minions
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Post by minions on Apr 20, 2010 13:25:35 GMT -5
Does anyone know if a dual pot with one side being normal and one side being reverse exists? Is there a way to wire a dual pot this way?
Thanks.
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Post by ashcatlt on Apr 20, 2010 17:25:17 GMT -5
Are you talking about a dual-ganged pot where both wipers turn with the one knob, or a concentric pot with two seperate adjustment deals? Either way, I think you can find what you're looking for, but I'm going to reserve my recomendations till I know exactly what you want. You could get both elements as linear and apply tapering resistors around each to accomplish what you're looking for as well. The Secret Life of Pots
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minions
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Post by minions on Apr 21, 2010 10:05:41 GMT -5
Yeah it's a dual-ganged pot with one shaft.
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Post by newey on Apr 21, 2010 12:14:20 GMT -5
minnie-
Do you mean that you want the 2 halves to have log/antilog taper? Or that you just want the one half to operate in the opposite direction as the other?
A pan pot (or a true blend pot) has log and antilog elements, with a center detent, with one half operative CW, the other half when CCW.
As Ash noted, resistors can turn a regular linear pot into an approximation of a log taper. Wiring a regular dual-gang pot with one half wired "lefty" would accomplish the reverse operation, if that's what you're after here.
What is this pot going to be doing, exactly?
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minions
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Post by minions on Apr 22, 2010 10:09:08 GMT -5
Yeah, I want one half to work in the opposite direction. What I would be using it for is one pot for a volume in an amp, and the other to change the value of the cathode resistor on one of the triodes of a 12AX7. When the volume is turned up, the gain of the triode is increased. On the other hand, I could use a normal dual pot and make it so that when the volume is turned down, the gain is increased, making breakup quicker even when the volume is backed up. Although, I'm not sure I would want to sign away the clean, undistorted signal. Plus, one of the pots would have to be 1 meg and the other would have to be around 25k at the most, which I'm not sure exists.
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Post by sumgai on Apr 22, 2010 14:14:12 GMT -5
minnie, ...... and the other to change the value of the cathode resistor on one of the triodes of a 12AX7. You can guess, can't you, that the very fact that I'm jumping here is a righteous clue as to how this is gonna turn out. Bring plenty of Kleenex, 'cause there's gonna be tears aflowin'. In short, the non-engineering version (if I can pull this off): What you are contemplating is adjusting the tube's bias, not it's gain factor. Doing this will not appreciably change the tube's gain, but it will certainly change its ability to amplify the signal in a linear fashion. IOW, if the tube is made to operate out of its normal specs, then it will distort the signal. In a good way? Probably not. More likely, you'd consider it to be "squashed", or perhaps tinny sounding. I'd bet shotguns to shillelaughs that you won't approve of the tonal output. If the tube survives your intended abuse, that is. HTH sumgai
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minions
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Post by minions on Apr 22, 2010 15:03:38 GMT -5
Hmm... okay. Haha I guess that means this thread is now rendered pointless and finished.
Thanks! And don't worry about offending me (which your post did not do), I needed that explained to me.
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Post by JohnH on Apr 22, 2010 15:20:19 GMT -5
What you are proposing sounds similar to something that Ive tried using Field Effect Transistors. These work in a very analogous way to valves, and in fact you can often use simliar configurations of components in simple amp stages.
And, as Sumgai points out, what you propose there will mess up the bias of the tube. Another factor is, any time you have dc running through a pot in an audio circuit, there is a high probability of loud scratching noises as you turn the pot, even if the pot is new. In all good audio circuits, the dc component is blocked by a cap before running through, say a volume pot.
I have had success with in my JFET circuits in a slightly different arrangement however, to change the gain of a JFEt amp stage using a pot. The source resistor (think similar to cathode resistor) is fixed, so bias is set, but it is bypassed by a cap which increases the gain of the stage. I put my pot in series with this cap, so it changes the gain without changing bias, and has no dc running through it so does not crackle. I have a handy little booster stompbox that works this way. An antilog pot is definately what it should have, but I have it wired with a linear pot which is OK but is a bit too sudden a change in gain at the end of the travel.
So Im wondering (and I dont know but I think it might), would that idea translate to a tube circuit?
John
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Post by sumgai on Apr 22, 2010 20:14:38 GMT -5
John, Short answer: yes, it could probably translate over pretty well. (And you hear a "But..." in there, doncha? ) However, think a moment - what's a capacitor most famous for? Right, it's a frequency sensitive component. That means that it has an effect on how much or how little of any given frequency passes through it. So, what you've described is a bias circuit that is said to be AC-grounded and DC biased in the negative region (the latter part of which the tube requires). This does give the active device the best chance of working on the whole signal (from peak positive to peak negative) without distorting it. Look inside of many Fender and Marshall circuits, you'll find this very setup on the first one or two pre-amp stages, if not more stages beyond. But wait, I've taken us off track for a moment....... Well, let's go back and compare your modified bias circuit with a guitar's tone (treble-cut) circuit. Oooooh, those look a lot alike, don't they? Indeed, by decreasing the efficacy of the AC bypass cap, you've changed the tube/JFET's frequency response. Equally along with that, you've also just eliminated the AC-grounded bias that was giving the tube its increased gain. Or to be more precise, you've made a simple equation into a quite complex one. I'd probably have to get out my notes on Thevenin's Theory to calculate the component values needed. So long as you're aware of these phenomena, and ready to deal with them at some point in the circuit, then there's really no reason not to implement this mod. OTOH, there's the school of thought that subscribes to the fact that over-driving a pre-amp tube (or a JFET) is about as pleasing a tone as, say, a cat in heat. I think we all know that I firmly believe (and I believe that I can prove) that without a certain level of magnetics in the picture, any simple active or passive distortion device is only a pale imitation of the real thing. But that's just me. And I'm too old to muster up enough pee for anyone's bowl of Cheerios, so if anyone wants to go ahead and perform this mod, you have my blessing. ;D HTH sumgai
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