500k vs. 250k volume pots

Herb Steiner · From: Spicewood TX 78669

Exhibiting great self-confidence and bravery, I changed out the volume pot in my Rube-Goldberg-esque contraption known as a Bigsby Volume/Tone Pedal. Not easy access like a Sho~Bud or Emmons pedal, the manuever (with the assurance from Lee Jeffries that it was not rocket science) required about 75% disassembly of the pedal.

The procedure took about 1/2 hour and with a minimum of cursing, at that. About the same number of swear words that would be contained in a normal barroom conversation of the same length of time, in fact.

However, since it was the only replacement I had at the time, I installed a Clarostat 500K pot (available from George L) in place of the original 250K no-name pot.

My question: is there a tonal difference between the two controllers that I am not noticing? Since the old pot was kaput, I don't have a clear picture in my mind of what it sounded like before the surgery.

Will anyone out there give me a quick, non-technical, layman's answer to this tonal inquiry?

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Herb's Steel Guitar Homesite

Jim Smith · From: Midlothian, TX, USA

Chick Donner · From: North Ridgeville, OH USA

You won't get a tone change because of the change of value . . . however, since that's a series wired foot control to begin with, you've got quite a tone change with volume level anyway. If you rewired to a parallel system, any tone change goes away.

Terry Downs · From: Wylie, TX US

quote:
a quick, non-technical, layman's answer to this tonal
inquiry?

I'm not sure if all that is possible, but I'll try.

Simplified volume control schematic






     -----   A


      |    |     CW


      |    /


  Pickup   \  B


      |    /<-------->  To amplifier


      |    \


      |    |     CCW


      ---------------

Take a magnet and wind hundreds of turns of wire around it. Move a magnetic object (steel string) through its field and it will produce a voltage across the turns of wire. The more windings, the more voltage. Unfortunately, when you wind wire in a coil you make an inductor (or coil). An inductor is like a resistance that varies with frequency. It is very low resistance at DC (just the wire resistance). But at high frequencies, it is like a big resistor. If fact we replace the word resistance with reactance since it "reacts" with frequency. The pickup is like an AC voltage source with an inductor in series. With a load on the output like an amplifier or volume pot, high frequencies will be reduced through the inductor. The lower the load resistance, the more high frequency loss. OK, why don't we just make the volume pot a gzillion ohms and eliminate this effect? Well we have a problem however on the other end. CAPACITANCE. Capacitance exists when two plates are placed in parallel with a dielectric between them. The capacitor is exactly opposite to the inductor. Its reactance is very high at DC, but gets smaller at high frequencies. Your cable is a conductor with a shield running over it. The longer the cable the more capacitance. You amplifier has some capacitance too. With that said, if you could drive the capacitance with a low resistance, the high frequency loss would be neglible. But using a very high resistance pot with make a high cut filter.

500K has become the standard for most applications. 250K will load pickups noticably in most cases. To eliminate the problem, use a preamp before and after the volume pot. The low output impedance will over come these efects. The pickup inductance is the significant problem. As you know the Goodrich driver is a preamp before the volume pot only. The high frequency loss due to the pot being turned down (inserting resistance) is not so bad. This effect can be reduced by adding a "treble bypass capacitor" between point A & B. This provides a path for high frequencies when the pot is turned down compensating for the cable capacitance.

Some people don't use preamps, but the certianly have good cables, volume pedal, and low capacitance amp input. I've never been able to make that happen without a preamp. I've heard the Goodrich active pedals and the newer infared pedals are good. But like anything, the introduce a likelyhood of failure or battery loss when you don't need it. A solid passive pedal at least gives you months of low volume scratches before you must do maintenance. An all encompassing active pedal failure can leave you in a bad position. I use the Goodrich driver and passive pedal. Those drivers are highly reliable, but if it failed, I could play through the pedal anyway. If I decide to go with one of the newer type pedals I will carry another pedal for backup. I'm not saying any of these products are unreliable, I just seem to meet Murphy a lot.

As far as predicting what the difference is it would depend on how you had it wired. As someone previously posted saying the pot was series wired makes wonder how well it worked. Someday I will post a topic on all the stupid volume contol circuits I have seen. I have an Ernie Ball had a foolish wiring.

I hope this helps Herb. I enjoy your playing and hope to see you in Texas next March.

[This message was edited by Terry Downs on 08 October 2000 at 05:30 PM.]

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