Output drop

J

Jarrodpimentals

New member
Hello. I’m losing my mind. I’ve installed a jb Trembucker and it sounds off. So I checked the output signal at the end of my guitar cable and it’s only reading 16.9 instead of advertised 17.6k. So I uninstall the pickup and check it and it’s 17.6 uninstalled. If I just solder it to a volume pot and check the other side of the pot it’s 16.9. I’ve tried 3 different pots now and as soon as I solder it I lose output. What in the world is happening. None of my other guitar pickups are lower than advertised.
 
Unsolder them and measure again. 17 600 Ohms in parallel with 500 000 Ohm would be 17 001.55 Ohms. In real world all that numbers will be slightly different (up to 20% difference) because of manufacturing tolerances, environment temperature and multimeter calibration.
 
A 0.7k resistance drop shouldn't be noticeable in output. It shouldn't "sound off". So what is it, really, that you are complaining about?
 
...not to mention that DCR of passive pickups is not indicative of their output level. ;-)

To extrapolate on what has already been explained: if a 17.6k pickup drops @ 16.9k when paired with a single pot, then this single pot measures 424.912k (IOW, it's a 500k pot under its official specs, as it's often the case).

Useful tool: https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/

Enter 17600 for R1, 424912 for R2, and see what is the.... "output" (!). :-)
 
Have you measured the pot resistance? If it's around 480Kohm it sounds about right. What is your concern?
 
The issue is 2 things. 1 my guitar sounds mushy. It’s a jb so it shouldn’t be bassy and mushy. 2 I have another guitar with a jb in it and it does not lose any output after it’s connected. It still reads 16.6 at the jack and my other guitar is 8.4K from the factory specs and it reads 8.4K at the guitar jack. I currently have the guitar in question wired straight to the input jack and it’s only 17.1k. As soon as I disconnect the jack the pickup goes back to 17.4k. Not sure I mentioned this one is a Trembucker so it’s 17.4
 
Last edited:
Jarrodpimentals said:
The issue is 2 things. 1 my guitar sounds mushy. It’s a jb so it shouldn’t be bassy and mushy. 2 I have another guitar with a jb in it and it does not lose any output after it’s connected. It still reads 16.6 at the jack and my other guitar is 8.4K from the factory specs and it reads 8.4K at the guitar jack.
Click to expand...

1-Regarding the mushy sound, keep in mind that a passive pickup sonically depends on the resistive load of pots but also on parasitic capacitance of the wiring + cable (and on the acoustic resonance of the guitar, of course). And it's objectively possible for a JB to sound bassy and mushy since it's a high inductance pickup...

2-It's physically not possible for a passive pickup to measure the same DCR alone then in parallel with some pot(s)*. If a pickup reads 8.4k when paired to a couple of 500k pots, it means that in measures 8.7k when disconnected.

3-The 17.4k of the trembucker alone vs when connected at the output jack can be explained as following:
a)you warmed it by holding it in your hand while it was not mounted so its DCR got higher.
b)there"s a problem with the output jack, since it behaves like a 1M resistor (17.4k in parallel with 1M = 17.1k)... Defective components can exhibit parasitic resistive bleeding or capacitive bleeding explaining such results. And a capacitive bleeding might certainly make the sound mushy... So I'd try another output jack plug, there.




*I've already explained this in the post 8 but if further explanations from other folks are necessary, here we go:

https://youtu.be/gKIhgfpyTVA?si=3GZ0T85aPUzvXzsN&t=473

​
 
Just re assembled my guitar so I can sell it and it still sounds terrible. But now my output reading is 19.4k on a Trembucker that is supposed to be 17.4. FOR THE RECORD I have a guitar right next to it that Has a normal JB(16.6k) and with the same multimeter this guitar reads 16.64k. Anyone want to try and explain this all to me?
 
Is there a chance you're holding the meter leads with your fingers? Or are they clip-on's? Skin resistance, at different perspiration levels, could cause that.
 
