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trying to test guitar pots - someone help the moron


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12/12/1999 8:57 PM
Tarrie
trying to test guitar pots - someone help the moron
I'm trying to test the pots in my Hamer Studio. They are unmarked. After reading some of Mark Hammer's posts and others, I took my meter and went into my guitar. Here's what both volume and the one tone knob does: starts off at close to nothing (about 8ohms) goes up to close to 250K when the knob is around 6 or 7, then goes back to nothing when the knob is turned up to 10. Please help the moron. Before I change these, I probably want to know what they are so I can better judge what values to get. Thanks for any help.
 
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12/13/1999 4:32 AM
Steve B

What lugs did you hook the meter to? Should be the middle & one of the outer lugs, I think.
 
12/13/1999 6:48 AM
Mark Hammer
Your description sounds like either the pot or the measurement process is suspect. If the volume control works fine when you plug in, then it is likely your measurement that is the culprit.  
 
With apologies to those who know more this reply suggests...  
 
1) The two outside lugs of the pot represent a big resistor between the pickup output and ground, typically either 250k or 500k, and sometimes (though rarely) 1M.  
 
2) The wiper of the pot "taps" the signal at some selectable point in between the two outside lugs. This will result in a measurable resistance between the wiper and each of the outside lugs that (all other things being equal) *should* add up to the total resistance of the pot. The resistance measured between the two outside lugs should not vary as you move the wiper with the control knob or pot shaft (although there are some circumstances where it might, but that's not a stock arrangement).  
 
3) The output level is a function of the ratio between the two measured resistances. As the wiper moves towards the ground lug, the input-to-wiper resistance becomes much bigger than the wiper-to-ground resistance, and the signal tapped at the wiper is a small fraction of the level fed to the input of the pot. This is why we refer to it as a "voltage divider".  
 
4) As the wiper moves towards the pot input, the pot starts to act like just a big value resistor going to ground, and stops being a divider. At this point, the next device in line sees essentially two parallel resistances between its input and ground: one is the total pot value (because the wiper is all the way at one end), and the other is the resistance of the pickup(s) selected (actually it sees *3* resistances, but we'll leave that aside for now). All other things being equal, when the pot value is larger, more of the treble frequencies of the pickup(s) are "seen" by the next stage, which is why people vary pot values to tailor the high end response which is optimal for the pickup in question and player's tonal preference. Typically, folks tend to use 500k pots for double coil humbuckers, and 250k for Fender-style single coils, though this is not a requirement by any means.  
 
5) Some companies and players install a small value capacitor between the input lug and wiper on the volume pot to compensate for the treble loss that occurs when the volume gets turned down on lower resistance pots. How does it work? Capacitors can be thought of as being like frequency-dependent resistors - they form a lower resistance path as the AC frequency of the signal applied goes up. Remember that when your volume control is turned down from maximum, the output of the signal in general is set by the ratio of the two resistances on each side of the wiper. If one of those resistances is paralleled by a cap, then the effective resistance for *high* frequencies (between input and wiper) may be much less than what the pot reads without the cap. The result is that the ratio of the path provided by the cap, and the wiper-to-ground path, is very much like having the volume control turned up - but JUST FOR HIGH FREQUENCIES. This results in the volume control having an effect on mids and lows before it has an effect on treble (as you turn it down). If the cap value is small enough, then the effect is to simply keep a little bit of sheen in the signal, even when turned down. If the cap value is large enough, then only bass will initially be turned down, so that the volume control starts to have a dual function as both bass-cut (first 1/4 turn or so) and volume control (for remainder of rotation). I find this latter mod great for turning a beefy bridge pickup into something more suitable for "chicken pickin" tone when you want it, and recommend it for dual volume control setups. A suitable value here might be 1000-1500pf for a 500k pickup.  
 
Hope this sets things straight.
 
12/13/1999 10:34 AM
Dave H.

Did you measure zero resistance with the pot on 10 or was it really a few k ohms? If youíre measuring with the pot wired up in the guitar it will have the pu coil in parallel with it when the knob is set to 10. The pu is only 6-7k for a single coil so you canít measure the pot with the pu connected. Youíll have to disconnect two lugs to get the correct reading.  
 
Dave  
 
12/14/1999 1:07 AM
Tarrie
Thanks, Guys. Now I know...
how the bass cut works on my G&L (never really liked it) and now I know why I have to disconnect the pots to get the correct reading. Now I need some recommendations on how to set up two volume pots with only one tone.
 
12/14/1999 6:13 PM
Mark Hammer

A wee bit trickier. The "simplest" thing, in terms of usability, is just to install a dual-ganged 500k pot where the original tone pot used to be, and wire it up like any self-respecting dual-volume/dual-tone guitar (with Hammer's preferred bridge pickup cap change - a .0068-.01 instead of .02). If you want to futz around with taper using parallel resistors, 500k should be big enough, but you may want to go with a dual 1meg.  
 
The alternatives to this include:  
1) installation of rotary or toggle switch selection of tone caps going between volume-pot-input and ground  
2) a single linear 1meg pot with outside lugs connected to each volume pot input lug, and the single tone cap between tone pot wiper and ground (this will serve adequately as a single treble cut for *each* pickup, but by its design cannot cut treble for both simultaneously)  
3) a single conventional tone pot at the junction of the two volume pot outputs (works, but if you thought two passive volume controls interacted...HOOOOOO, boy!!)
 

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