CTS push-pull (which was only a SPST anyway, so not suitable for many functions) was discontinued a year or so ago. Most "Log" mini-pots are in fact made from two sections of linear track, since this is much cheaper - but the result is two sections of linear taper too, and you can easily hear this when turning the Treble Tone knob on a 4003 with the Vintage switch... it does almost nothing from full down to halfway, then drops to an intermediate tone which does not offer much fine control, and finally cuts off sharply below 1/4. As a result I will not use push-pulls on volume controls, they simply aren't good enough - and only tolerable on tone controls because most people don't use them that much.

Mini-pots seem to suck some of the life out of the tone too - I know this sounds silly (or like snobbery) but there is a potential cause in the internal capacitance of the pot, which minis seem to be much worse for. I find this a more important difference than the variation in actual track value (which is usually 20% even on high-quality pots) and which some people think makes a significant difference to the tone too. I must admit I can't really hear the difference between a 450K and a 550K "500K" pot, but I can certainly hear the difference if I rewire a guitar from mini-pots to something decent.

It's sad that such a fundamental component is no longer commonly manufactured to anything like the standard they were a couple of decades ago, and frustrating for me to see many nice old instruments and amps that have had their original (and probably perfectly good, if cleaned) pots replaced with low-quality modern junk by other techs who don't understand this.


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 Post subject: Re: potentiometer differences
 Post Posted: Wed Dec 10, 2008 11:36 pm 
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BlueAngel, thank you very much for that. With your experience, how would pots of different values effect the tone of a 1999v63? Thank's Mark


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 Post subject: Re: potentiometer differences
 Post Posted: Thu Dec 11, 2008 12:26 am 
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OK, and now for something completely different: I like a hybrid curve on my controls between straight linear and pure log audio. So, I usually purchase the highest quality 500kohm audio taper pots I can find, leave them be for overwound humbuckers, and add bridge resistors to bring the overall value down as the application requires. Remember your ohm's law about the reciprocal of the sum of the reciprocals in parallel to get the effectve value. Bridging with:
a 2 meg resistor gets you a 400kohm value;
a 1.5 meg resistor gets you a 375kohm kohm value;
a 1 meg resistor gets you a 333 kohm value;
and, of course, a 500kohm bridge resistor gets you a 250 kohm value, which is good for a tone control, but starts getting too linear to use for a volume control.

And if the pot is not exactly "spec," as most pots have a 20% tolerance, then I can fine tune the resistor accordingly to get what I need.


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 Post subject: Re: potentiometer differences
 Post Posted: Thu Dec 11, 2008 12:53 am 
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iiipopes, how would I apply that? Do you have some rules of thumb that I might apply to tonal applications? Thank's Mark


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 Post subject: Re: potentiometer differences
 Post Posted: Thu Dec 11, 2008 1:03 am 
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Quote:
BlueAngel, thank you very much for that. With your experience, how would pots of different values effect the tone of a 1999v63?

Higher resistance pots give a brighter tone, as cassius987 posted - both for tone and (slightly less obviously) for volume controls.

For a tone pot, it's simply that the more resistance there is between the pickup and the tone cap, the less highs are bled away. For a volume pot, it's because the load on the pickup is reduced, so the effect of the coil's inductance (very roughly related to the number of windings and the coil resistance, and which reduces the amount of highs sent to, or 'seen', by the amp) is reduced.

The two changes are slightly different in sound. Although it's normal to have both the volume and tone pots the same value, you don't have to, and you may prefer to change one rather than the other if you're after a particular result - increasing the tone control value is a 'simple' change that just increases brightness, but increasing the volume control value has some other effects as well - it can make the pickup sound more 'peaky' or 'spiky' as well as brighter, and it increases the treble LOSS as you turn down the control, which is a well-known effect (caused by the resistance of the pot and the capacitance of the guitar cable).

So for example if you want a bold, aggressive and fat tone and don't use the volume control for subtle adjustments, you may want to increase the volume pot value rather than the tone; conversely if you want a smoother sweep but clearer tone you may want to increase the tone but leave the volume.

The best values give the widest range of tones with the smoothest sweep of the controls, and depend on the type of pickups. Traditionally, 250K is the value for "single coils" and 500K for "humbuckers", but this can be misleading because although those values are well-chosen for Fender-style single coils and Gibson-style humbuckers, it isn't determined by the type of pickup but by its inductance - so a high-wound single coil like a Gibson P90 needs a 500K pot, not a 250. A Rickenbacker pickup is somewhere between the two, so RIC's choice of 330K is logical. Some unusually low- or high-inductance pickups may need very different values, eg 100K or 1M. (Fender did use 1M for many of its single-coil guitars in the 70s too, and almost no-one likes the sound of them - it's far too high for a normal single coil unless you really want that icepick-in-the-ear tone!)

"Log" (logarithmic) and "Lin" (linear) refer to the rate of change of value as you turn the pot. Because of the way the human ear works, a linear change in electrical volume or frequency response does not sound like that, so this is modified in a Log pot - a true Log pot has a value of 1/10th at the halfway point, whereas a linear has a value of 1/2. (There are other ratios of Log pot too, as Greg said.) Generally you always want a Log pot for a tone control as otherwise it tends to do nothing until it's almost all the way down, then suddenly cut off, but volume controls can usefully be either Log or Lin depending on how you want the sweep to work - either more or less change at either the top or bottom end of the turn.

Finally, the value of tone cap makes a difference too, with a larger value rolling off a wider range of frequencies - not really more treble, but further down into the midrange. The 'best' values traditionally are .022uF for "humbuckers" and .047uF or .1uF for "single coils" (Rickenbacker uses .047uF), but as usual it depends on both the exact type of pickup and what sound you're aiming for - a lot of people nowadays prefer smaller values because you can roll off more of the highs without getting into really deep sucked-out boomy tones. The cap value has a very small effect on the tone even when the control is up full too, but you have to listen closely to hear it.

You may also read a lot about different types or brands of cap affecting the tone, but I don't believe this - both theoretically and from practical experiments, it can be shown that it doesn't - not even the expensive hyped 'Bumblebee' and 'paper in oil' caps that a lot of people swear sound better. Any cap of the right value will sound as good as any other. Capacitance certainly does affect tone, both where it's meant to be (ie the tone cap) and where it's not (ie the pots, the wiring in the guitar and especially the cable to the amp), but it's the value that's important not the type or brand of cap.

(Types and even brands of cap DO make a significant difference in high-voltage amp circuits by the way, which is where this idea comes from - but it really doesn't apply in passive guitar circuits.)

Hope this is not too much info all at once! :-) It should at least give you an idea of what you may want to change (or not).


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