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Post by ChrisK on Dec 16, 2007 1:11:56 GMT -5
Well, you know, I started to build a generic pot value test fixture three years ago. It already has a chassis and a scale (generated in AutoCAD) that has both +/-150 degrees as well as 300 degrees of travel.
It was to have 6 5-way binding posts for a dual element pot, or two 5-way binding posts and a 3PST for left/total/right resistance and a 3PDT for element A/B. Either one will have 6 clip leads for the terminals. I'd probably do the latter since it's easier to switch than plug, and best to make all measurements for each mechanical rotation step.
There were some unresolved issues regarding the centering pots of different bushing sizes and knobs for different shaft sizes. I'm sure that they can be resolved.
The good news is that I know where the parts are!
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Post by sumgai on Dec 16, 2007 12:49:34 GMT -5
......... The good news is that I know where the parts are! Yeah, yeah, we know....... they're in a different city! ;D Seriously, thanks, that'd be a godsend, not only to forestall any unfounded discussion, but to offer sort of a "quality check" on what's being sold across the counter by Company X. Provided that the test requestor does something nice for the test requestee... it does take more than a fraction of a moment to effect these tests, to be sure. sumgai
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Post by ChrisK on Dec 17, 2007 0:40:07 GMT -5
No, they were in the parts cabinet drawers in another room. (I'd come across them a few weeks ago).
I ran tests on both a 500K pan pot (A500K/C500K curves) as well as a Fender 250K bass blend pot.
As soon as I get the readings into Excel, I'll post them with graphs.
You're right about the time, it took about an hour to do both (31 position settings with 6 readings each per pot). I do need to put the switches in with the mini clip leads to make it easier/faster.
I was surprised how accurately I could set a position.
The solution for the different bushing and shaft sizes was blindingly simple, uh, like different holes with scales and knobs fer each (d'oh).
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Post by ChrisK on Dec 17, 2007 22:35:33 GMT -5
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Post by sumgai on Dec 17, 2007 23:24:56 GMT -5
Chris,
Much Grass! ;D
sumgai
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David Schwab
Rookie Solder Flinger
SGD Lutherie
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Post by David Schwab on Dec 22, 2007 16:25:48 GMT -5
If one interpolates at 50% of rotation, one sees that the response is aboot 16% of full scale. This correlates to 40 K at 250 K and 80 K at 500 K. There's more tapers than that though... and that is pretty much an audio taper... Personally, I don't like audio taper pots, even for volume controls. The big problem with the Stew-Mac pots is that both ganged sections of the pot have the taper going in the SAME DIRECTION! They are stereo audio taper volume controls. Not pan or blend pots. That's the big problem more than anything. So when one pickup might be seeing 200K to ground, and 50K in series at the 50% point, the other pickup is seeing 50K to ground, and 200K in series! That's never going to give you what you want. If it were made correctly, with one gang being reversed, it would work a lot better. You'd still have some resistance in series, but that's not as bad a low resistance to ground. The correct pots to use as blend controls are marked M/N taper. I beleive these are made by Noble (www.nobleusa.com), and you can get them from Bartolini and maybe Duncan. The tapers below are available from www.potentiometers.com
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Post by flateric on Dec 24, 2007 4:30:02 GMT -5
From me: Dear sirs, I see you have blend or pan pots with centre detent for sale on your website, can you advise if these are true blend pots with zero resistance for each track at the centre position? I have bought pan pots in the past for blending 2 pickups but due to residual resistance the centre position gives a significant loss of output volume, which makes them unsuitable for use in blending 2 pickup signals together. The true blend pots should have M/N250k or M/N500k marking on them. Thanks in advance,
"We do not have this exact marking on our blend pots,but I believe that the pot is not the problem.Whether a pot or a switch is employed there will be a volume drop when both pickups are activated.This is due to them being connected in paralell.The impedance load drops to one half that of a single pickup when both are activated.
Is this correct?
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Post by JohnH on Dec 24, 2007 15:00:58 GMT -5
flateric - Ive heard that said about reduced volume in parallel. But I dont believe it and it doesn't happen on any guitar I have. You get a different sound, and being a mixture of two pups, the tone aspects where each pup is different are smoothed in the combination - eg, the bite provided by the bridge pup is reduced when the neck is also in circuit. But the underlying volume of the note is basically an average of the two pups rtaher than a reduction of them.
I think of pickups as like little tone-generating batteries. If you put two 1.5V cells in parallel, the output is still 1.5V.
cheers
John
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Post by flateric on Dec 24, 2007 15:50:02 GMT -5
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Post by sumgai on Dec 24, 2007 22:27:41 GMT -5
♭eric, You were given a bill of goods, and I don't mean the usable kind, either. In essense, the technical portion was correct, impedances do go down as they are connected in parallel. However, that's misleading - impedance has nothing to do with a pup's voltage ouput! It's merely an indicator of how much wire might wound on the coil-form, and that's about all. In general, more wire should mean more output, but like we've said all along, tain't necessarily so, there's a boatload of other factors to consider. Long story short, thank him for being honest in that he didn't have the part(s) you requested, and leave him alone to his delusions - life's too short to edjumicate every clown that trips across your front stoop. Next supplier, please. ;D HTH sumgai
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servant
Meter Reader 1st Class
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Post by servant on Jan 9, 2008 23:38:33 GMT -5
Hi guys,
I'm a little late in joining in here. I have been interested in rewiring my Jazz Bass for years but finally got around to digging into the research here. I just got some DiMarzio pups for it so now's the time to do the update.
There are a couple threads ongoing regarding blend pots so this might not have been the best choice...
I emailed Stew-Mac about their blend pots and asked questions such as:
Is the center detent zero ohms on each pot?
Is the taper the same on each side? (i.e. 2.5 on the dial on one pot is the same value as 7.5 on the dial measuring the other pot)
Here is their response:
> Thank you for contacting us. Unfortunately, our #4137 and #4138 > Blend Pots are no longer available. They have been out of stock > for over 6 months and we are looking for a different supplier. > > Neither one of these pots measure 0 at the center detent. The > taper is correct - they function opposite of one another.
Other than the Fender Blend pot referred to, are there other known sources of the proper part?
Thanks, Dean
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