Jarrodpimentals said:
Just re assembled my guitar so I can sell it and it still sounds terrible. But now my output reading is 19.4k on a Trembucker that is supposed to be 17.4. FOR THE RECORD I have a guitar right next to it that Has a normal JB(16.6k) and with the same multimeter this guitar reads 16.64k. Anyone want to try and explain this all to me?
Click to expand...

Can you hear the differences between them?
 
Jarrodpimentals said:
Just re assembled my guitar so I can sell it and it still sounds terrible. But now my output reading is 19.4k on a Trembucker that is supposed to be 17.4. FOR THE RECORD I have a guitar right next to it that Has a normal JB(16.6k) and with the same multimeter this guitar reads 16.64k. Anyone want to try and explain this all to me?
Click to expand...

IF this "new" measurement is not simply due to ambient heat (of summer or human body) or to the multimeter, some bad soldering might explain a higher resistance than expected, as would do defective components - the probes of the meter would also exhibit a potentially variable resistance if they're not intact, the volume pot might add a bit of its own resistance to the pickup if a protuding blob of solder forbids it to be really set full up and/or... the pickup itself might ramble, if it has been harmed by multiple manipulations: fluctuating resistance is the sign of a twisted wire going to separate itself from a coil, for instance.

Regarding the JB measured at 16.64k in a guitar: as already mentionned in my previous posts, its own resistance would/should therefore be higher without pot(s), according to the explanations and formulas that I've shared.
Checking empirically these explanations and formulas is easy: connect then disconnect a pickup from its pot(s) with a set of alligator clips and while doing this, watch its DCR changing in direct on the meter...

The most probable explanations to me in this "case" remain ambient heat or a problem with the meter. I'd start by putting a new battery in it and by using it in "normal" conditions of humidity / temperature, without moving anything during the measurements.
 
Last edited:
Jarrodpimentals said:
Just re assembled my guitar so I can sell it and it still sounds terrible. But now my output reading is 19.4k on a Trembucker that is supposed to be 17.4. FOR THE RECORD I have a guitar right next to it that Has a normal JB(16.6k) and with the same multimeter this guitar reads 16.64k. Anyone want to try and explain this all to me?
Click to expand...

Additional series resistance because of bad soldering. Or Fender TBX instead of the usual tone control.
 
For the record, a Fender TBX control can't increase the measured DCR of a passive pickup: it just decreases it less than a 250k or 500k pot would do... :-)
 
freefrog said:
For the record, a Fender TBX control can't increase the measured DCR of a passive pickup: it just decreases it less than a 250k or 500k pot would do... :-)
Click to expand...

Not at all. 1M part of this pot is not a zero Ohm from the center to the CCW position, it is something about 1K. And I forgot to mention that I wired it completely differently, like Phostenix mod. So that 1K is always in series with my pickups.

TBXModSchematic.jpg
 
htrd said:
Not at all. 1M part of this pot is not a zero Ohm from the center to the CCW position, it is something about 1K. And I forgot to mention that I wired it completely differently, like Phostenix mod. So that 1K is always in series with my pickups.

Click to expand...

Ahhh, OK...

Now, the standard (Fender) wiring of a TBX varies the resistive load in parallel with pickups and exhibits a maximum resistance of 1M + 82k = 1082k, if memory serves me.
If I put 1082k in parallel with a 16.6k pickup, it will measure 16.34k (without taking in account the presence of the volume pot, of course). So, a TBX wired as Fender intented it to be doesn't increase the measured DCR.

A TBX "modified for high and low cut" is obviously another story: I see it as a variation on the G&L PTB circuit implemented on a TBX pot.
It's a clever idea, objectively more efficient as a "Treble Bass Expander" than the Fender original TBX recipe, and that I've applied a few times as well. :-)
But in such a case, it's no more a "TBX control" as Fender designed / named it and being not a telepath, I can't guess when someone did "forgot to mention" that it was a "modified" one... ;-)
 
You must log in or register to reply here.
Back
Top