Brittle tone from JB when volume pot rolled back - need opposite of treble bleed cap

[tonesnack]

Looking for a way to deal with the sound I'm getting from a JB is a Les Paul bridge position. When i roll down the volume, the sound really thins out and sounds brittle. This is in the gain channel of any Peavey, Mesa or Marshall. I know that the 16.4k JB will not lose highs when rolled down, but it's too much. I'm looking for something that's the opposite of a bright cap, a low pass filter that becomes effective as I roll down the volume - or any other trick. Not interested in 250k pots - they kill too much high end.
Anybody understand exactly how a high resistance PU causes this? A high pass filter is developing as the pot is rolled down. This does not happen with 8k PUs.

 

[Clint 55]

330k volume?

https://www.guitarfetish.com/XGP-Cu...30K-Single-Coil-Volume-Our-Best-_p_22997.html

 

[beaubrummels]

Welcome to the forum

Does your Les Paul have a treble bleed built in? If so, just disconnect it. I've haven't ever had a JB get thin when turning down in my LPs, so if there isn't a modern treble bleed in there, I can't think of what might cause it.

 

[tonesnack]

No treble bleed cap on the LP. Just a 500k Bournes pot.

 

[tonesnack]

Has anyone ever torn apart a JB to see what makes it tick? It's a fairly unique PU. Is it possible that there is some high pass filtering built in to it?

 

[beaubrummels]

A JB is two coils of wire and a magnet. There's no trickery. It's just the particular wire gauge, particular number of turns and magnet make for a workhorse recipe.

 

[Mincer]

The JB sort of gets that 'hollow' sound as you turn the volume down. At least every JB I've tried gets that. I think it is just the way that pickup sounds. It wants to be up full all the time.

 

[larryguitar]

I've torn one down in an attempt to better understand it; Beau is right, two coils and a magnet, along with the connections and hardware required to make it meet the required form factor; nickel silver baseplate, brass (or brass-plated) bobbin screws and what seem to be plastic (not butyrate) bobbins. Looks like 44AWG wire, reported to be an A5 magnet. Just a pickup, in the sense that the best meal you ever ate was 'just food.'

Larry

 

[eclecticsynergy]

Is the guitar wired 50s style? That retains highs when you roll back the volume pot.
Not as much as with a treble bleed, but it's definitely noticeable.
Especially if your LP is a bright/thin one - they can vary a lot from one to another and a few are super bright.

With modern wiring, you get more darkening when you roll back.

 

[tonesnack]

I checked. It's modern wiring.

 

[freefrog]

I've no SH4 at disposal for the moment to check experimentaly my statement but it might be a side effect of the 4 conductors cable and its parasitic capacitance.

As this phenomenon is never mentioned online (and probably not conscientized in most cases), see the posts 2, 18, 21, 22 and 23 in a topic that I've devoted to the question there:

https://music-electronics-forum.com/...ts?view=thread

If my intuition is right, the effect that you mention might be attenuated by a simple low value capacitor to ground located between the coils (at the junction between red and white wires). I insist on "low value" (low capacitance) or the cap will create a kind of inner VariTone circuit affecting fundamental notes. Try with 10pF to 220pF, with increments of 10 or 20pF only between caps - and don't hesitate to reverse the wiring + swapping the mag: it might even solve the issue without added capacitor, by redistributing the stray capacitance of each coil + its own wires...

 

[freefrog]

As a footnote to my previous answer, here is the response of a SH4 found in my experimental archives.



Red line = the PU played in bridge position, direct to the board through a 1M input, in single notes, from unfretted low E to 22th fret of high E string, all the frequencies produced being stacked... I've put a vertical line to differentiate fundamental notes (with peaks and dips mostly due to the resonance of the guitar and its dead spots) from harmonics - mainly shaped by the comb filtering theorized by Don Tillman + the resonant peaks of the pickup.

Pink and black lines = the electrically induced response of each coil, as reshaped by stray capacitance. If the pickup had a short connection between coils and a coaxial cable, the peaks and dips beyond 2750hz (main resonance of the PU's) would be way higher pitched and probably not detectable on the screen.

My hypothesis is that when the volume control is lowered, the secondary peak @ 8khz in the pink line becomes more perceptible.

Of course, this explanation won't be valid as such if the pickup has ALREADY a coaxial 2 cond. cable. But I suppose it's not the case (?)...

 

[eclecticsynergy]

I've no SH4 at disposal for the moment to check experimentaly my statement but it might be a side effect of the 4 conductors cable and its parasitic capacitance.

As this phenomenon is never mentioned online (and probably not conscientized in most cases), see the posts 2, 18, 21, 22 and 23 in a topic that I've devoted to the question there:

https://music-electronics-forum.com/...ts?view=thread

If my intuition is right, the effect that you mention might be attenuated by a simple low value capacitor to ground located between the coils (at the junction between red and white wires). I insist on "low value" (low capacitance) or the cap will create a kind of inner VariTone circuit affecting fundamental notes. Try with 10pF to 220pF, with increments of 10 or 20pF only between caps - and don't hesitate to reverse the wiring + swapping the mag: it might even solve the issue without added capacitor, by redistributing the stray capacitance of each coil + its own wires...

