Different value pots, how much difference do they make on a bass?

[Guest MoJo]

I recently swapped out the stock pickups in my Peavey Zephyr V, replacing them with Seymour Duncan SSB-5's. The difference is incredible. It's like I've had my ears syringed. I left the stock 500k pots in but noticed that the instructions, included with the pickups, recommended 250k pots. Playing the bass with it's new pickups through my Tascam GB-10 bass trainer into headphones, I though that maybe the top end was a bit harsh so I ordered a set of 250k pots. Playing the bass through my amp at a gig however, I couldn't detect any harshness so I haven't fitted the new pots just yet.
My question is, how much difference to the top end 'zing' would 250k pots really make? I'm tempted to try them just in case they warm the timbre slightly to give a more pleasing aural experience but, at the back of my mind, I'm thinking, it's a lot of work if it knocks a lot of the top end off and I have to take them all out again.

Edited June 17, 2015 by MoJo

[KiOgon]

SD's chart shows the resonant peak of the SSBs to be 9.2KHz for the Neck & 8.1KHz Bridge http://www.seymourduncan.com/comparetones
That's pretty high for bass pups so I certainly wouldn't normally use 500K pots with them, best stick to 250Ks, however I can't tell you how much percievable difference that would make to you Please let us know if you try changing them

Cheerz, John

[paul_5]

I put 500kΩ pots on my Jazz (one single coil, one humbucker) and it's really opened up the top end. Even when I run the humbucker in 'single coil mode' it's a lovely, biting edgy sound. I like that. I fully understand that some folks might not though.

[brensabre79]

Higher value pots (500k) traditionally were used on guitars like Gibsons that have Humbuckers because they allowed more treble through - the humbuckers being naturally darker sounding, 500k being brighter sounding than 250k, balanced out well. Fender guitars (and basses) with single coils used 250k because they are naturally much brighter pickups anyway. Rickenbacker used 330k (or something like) as somewhere in between.

Back in the days these things were first invented amps did not have a very wide range of EQ capabilities, so it was important to unify the sound from the instrument a little. Although to my ears a Strat through a Marshall amp still sounds like breaking glass.

Anyway, same principles apply with bass pickups.

Now, you've gone and got yourself some brighter sounding pickups, the recommended 250k pots will take away a little bit of the harshness you're experiencing. This is probably the reason they recommend them.

In reality though, as long as you know that it may be sounding a little harsh you can use EQ on your amp / desk to do the same job as changing the pots.

[dannybuoy]

When I get some free time I intend to upgrade the pots in my Warwick Rockbass Streamer, which oddly comes with 25K pots wired up to 2 humbuckers, all passive. No wonder it sounds crap when plugged straight in (but awesome when boosted/EQed with various pedals).

[brensabre79]

[quote name='dannybuoy' timestamp='1434539451' post='2800523']
When I get some free time I intend to upgrade the pots in my Warwick Rockbass Streamer, which oddly comes with 25K pots wired up to 2 humbuckers, all passive. No wonder it sounds crap when plugged straight in (but awesome when boosted/EQed with various pedals).
[/quote]

Some passive pickups do require 25k pots due to high impedence. I had a passive Ibanez bass with 25k pots and when i replaced the pots with 250k it sounded very thin and clanky. But generally 25k pots are used on instruments with active pickups.

[Guest MoJo]

[quote name='KiOgon' timestamp='1434529419' post='2800370']
SD's chart shows the resonant peak of the SSBs to be 9.2KHz for the Neck & 8.1KHz Bridge http://www.seymourduncan.com/comparetones
That's pretty high for bass pups so I certainly wouldn't normally use 500K pots with them, best stick to 250Ks, however I can't tell you how much percievable difference that would make to you Please let us know if you try changing them

Cheerz, John
[/quote]

Thanks for the info John. I've decided to swap them over at the weekend. I'll record the bass before and after and post the results on here

[alyctes]

Would changing only one pot be a way of getting an ear for the difference?
(Disclaimer: this may be an electrically stupid idea

[ikay]

This thread from TB has a link (in the first post) to some audio clips comparing different pot values and types -
[url="http://www.talkbass.com/threads/pot-shootout-250k-vs-500k-audio-vs-linear.671456/"]http://www.talkbass.com/threads/pot-shootout-250k-vs-500k-audio-vs-linear.671456/[/url]

[Lozz196]

I took a Precision to a luthier for a change of pickups and a dodgy pot, he replaced the dodgy but regular 250 with a 500 and suddenly the bass was treble-city. Only found out when I took it to another luthier for yet another pickup swap and after discussing the sound I wanted he asked if I still wanted a 500. I`d thought it was the pickup that was the issue, but he advised that the 500 would let a lot more top end through. As I swapped the pickup out at the same time never got a chance to put that to the test, but everything else he`s told me re the workings of basses/guitars has panned out to be true so no reason to doubt him on this.

[tauzero]

There's an article on the reasons behind the value of pots affecting the tone here: [url="http://www.seymourduncan.com/blog/tips-and-tricks/250k-pots-versus-500k-pots-going-deeper-into-the-subject/"]http://www.seymourduncan.com/blog/tips-and-tricks/250k-pots-versus-500k-pots-going-deeper-into-the-subject/[/url] and very interesting it is too.

