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Post by ashcatlt on Aug 7, 2008 12:34:27 GMT -5
What I need is a 500K linear taper Les Paul pot (you know, the ones with the long threads) which reaches the extremes of its resistance before it reaches the extremes of its rotation. The idea I've got is, at each end of the wafer, run a little bit of conductive paint from the metal at the end along the edge of and contacting the resistive strip. It needs to be carefully applied so the wiper doesn't actually contact the paint. I'm thinking that could be bad. This is (obviously) going to require dissambling and reassembling the pot. And that's where my problem comes. I'm pretty sure I could get it apart, but I doubt my ability to get it back together in or working order. Also, I don't have any conductive paint and don't really need it for anything other than the couple dabs this project would take. Until, that is, I decide I need another of these... I'm well aware of the fact that it will end up a bit less than 500K. That's fine with me. At least this way my passive volume pedal will go from all the way off to all the way on, and avoid putting undue rotational stress on the pot. I have yet to do the measurements to figure out how much of each end I need to short out. Details, details... So, if anybody's got the paint and the patience, I'll buy the pots, pay shipping, even buy you a couple beers to help steady your hand.  |
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Post by JohnH on Aug 7, 2008 16:01:07 GMT -5
Just go for it yourself! I've been opening up and putting back together quite a few pots over the last couple of years with no problem, even with the first. Just prise up the case lugs with long nose pliers without bending them too much, to lift off the case back, then sqeeze them down carefully when you are done - and the pot is perfect. I do this to cut the track, or scrape the sides a little to correct a too-low resistance.
But why do you want to do what you want to do?
John
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Post by ashcatlt on Aug 7, 2008 21:49:31 GMT -5
Maybe I will. Order a couple spares just in case I mess something up. Still end up with a whole can/jar of metallic paint I don't need...
I need it for my "passive" volume pedal. My strat doesn't have knobs, you see. So, after confirming here that it doesn't matter which end of the cable the volume/tone knobs sit, I took the guts out of a Crybaby. Replaced the electronics with a variable-Z buffer. Replaced the pot with the tone pot which was eliminated when I converted my LP to Master Tone. (Replaced that pot with a 1/4" bolt to hold the shiny new tele style knob.) The pot is wired as a volume control pre-buffer. The switch connects a cap to ground (taken from the very same tone control) for tone kill. Thinking about changing that by adding a 470K resitor between the wiper and the cap, and using the switch to short past that new resistor.
Anyway, the rocker pedal just doesn't achieve the full rotation of the pot. I guess they do this on purpose to avoid the user attempting to torque the pot past it's endpoint. When I installed it, I decided to err on the side of "all on". So not only do I risk irreperable damage to the pot, but - far more importantly - it doesn't go all the way off! Kind of messes up my spacey volume swells.
I could always twist it back the other way, but then I've got that resistance in series with the pickups, and I'd still have the whole damage issue.
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Post by JohnH on Aug 8, 2008 1:45:57 GMT -5
I’ve never used conductive paint of the type used for shielding cavities, since I can’t find a local source for it. But I have seen silver conductive paint, in small bottles and pen applicators which is normally used to fix pcb traces. That might work quite well for this since the tracks are basically like pcbs.
I’m also thinking that this technique might solve a need I have, for an extreme ‘antilog’ pot, which as you rotate clockwise, has a resistance that falls rapidly, and then has a slow approach to a zero resistance.as you turn it to 10 It would be for a gain control in a booster pedal, in which lower resistance leads to higher gain (electronics note - to control source bypass in a JFET amp stage). This might work with a series of short dashes of conductive paint, of variable spacing around part of a linear pot track .
John
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Post by sumgai on Aug 8, 2008 13:00:34 GMT -5
Interesting problem......... The issue derives from the physical limitations of how far the ankle will let one's foot rotate (vertically), and several mechanical limitations within the pedal itself. (Size of the gear's teeth, etc.) Wah pedals have not needed a pot's full travel (≈300°) because the filter gave a very broad sweep with only a little resistance change. Not so with volume pedals, unfortunately. However, steel guitar players used to have the same problem. Many of them solved it by using steep log pots, and connecting them up backwards - then they'd reverse the mechanical mechanism so that the pot turned "backwards", but the end result was the one desired - lots of effect over a small range of motion. Those were the days of "tinkering" out of necessity, not for the sheer joy of it, for which we Nutz are noted. ;D There are several possible solutions. But, since you're playing with active electronics anyway, I'd investigate a different method of controlling the volume.......... Set the input resistor to some fixed level, and vary the feedback value with the pedal's pot. Effectively, you're multiplying the bare pot's actual value, perhaps by a large amount. (An order of magnitude is not uncommon.) Depending on your initial values, the ratio of R f / R i will give you substantial control over the signal level. With only a little physical rotation, the pot will change the gain of the amplifier stage to the point where you've pretty much killed off the signal, or you've nearly blown the amp's input stage with too much signal! Be careful when testing a setup like this, or you could be cursing me for days on end.  Do you need some math for demonstration purposes, or are you comfortable enough to get started now?  HTH sumgai |
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Post by ashcatlt on Aug 8, 2008 16:10:57 GMT -5
I appreciate that idea, but it has already been considered and rejected. I want a passive volume control complete with all the "problems" normally associated therein. I would very much like for the treble to fall away as the pot is rotated and all that other happy crap. Won't the opamp stage defeat this action?
I'm sure we could work out some way to fake it, but it won't likely be the same thing as just plain loading down the pickups.
BTW - You'd ought to know by now that I don't mess with real amplifiers. I'm pretty sure my V-amps can handle the output of a 9V TL071 amp stage without smoking.
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Post by sumgai on Aug 9, 2008 2:59:50 GMT -5
ash, Guess which part of the foregoing is making my head spin? Come on buddy, are you going passive, or are you inserting an op-amp? If it's the latter, then you're pretty much prohibited from calling it a passive circuit, even if the volume pot is out front of the amp, and could effectively be used/useful all by itself. Remember, the whole point of going active is to overcome the shortcomings of passive components. Even if the control is connected only at one end to the op-amp, it's still part of the circuit. In fact, the only difference between my idea and yours is that I'm adjusting the feedback value, and you're adjusting the input value. Given the formula shown above, it should be easy to see which way will achieve the stated goal of having a large swing in the output strength......... True, one op-amp isn't usually powerful enough to damage the input of another op-amp, unless the power supply voltages are drastically mis-matched. (Very high on the first stage, very low on the following stage.) Yep, sure will, in what we might call a "normal" layout. (Engineers? Normal? Surely we jest! ;D) It's exactly what JohnH has been harping at us aboot for the last couple of years - adding an op-amp to the guitar's output eliminates the interaction between the cable's capacitance and the pickup/pot combination, and in turn we get more treble even when the pot is turned down. Restoring that passive tonal behavior in exacting detail will require some sturdy engineering. Getting a close approximation won't be nearly so difficult. See any web page that deals with active filters in the audio frequency range for all the gory details. HTH sumgai |
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