"Faking" the presence of a pot in a circuit? Line6man, help....

[stubhead]

O.K., I have gotten very used to the classic four-control Gibson setup, leaving the 3-way selector switch in the center and manipulating the two volumes. If you set your amp to sound it's best when both the guitar's volume knobs are at "7" or so, there's a whole world of sound in there. Just ask Duane Allman, Jimmy Page, Carlos & Eric (back when they sounded best, using Les Pauls :laughing3 and on and on. It's a very versatile setup once you're used to it.

I also use the bridge pickup's tone knob a lot, when you start with a fire-breathing Lawrence L500 pickup you'd better... but what I don't use is the neck pickup's tone control, it's always full up. So, I'm only using three knobs. But when I tried wiring a guitar up with that - just a volume control and no tone control on the neck pickup, and both tone and volume on the bridge pickup - it doesn't work right. The neck pickup's tone pot (500K) is doing something in there, even though it's all the way "open", supposedly.

On my upcoming Jagenstein (code name: "Peaches"), control space is going to be limited, at least in the first iteration. I would of course like to wire it with only three knobs. Is it as simple as just dropping the neck pickup's volume control down to a 250K pot, or wiring a resistor across the outside lugs or something? I understand the whole four-pot/two-pickup system is interactive (that's its joy) but I want to substitute some smaller component than a pot, since all it's going to do is sit there. I just don't quite understand what a 500K pot is, when it's not actively potting away.

 

[Cagey]

The easiest thing to do would be to just get a 250K or 500K 1/4 watt resistor (match the pot size you're used to using) and a .022μf or .033μf capacitor (match the size you're used to using) and wire them in series from the hot lead on your neck pickup to ground. The circuit will act like you have a tone pot on "10" all the time.

 

[Cagey]

Another thing you could do is put a small 250K or 500K trim pot in place of the 1/4 watt resistor and just tuck it away someplace in the control cavity in case you want to fine tune how that filter works. It wouldn't take up much room, and it would give you options.

If you were real crafty, you could mount the trim pot solidly someplace and drill a hole through the control cavity cover so you could tweak it without removing the cover.

The only reason the big 2 watt size pots are used is for their mechanical integrity. There's no power in the there to speak of.

 

[mayfly]

The easiest thing to do would be to just get a 250K or 500K 1/4 watt resistor (match the pot size you're used to using) and a .022μf or .033μf capacitor (match the size you're used to using) and wire them in series from the hot lead on your neck pickup to ground. The circuit will act like you have a tone pot on "10" all the time.

Cagey has it. That's all you need to do.  It's an equivalent circuit.

 

[line6man]

Yeah, if you want the sound of a tone pot on "10," with no actual pot, you simply replace the pot with a resistor whose value is equal to the full resistance of the pot. If you have 250k pots, that's 250k, usually +/- 20%, which would best be substituted with a 220k resistor, since 250k is an uncommon value for resistors. If you have 500k pots, on the other hand, in my experience, the resistance is too high to really hear very much difference between having a tone pot on "10" and removed from the circuit, with the average pickup impedance. But I've only experimented with tone pots on Jazz basses, so your results may vary. In any case, 500k pots would be replaced by easy-to-find 470k resistors.

1/4W and 1/2W carbon and metal films are most common. You might argue the higher wattage to have less contact noise with it's larger resistive area, and such, but it makes no difference. Any non-inductive resistor with a decent tolerance will sound fine.

As Cagey said, perhaps you might consider using a trimmer pot, mounted inside the control cavity, so you can tweak the tone pot to exactly where you want it, and compensate for different pickups if you are a pickup swapper.  That's what I would do.

 

[stubhead]

If you have 500k pots, on the other hand, in my experience, the resistance is too high to really hear very much difference between having a tone pot on "10" and removed from the circuit, with the average pickup impedance.

I'm kind of surprised by how big a difference it does seem to make, I thought a 500K pot "on" was pretty close to wide open. Of course, this sensitivity is based on the other pots being turned down some too, so I guess it's like pinching a garden hose in a few different places - every one counts for more. And after I started testing all the pots, I was surprised to see how often they were low. The specs may say (+ - 20%) but I've never had one go over, and most of the 500's are between 480 and 440. the "250's" often dip below 200, which has got to make a difference. And this is the reputable CTS pots, I may even have to go the Bourns route.

I like the idea for a trimpot, as this is basically going to be a test run for the "real" build... I picked up a Squier "Jagmaster" body on Ebay for $76 specifically to toy with, though as I can't decide between hardtail or whammy, 24" scale and 24.75" scale, rocket-science or streamlined wiring, I 'll probably end up doing the normal Warmoth thing - build one of each.

 

[line6man]

And after I started testing all the pots, I was surprised to see how often they were low. The specs may say (+ - 20%) but I've never had one go over, and most of the 500's are between 480 and 440. the "250's" often dip below 200

440k to 480k on a 500k pot is fine. That's within 12%, when tolerances are often 20%.
Below 200k on a 250k pot exceeds a 20% tolerance, however, so I'd trash those pots and find something better.