[pfretrock]

Being a complete anorak and wanting to put some [url="http://www.linear.com/designtools/software/"]Spice[/url] in my dull life, I was not convinced of the pot change thing. So I did the science bit. Taking a SPB-3 pickup, the inductance came out at around 7.8H and the DC resistanvce measured 11.56k (Seymour don't give the inductance but give resistance as 12.5K but it is temperature sensitive). I also estimated the self capacitance to be about 50pf (I'll spare the details on these)
Using a standard 47n tone cap, and a 250k pot wide open, a Spice model gave the -3dB point at 2760Hz. Substituting a 500k pot gives 3577Hz. So quite a difference.
I'm not convinced changing the volume pot does much, but it depends how it is wired (single or two volume pup). I may come back to this.

Perhaps someone can help me with this? I don't have a J only a P and a P/J. The capacitance of a J pup is much higher than a split P, around 200pf for the J. The reason is the P has the wire spread over two coils whereas the J is bunched up on one coil. This gives the J (in theory) a lot more top cut. The SJB-3 resonance is 4300Hz and the SPB-3 10200Hz, due to the difference in self capacitance. They have similar inductance.

(Resonance point will not give the highest output, it is the point where the output drops to its lowest, since all the energy at that frequency is absorbed and does not get out. The output naturally decreases up to the resonance point)

So is a P more trebly than a J - in a similar position?

The gain from using a 500k pot on J may be less (due to the higher capacitance), I've not worked it out yet, but probably worthwhile.

(end of science lesson )

[ikay]

pfretrock, can you clarify the point you make above about the resonance point? I always thought that this referred to the resonant peak of the pickup which is a distinct peak in output, not where it drops to it's lowest (as on the frequency chart below which shows the peak for various different loads). Thanks

Edited July 1, 2015 by ikay

[pfretrock]

it is common to get peaks like your diagram and the lead capacitance seems to be a significant factor, together with the other components. However that peak is not the pickup resonance point, which is 7kHz - determined only by the inductance of 3.2H and capacitance of 160pf. These form a parallel tuned circuit which resonates at 7kHz ( Google will give the method of calculating this). So the pickup resonance point is over to the right in the plot and the other components are already attenuating the signal at this frequency. At the resonant frequency of a parallel tuned circuit, the impedance is highest, which will give a minimum signal. I'll try and post a screen snap of the Spice analysys to show this

[pfretrock]

Attached spice screen shot. Pickup inductance (L1) is 7.8H, capacitance (C6) 43p and resistance (R11) 11.6k. This resonates at 9kHz. The Seymore Duncan data says this should be 10.2kHz, so I may have some errors in my measurement (they may also!).

R12 and C5 is the tone cap and pot, wound fully on. R14 represents the vol pot, wound fully on. C3 is a bit of cable capacitance. The rest of the circuit is the front end of my amp.
The upper solid trace shows the amp output as the circuit analysis is swept from 10Hz to 12 kHz.
With tone flat out, the signal starts to drop at around 3kHz. When the pot is wound back, this point moves to lower frequency.

The minimum response occurrs at 9kHz, which is the pickup resonance frequency since it is at its highest impedance at this point and forms a low pass filter with the other components.Usually the tone pot kicks in before this frequency.
It is possible to get a peak like in ikay's post by fiddling with the capacitor values, so that the signal peaks before it starts to roll off.

Incidentally the pickup DC resistance (11.6k) only has a very minor effect on the circuit, it has no effect on resonance frequency.

Edited July 5, 2015 by pfretrock

[pfretrock]

For some reason the thumbnail is not opening to full resolution - or maybe its my PC!

[pfretrock]

[quote name='ikay' timestamp='1435736938' post='2811860']
pfretrock, can you clarify the point you make above about the resonance point? I always thought that this referred to the resonant peak of the pickup which is a distinct peak in output, not where it drops to it's lowest (as on the frequency chart below which shows the peak for various different loads). Thanks
[/quote]

Attached a Spice run of the data in your plot. The peak (about 5dB high) at around 3.5kHz depends on the capacitance on the input (i've guessed a 10ft lead at 47pf/ft). You can also see the resonance dip at a little over 7kHz due to the L and C values of the pickup.
If the lead capacitance is reduced to low values, you don't get the peak (so some leads may sound different with flat EQ). Also the peak is much higher with 500k pots rather than 250k, so things will get brighter with 500k pots (and a suitable lead?)

Sorry the plots are not clear - not sure what is happening. I'll try and fix them.

[attachment=195678:basschat.jpg]

Edited July 5, 2015 by pfretrock

[ikay]

Ah. ok thanks for explaining that. The plot in your chart shows an incredibly flat response up to the resonance point which I guess is down to it being a simulation. I think that in most practical cases (with all the other various loads and capacitance etc) there would be a peak just before the cut-off point which is what gives a pickup it's distinctive tonal character. What this really tells me is that I don't understand enough about the science here and that there's a good amount of voodoo involved in pickup design and electronics!

[pfretrock]

If I add capacitance to the input of my circuit, I get a peak in the simulation. The peak is greater with 500k pots and lower pickup inductance. Of course as soon as you wind the tone pot back, the peak drops.

[pfretrock]

One more plot, from your data, for lead lengths of 5 (red), 10(orange), 15 (yellow), 20 (green) and 25(purple) feet.
The dip at ~7Khz is due to the parallel resonance between the inductance and capacitance of the pickup (what the pickup manufacturer gives a resonance).
The peaks positions are a series resonance between the inductance and capacitance of the lead.
I'm assuming 33pF per foot.
However the best measure of anything is ears!

EDIT added parallel, series.

Edited July 5, 2015 by pfretrock