Joseph Barcus
From:
Volga West Virginia
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Posted 25 Jul 2010 12:27 pm |
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I will try to ask this the best I can lol. say your pick-up is a oh lets use 19,000 winding, now hook your volume pedal to this and check the end coming out of the pedal. should it come out to be the same wide open? the reason I ask this is I have been having some trouble not only with tone but power. I can check the ohms on the other side of the volume pedal and its up in the 30s. now why have a pick up at 19,000 but come out almost twice as much out of the pedal. would that also be my hum when I turn the amp on push volume pedal down without playing nothing has this hum. I had a new pot put in the pedal some time back. but again my main question is, should the pickup value be the same passing though a volume pedal????
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https://www.youtube.com/channel/UCvuH7H8BajODaL_wy3_HSJQ |
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Marvin Born
From:
Ohio, USA
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Posted 26 Jul 2010 6:35 pm Volulme pedal
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You did not say if you were using an electronic pedal or a "pot" pedal. So assuming it is a pot pedal, when the pedal is full open, the wiper is at the end of the travel to one end, the high end, making the value of the pot input and output the same. And that value is equal the the value of the pot itself. Usually, 500,000 or 250,000 ohms. The value is usually stamped on the back of the pot.
Since the pick-up is connected to the high side of the pot and ground, (the output is connected to the wiper which can vary from zero to 500K) when you plug the pickup into the input of the pedal, you have placed the pick up in parallel with the resistance of the pot.
The formula for resistors in parallel is
R1 X R2 / R1 + R2 in other words, multiply the two values and divide that number by the sum of the two values. Generally speaking, that answer is lower than the lowest value, so your number of 30,000 indicates some thing is wrong in your pedal or pickup or your are using an electronic pedal.
Since you have an ohm meter, measure the input (pedal only ) resistance and it should be equal to the value of the pot, something close to 500k (or 250k) if you have a 250k post. It should not change when you move the pedal.
Then connect to the output and move the pedal through its range; the value should go from Zero to what ever the input was measured. 250k or 500k.
If these measurements are wrong, check out the connections inside the pedal, especially the ground connections.
So the answer to your question for a pot pedal is, no the value of the pick up will not be reflected on the output exactly, close, (but lower.)
However, when you do the math, what you measure will be somewhat lower than the value of the pickup its self.
Try the formula with simple numbers like 8 ohms, 16 ohms, etc and see how the numbers react. If R1 and R2 are the same, then the answer is exactly 1/2 one of the resistors. Such as with two 8 ohm speakers,
8 X 8 =64 and
8 + 8 = 16.
64 divided by 16 = 4. So two 8 ohm speakers in parallel is 4 ohms.
If you use an 8 ohm speaker and a 16 ohm speaker the answer will be 5.33 ohms. So when you use, 19,000 and 500,000 the answer will be 18304 very close to your pick up, but slightly below.
If you are using an electronic pedal, this does not apply as there are multiple stages of electronics between the input and output. And yes it could be 30000 or more or less.
M
Marvin |
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Richard Damron
From:
Gallatin, Tennessee, USA (deceased)
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Posted 27 Jul 2010 10:47 am
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Joseph -
Allow me to add a tidbit or two to Marvin Born's post.
He has suggested that you make measurements of the pot in the pedal both at the input and at the output. If, for example, the input measured 500K ohms - where "K" = 1000 ohms - and the output is anything LESS than that, then it begs a simple qustion. Has the pot been installed correctly such that the wiper travels its' FULL range when the pedal is actuated? If not, then the portion of pot resistance from the wiper to the "top" of the pot will be effectively added to the measured resistance of the pickup since this now places the pickup plus that "missing" portion of the pot in parallel with the bulk of the pot resistance as measured at the wiper. It is now this parallel combination of resistance which you are measuring at the output of the pedal.
His formula for finding the equivalent resistance in parallel resistor combinations will show that, should the wiper NOT return to the maximum - say only to 475K (K = 1000 ohms) - then the measured resistance will be the product of (19K + 25K)X 475K divided by (44K + 475K) or 44K X 475K divided by 519K = 40.3K ohms of resistance as measured at the output of the pedal. It now becomes clear that it doesn't take much of an error in pot wiper travel to confuse the issue when making a measurement such as you describe. In the example that I've just given the failure of the wiper to move to its' maximum position is an "error" of 5%. This will manifest itself, first and foremost, in a slightly reduced output from the theoretical maximum. After all, the pickup is - in the simplest of terms - an AC generator whose output is applied to the entire resistance of the pot. If the wiper is mechanically unable to access that uppermost portion of the pot, then the output will be slightly reduced.
It should be noted - and as Marvin has pointed out - that, should the wiper indeed travel to the top of the pot, then the measured resistance of the parallel combination of pickup and pot will be 18.3K ohms - slightly less than the 19K as measured across the pickup alone.
One other item which should not go unnoticed is your observed evidence of hum. Poor solder joints - anywhere - can quite often be the culprit. Check them all - resolder if necessary.
Hope that this humble addition to Marvin's post will serve to further clear up the confusion.
Respectfully
Richard |
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