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Volume pot wiper dillemna

Discussion in 'Shock Brother's DIY Amps' started by smokebreak, Feb 9, 2013.

  1. smokebreak

    smokebreak Tele-Meister

    115
    Jul 24, 2011
    Austin, TX
    I'm a bit confused about different volume pot wiring arrangements. What's the difference between the vol pot on channel 1(up top between to .o1 cap and 470k) : http://www.gibson.com/Files/schematics/GA40LesPaul.JPG

    and what I'm familiar with : http://www.drtube.com/schematics/fender/champ-5c1-schem.gif

    I've always looked at the wiper as the "out" of the pot, as resistance is varied, as in the wiper takes signal to the next stage.

    What is going on in the first example, where signal goes to wiper, then signal goes out through one of the outer legs, with the other to ground?

    Would both of these configurations basically act the same way?

    Here's a simple drawing with both : http://www.whiteghostshivers.com/images/smokebreak/pot%20001.jpg
     
  2. LeroyBlues

    LeroyBlues Tele-Holic

    928
    Apr 23, 2008
    Michigan
    I'm no expert but, from what I can tell the Gibson schematic shows a volume control for a channel of the amp, like what's used in a 5e3. The Champ schematic, because it is tied directly to a valve, is designed like a master volume would be. The champ, only having one channel, can do this.

    Hope that helps, and if I'm incorrect someone will let us know.
     
  3. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Don't know why Gibson decided to go this arrangement, it makes a bit of sense with a 5E3 although nobody would do it that way now days (Unless you are trying to recreate a 5E3). The normal configuration with the signal going out the wiper is a voltage divider. It gives a high constant load on the preceding stage but a varying source impedance to the next stage. The configuration with the signal going into the wiper puts a varying load on the first stage and the next stage impedance varies but not in the way the previous on does.

    Fender probably hooked up the volume controls in the 5E3 the second way because it sort of isolated the preamp tubes from each other and he saved using two resistors. For a single channel amp it does not make much sense.
     
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  5. andyfromdenver

    andyfromdenver Friend of Leo's

    Dec 21, 2011
    grandpa's
  6. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    Yeah but I can't tie my shoes.
     
  7. smokebreak

    smokebreak Tele-Meister

    115
    Jul 24, 2011
    Austin, TX
    Thanks. So besides the different loading, would they both behave(for lack of better term) the same way, in terms of being a volume control?
     
  8. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    No, they behave differently. The voltage divider arrangement just cuts down the signal from the first stage but does not effect the first stage,

    The other circuit starts at no load on the first stage (full volume, full signal going to the next stage) and as you turn it down you reduce the load on the first stage from 1M down to zero. As this happens you resistance that is not going from the wiper to ground is going from the wiper to the input of the next stage. This resistance acts as a grid stopper for the second stage (not a bad thing) but as it rises this resistance combines with the capacitance in the tube and acts as a high cut.

    http://www.ampbooks.com/home/amplifier-calculators/grid-stopper/

    When you are on '8' for example (0-10 not the regular 0-12 of the 5E3) you might have 70k in line with the next stage and the roll off at 10kHz is about 2dB. Now with 220k ohms on the second stage side you have a roll off of 6dB, and at 500k you have 12dB. So at lower volumes the amp may sound darker

    On top of that as you reduce the load on the first stage it produces less gain (well you are turning it down for a reason right?) but it also distorts a bit more. Not necessarily a bad thing, that is if that is what you want. Also the rotation is not all that friendly if you want smooth control, You always hear people complain that all the control is at the bottom 1/4 of the volume control and that not much happens at the top.

    I once set up two volume controls like the 5E3 and fed a signal in one of them. The following chart is the result. The two lines lowest lines that curves up are the two pot's resistance compared to rotation. Sort of a log relationship which is what you want for volume pots. The next two are the voltage out as they are rotated, the top one is with the second control on maximum, the next one is with the other pot at one value loading down the signal (can't remember the position though) The top trace, can't remember either.

    [​IMG]

    Now with the pots arranged as voltage dividers the voltage out of them would mirror the resistance plot. This would feel like your regular volume knob on most audio equipment.

    Now why would we want to use the 5E3 arrangement if it has all these quirks? Well they have an effect on the sound that some like. Mind you this may not translate well to a single channel amp, but I will just leave it at that.
     
  9. smokebreak

    smokebreak Tele-Meister

    115
    Jul 24, 2011
    Austin, TX
    Great explanation! Thanks a bunch.
     
  10. andyfromdenver

    andyfromdenver Friend of Leo's

    Dec 21, 2011
    grandpa's
    Lol :)
     
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