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Topic: Resistance in the open position of pot vol. pedals |
Tom Wolverton
From:
Carpinteria, CA
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Posted 6 Sep 2012 8:16 pm |
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I have a basic question about my regular Goodrich VPs. (I have a 120 and an L-120) I adjust the pot in them so the signal is off in the heel-down position. With that adjustment, the pot does not fully open (electrically) when the VP is floored (toe down position). With a meter, I measure about 10k ohms of resistance in this open position. Isn't that a bit of headroom loss for the signal to my amp? Shouldn't the floored resistance be more close to zero ohms? What does everybody else do? (get a Telonics, maybe?). : )
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To write with a broken pencil is pointless. |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 6 Sep 2012 8:50 pm
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Tom, if you see 10k there, that means that there's about 490k of resistance to ground, assuming a 500k pot. It's the ratio of these two that helps you find the gain lost by not being at 100% full. The math suggests that you are at about 98% of full volume with the toe down. Really not much loss there to be worried about. Also, since we operate most of the time in the 20% to 80% range when we play steel, it's kind of a non-issue really. But don't let that deter you from getting a Telonics pedal.
The reason this happens is that pot-type pedal designers make that brass collar just the right diameter so that we don't mechanically slam the pot arm against the stop as that can damage the pot. They make it a hint under the full 300 degrees of rotation so that we can protect the pot from internal mechanical damage. It's important that we set our pot pedals so that the pot doesn't reach either end's limit before the pedal does.
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Jack Stoner
From:
Kansas City, MO
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Posted 7 Sep 2012 2:18 am
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| The Hilton pedal, has "no loss" if that is what you are looking for. It has adjustments for the "off" position and the gain (and tone). |
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Jerry Jones
From:
Franklin, Tenn.
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Posted 7 Sep 2012 5:17 am
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| My Goodrich measures 400K when fully opened....are you sure you don't have a 10K pot installed? Did you disconnect the pedal input for your test? |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 7 Sep 2012 6:12 am
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Thanks to you all for the info. I simply insert a short guitar cable into the inst jack, another into the amp jack and measure the resistance between the two free-end tips of these cables. I figure if I was bypassing the pedal, the effective tip to tip resistance would be nearly zero. I'm not sure what pots are in there. One was put in by Mr. Bradshaw, the other by Mr. Palenscar. The pedals seem to sound about the same. I guess that's what really matters at the end of the day.
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To write with a broken pencil is pointless. |
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Jerry Jones
From:
Franklin, Tenn.
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Posted 7 Sep 2012 7:11 am
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| I see what you're saying now. Measured your way, my tip-to-tip is 55K @ full pedal. I think this is just the unrealized full rotation of the pot. Probably trivial anyway....most PSG playing is well below full pedal anyway. |
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Jim Palenscar
From:
Oceanside, Calif, USA
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Posted 7 Sep 2012 7:21 am
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| In setting up the travel the first consideration is whether the player wants the pedal to be totally quiet when the pedal is off or wants to hear a little sound. The size of of the pulley around the shaft of the pot determines how far the pot opens given the travel that the pedal is able to utilize which is pretty much fixed based on the design of the pedal. The most important consideration bedsides that is to not let the pot stop the travel in either direction or premature failure will certainly occur. If there is substantial travel between the off position and the point where sound is beginning to be heard I will adjust it so that is minimized in order to get as much coming through the pot at full open as possible. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 7 Sep 2012 7:56 am
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| Quote: |
| With that adjustment, the pot does not fully open (electrically) when the VP is floored (toe down position). With a meter, I measure about 10k ohms of resistance in this open position. Isn't that a bit of headroom loss for the signal to my amp? Shouldn't the floored resistance be more close to zero ohms? |
In short, no. The isolation resistors on the inputs of most every amp are several times this value. When you consider that the pedal works the opposite of what you think it does, this will become clear. The pot is actually shorting the signal to control the output. Therefore, the maximum signal to the amp comes from the pots highest value, not its lowest!  |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 7 Sep 2012 8:57 am
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| Donny Hinson wrote: |
| Quote: |
| With that adjustment, the pot does not fully open (electrically) when the VP is floored (toe down position). With a meter, I measure about 10k ohms of resistance in this open position. Isn't that a bit of headroom loss for the signal to my amp? Shouldn't the floored resistance be more close to zero ohms? |
In short, no. The isolation resistors on the inputs of most every amp are several times this value. When you consider that the pedal works the opposite of what you think it does, this will become clear. The pot is actually shorting the signal to control the output. Therefore, the maximum signal to the amp comes from the pots highest value, not its lowest! |
He's measuring tip to tip, so a lower value means more signal... sort of. The input should be across the pot track... the output is taken from the wiper to one side (ground) so that the voltage is divided across the two parts of the pot track. You have to measure the tip-to-tip, then the output tip to ground, to get a full understanding of the voltage attenuation happening... with 50K tip-tip and 450K output tip-gnd you're getting 10% attenuation (50/500)... barely detectable.
If the input is to the wiper (as when connected backwards) then the pickup sees a varying resistance as the pedal is pressed... this can do funny things to the tone, as well as perhaps stressing the pickup (running a transformer into a short!).
Donny, if your pedal has adjustable stops, you can dial that 10K out... I don't think you'll get much more detectable signal, though... much less than 1dB change. You certainly don't want to bang the pot's internal stops at all. And if you want zero output at low pedal you may just have to live with it.
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Tom Wolverton
From:
Carpinteria, CA
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Posted 7 Sep 2012 10:35 am
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Check out the Vol control schemtic in this thread
http://bb.steelguitarforum.com/viewtopic.php?t=173031&highlight=
To me, it looks like the VP at full gain would read zero resistance between the hot wires to the guitar and amp. Assuming it was opened up all the way.
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To write with a broken pencil is pointless. |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 7 Sep 2012 11:03 am
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I think I like the way Brad explained it. My pedals have been adjusted (by Jim and Tom) to open up almost all the way, but there is some residual resistance in there. But only about 10k ohms. For a 500k ohm pot, this is only about 2%, so I’m getting 98% of the signal passing thru. Good enough. Also, I’m reminded that three half’s of the world’s population don’t understand fractions. : )
Thanks to all for your insight into this.
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To write with a broken pencil is pointless. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 7 Sep 2012 3:40 pm
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I looked through one of my guitar amp books, and sure enough, none of the amps had an input resistor less than 10k. Most were in the 39k to 68k range, but a few were as high as 220k.  Therefore it's probably safe to assume that you're not losing significant amounts of signal, even when the series resistance is quite high.
For all the bad rap that pot pedals get here (and that's a ton, believe me) it's strange that all electric straight guitars have volume pots...and they don't seem to bother their players very much?  I haven't seen any mass exodus of manufacturers to eliminate pots in Telecasters and install "electronic volume modulators" of any kind.
That fact (and a heckuva lot of personal experience) tells me that the horror stories you read here about pots affecting tone, lowering output, "killing the highs", and being generally detrimental are, to say the least, highly exaggerated and misleading (and pretty funny, when you really think about it.)
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