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View Full Version : 500k/250k pots -- what effect do they have on tone?


ari
11-29-2004, 01:52 PM
So, a regular Strat comes with 250k pots on volume and tone controls. Les Paul and humbuckers come with 500k pots, correct?

So are the 500k pots simply brighter than 250k? Are there any other tonal differences? What happens if 250k pot is used on volume but 500k is used on tone onctrols?

Thanks,

ari

Clorenzo
11-30-2004, 12:27 AM
http://www.thegearpage.net/board/showthread.php?s=&threadid=57716

Jim Collins
12-04-2004, 11:06 AM
One other thing to add is the fact that the higher the value of the volume pot, the greater the treble rolloff will be as you roll off the volume. A 1Meg volume pot in a Telecaster will sound annoyingly bright when the volume pot is full on, but as soon as you back off on the volume, the highs will roll off rather dramatically. This is why you frequently see a treble bypass cap on a guitar with a 1Meg pot.

The treble rolloff with a humbucker and a 500K volume pot won't be that bad, but the rolloff with a single coil and a 500K pot will be quite noticeable. This is one reason a PRS rotary switch guitar has a 180pF treble bypass cap on the volume pot. The two, parallel single coil positions in such a guitar have significant treble rolloff with the 500K volume pot -- too much, really, to be acceptable, so the byass cap is necessary.

Clorenzo
12-04-2004, 01:36 PM
Originally posted by Old Tele man
TONE control pot value doesn't matter too much because the overall "taper" or "rolloff" can be 'adjusted' by simply changing the cap value. This is not correct. The value of the tone pot when it is fully open has exactly the same loading effect as that of the volume pot, and in fact does not depend on the value of the cap (for usual values), which only affects its response when you turn it down. In other words, a single 250 k vol. pot will produce the same amount of treble rolloff as a 500k vol. pot and a 500k tone pot (in all cases with the pots at maximum) since the parallel of 500k and 500k equals 250k.

You can see this effect and also the one explained by Jim in my simulation spreadsheet: www.harryj.net/voltone.xls. The default pickup parameters are for a vintage humbucker, use pickup L = 2.8 H and pickup C = 60 pF for a typical single coil.

Clorenzo
12-05-2004, 02:12 AM
Originally posted by Old Tele man
...unless a "no-load" pot is used, the TONE control resistance shows up as frequency-sensitive (impedance) load on the pickup(s)...at LOW-frequencies, it's essentially non-exsistant; but at HIGH-frequencies, the capacitive-reactance becomes very small, effectively "shorting" the TONE pot directly across the pickup / VOL-pot circuitry. That's of course true, but given that it's only at high frequencies that the loading affects the frequency response (the whole circuit is effectively a low pass filter with a cutoff freq. of about 5 kHz), and given the practical cap values used, the effect is negligible. For example, for a vintage type humbucker with 500k tone controls, if you compare the point of maximum variation in the frequency response (the resonant peak at about 2.5 kHz) between using a 10 nF and a 100 nF tone cap, which are pretty much the extremes of the range of values normally used, you will see a variation of 0.01 dB. In fact this is not surprising if you consider that the reactance of a 10 nF cap @ 2.5 kHz is about 6 kohm and for a 100 nF it's 600 ohm, but that's in series with 500 kohm and the difference between 506kohm and 500.6 kohm is about 1%. And that's in parallel with the volume control, cable capacitance and amp. input impedance. To see just a 0.5 dB variation you would have to go down to a 0.47 nF cap, totally useless in a tone control.

It's only when you start actually using the tone control (by turning it down) that you see significant differences between different practical cap values, because then the resitance of the pot becames comparable to the reactance of the cap. And even then you will not see big differences before you set the control below 5 or so.

Clorenzo
12-06-2004, 04:07 AM
Originally posted by Old Tele man
...but the REAL "bottom-line" is that the "sum" of it all must pass through the speakers which seldom get 5-6KHz, making much of what we're discussing "mute." Don't forget that just before the upper cutoff frequency you have the cone breakup region with substantial resonances (10 dB peaks are quite normal) between about 1 and 5 kHz, precisely the region where the resonant frequency of the combined pickup - volume control - tone control - cable capacitance - amp. input impedance will be, and also the frequency band the ear is most sensitive to, so any variations in amplitude or frequency of that resonance will be easily audible and contribute substantially to the particular "voice" of the guitar - loudspeaker combination. Otherwise we would not be discussing pot values, would we? ;)