Thanks, freefrog. As usual, you share a wealth of information.

Given the stray capacitance issue on 4-wire humbuckers, I guess it would make sense to cut the leads as short as possible.
I had been leaving them long, in case of future swapping or sale.

 

[beaubrummels]

FWIW I've left 4-cond full length with every pickup I've used that wasn't already cut (that's 20-25+ pickups over 10 years at least) and I've not had a pickup behave unusually yet, even compared to 2-cond versions of the same pickup.

I really believe something else has to be wrong in the circuit.

 

[tonesnack]

Thanks Freefrog, I'm going to check that out. One thing I've wondered about is why this does not happen with pups of lower total coil resistance. With 8-9k PAFs I've had to use treble bleed caps to retain treble when turning down the guitar volume. I suppose that somewhere around 12k a perfect balance might be achieved. I know a 13kish super distortion I have sounds quite balanced when rolled down. (500k Bournes)

 

[freefrog]

Thanks, freefrog. As usual, you share a wealth of information.

Given the stray capacitance issue on 4-wire humbuckers, I guess it would make sense to cut the leads as short as possible.
I had been leaving them long, in case of future swapping or sale.

You"re welcome.

My merit is relative: without saying it (and maybe without conscientizing it), a famous US winder has actually based one of its well-known tricks on the use of assymetrical capacitive values. ;-)

And I wouldn't be able to share this kind of info if I hadn't helped a winder to correct one of his designs plagued by unrequested parasitic capacitance. So I thank him as you thank me.


That said: when it comes to humbuckers coils wound with a same wire gauge, the problem is that stray capacitance is narrowly dependant on the physical reality of each pickup and wiring.

In many cases, one coil compensates the other and the overall frequency response appears as flat, with minor deviations. That's why some people won't never notice any issue with 4 conductors wiring.

Furthermore, even the presence of covers, wax potting, shielding foils and so on can change the overall capacitive load AND "capacitive balance" between coils.

Reason why, to obtain the same correction on two pickups of a same model, it can be necessary to use capacitors of different values... A bit like with the resistors used to "tune" Duncan stacks.

Shortening 4 conductors cables can certainly help by shifting up the "cross frequency" between coils but it doesn't prevent the potential issue created by stray capacitance, for two reasons and/or in two situations:

1)when wound with more turns of thinner wire, high DCR pickups have inherently more capacitive coils. It shifts down their main resonant peak AND the "crossover frequency" that most normal humbuckers keep out of perception. It's not always "correctible" by cutting shorter the 4 cond. cable;

2) what makes stray capacitance annoying or not because of secondary peaks in the audio range is the capacitive proportion between coils: a difference of 10pF only can solve the problem or make it worse (!).
BTW and FWIW, that's what I tried to share there: https://music-electronics-forum.com...ransducers-a-few-thoughts?p=964626#post964626

That's why I've recommended to try capacitors from 10pF to 220pF with the smallest increments possible between capacitive values.

That's also why I've not claimed it's a miraculous solution. Ideally, one would need to measure the resonant peak of a pickup by exciting it electrically to check if a capacitive correction is efficient or not...

 

[freefrog]

Thanks Freefrog, I'm going to check that out. One thing I've wondered about is why this does not happen with pups of lower total coil resistance. With 8-9k PAFs I've had to use treble bleed caps to retain treble when turning down the guitar volume. I suppose that somewhere around 12k a perfect balance might be achieved. I know a 13kish super distortion I have sounds quite balanced when rolled down. (500k Bournes)

Low resistance pickups are not prone to develop the problem that I've tried to explain because, hosting less wire, they are inherently less capacitive than hot humbuckers.

That what the post 18 shows in my topic on the music electronics forum.

A DiMarzio Super Dist exhibits actually a noticeable stray capacitance BUT each of its coils compensates the other because they are symetrical (in DCR and by hosting identical hex screw poles).

As I said in the previous answer, IF a pickup is brittle sounding because of a secondary peak due to stray capacitance, it's due to the proportion between capacitive values of coils + their wires. And this proportion is potentially different for each pickup in each guitar. Even disconnecting the bare wire or using more or less solder changes that...

Hence the random nature of the issue and my advices: a 47pF to ground at the junction between white and red wires might solve the problem while a 68pF makes it worse... or conversely. Like reversing the magnet AND wiring of a high gain pickup might tame the issue... or create it.

That said, I don't know if my hypothesis is correct in your case (and I can't check it since your pickup and my lab gear are probably far from each others: I live in Europe).

But in my mind, there's not hundreds of possible explanations for what you experiment.

BTW, what DCR do you read on YOUR SH4? Feel free to mention the resistance of each coil, it might help.

 

[zionstrat]

I love Freefrog!!!!
Every time I learn something.