Trim Pot Help

[Drewmeyer]

Alright, so I'm looking to build a BYOC 2 knob tone bender. In their circuit they have a trim pot which acts as a bias control for the transistors. Effectively, I'm wondering if it would be possible to switch the trim pot out for a regular pot and put the control on the front of the pedal with the volume and gain controls, and if so, how do I wire up a regular pot to work the same as the trim pot (in terms of the lugs). The BYOC kit uses this kind of trim pot: http://buildyourownclone.com/parts/passives/potentiometers/pot3306trim.html. Any help here?

[Tony Forestiere]

Sure. Go ahead. A trimmer is the same beast as a regular pot, only smaller. Some boutique pedals put the bias available to intentionally mis-bias the tranny for farty/splatty sounds. Knock yourself out.

*edit* I would suggest breadboarding first, and see if playing with the bias trims might yield sounds you might find musical/useful to you before committing to something more permanent.  

[SmoothAction]

Yea you totally could. Lug one of your regular pot goes to pad one where the trim pot would be, 2nd lug of pot to the 2nd pad on pcb, 3rd lug to 3rd pad on pcb. Pretty easy. Im pretty sure most trimpots are linear taper, so if you're using another style taper of the same value it would have a different feel of how the pot sweeps. Thats about it. Also, put a voltage starve pot in there too for extra coolness.

Edit - Just for my own clarification, isn't the bias pot supposed to increase or decrease voltage anyway, acting like a voltage starve if you wanted it to? Would a starve pot be redundant here with a top mounted bias pot? Sorry for the slight thread derail Drew.

[Drewmeyer]

Yea you totally could. Lug one of your regular pot goes to pad one where the trim pot would be, 2nd lug of pot to the 2nd pad on pcb, 3rd lug to 3rd pad on pcb. Pretty easy. Im pretty sure most trimpots are linear taper, so if you're using another style taper of the same value it would have a different feel of how the pot sweeps. Thats about it. Also, put a voltage starve pot in there too for extra coolness.

Edit - Just for my own clarification, isn't the bias pot supposed to increase or decrease voltage anyway, acting like a voltage starve if you wanted it to? Would a starve pot be redundant here with a top mounted bias pot? Sorry for the slight thread derail Drew.

How can I tell which lug on the trimpot is which? As far as I can see, they aren't labeled on the trimpot nor the PCB.

[Drewmeyer]

Sure. Go ahead. A trimmer is the same beast as a regular pot, only smaller. Some boutique pedals put the bias available to intentionally mis-bias the tranny for farty/splatty sounds. Knock yourself out.

*edit* I would suggest breadboarding first, and see if playing with the bias trims might yield sounds you might find musical/useful to you before committing to something more permanent.  

Alright, good to know. Another thing, let's say I didn't want the bias control, would I be able to bypass it?

[SmoothAction]

With a bias control usually lug one goes to ground. Same as a volume pot, lug one is 'grounded', lug two is 'out', lug three is 'in'. This means in the full CCW position the signal goes to ground, the fully CW position the signal is at max. These are your lugs one and three, lug one being 'ground', lug three being 'input/max setting'. Lug 2 is output. A quick look at the pcb would be easy to see which pad goes to ground, that pad would be your 'lug one'.

[Tony Forestiere]

Alright, good to know. Another thing, let's say I didn't want the bias control, would I be able to bypass it?

Short answer...No. External pots for immediate access (mis-bias on-the -fly), or board mounted trims (set-and-forget), the circuit still needs voltage divider(s) so the transistors are properly biased to the circuit. Necessary.
I thought your original question was: Effectively, I'm wondering if it would be possible to switch the trim pot out for a regular pot and put the control on the front of the pedal with the volume and gain controls, and if so, how do I wire up a regular pot to work the same as the trim pot (in terms of the lugs).[/size]
Sorry if I misunderstood.

[Drewmeyer]

Alright, good to know. Another thing, let's say I didn't want the bias control, would I be able to bypass it?

Short answer...No. External pots for immediate access (mis-bias on-the -fly), or board mounted trims (set-and-forget), the circuit still needs voltage divider(s) so the transistors are properly biased to the circuit. Necessary.
I thought your original question was: Effectively, I'm wondering if it would be possible to switch the trim pot out for a regular pot and put the control on the front of the pedal with the volume and gain controls, and if so, how do I wire up a regular pot to work the same as the trim pot (in terms of the lugs).[/size]
Sorry if I misunderstood.

Oh, no that's exactly what I wanted to know and your answer was exactly what I needed. I was just wondering if I would be able to bypass the bias if I later decide I don't like the bias control's effect. Kind of digressed, sorry if it was confusing.

[duck_arse]

stick in a bias pot wired like a variable resistor. fiddle with it, see if there is ONE spot that you would like as your go-to setting. measure the value of the pot at that setting. you can then wire that value to one end of a switch, with the pot at the other end (sorry for the simplistic description of a switch, it's late). then you have your fave spot, and the vary pot as well. cake and eat.

[Drewmeyer]

stick in a bias pot wired like a variable resistor. fiddle with it, see if there is ONE spot that you would like as your go-to setting. measure the value of the pot at that setting. you can then wire that value to one end of a switch, with the pot at the other end (sorry for the simplistic description of a switch, it's late). then you have your fave spot, and the vary pot as well. cake and eat.

Oh, nice. That sounds like good fun. Thanks for the idea! By a switch do you mean something like a SPDT toggle switch?

[duck_arse]

something exactly like a spdt, yes.

I was going to do an ascii drawing, but now I'm not. wire your go-to fixed resistor in series with your variable resistor. the common of the switch goes to the junction of fixed and variable. the either end of the switch goes to the other end of the fixed, and the other end of the variable. this way, you simply short-out the one you aren't using, but you never open-circuit the collector leg.

let me know if you need a diagram ....

[Drewmeyer]

something exactly like a spdt, yes.

I was going to do an ascii drawing, but now I'm not. wire your go-to fixed resistor in series with your variable resistor. the common of the switch goes to the junction of fixed and variable. the either end of the switch goes to the other end of the fixed, and the other end of the variable. this way, you simply short-out the one you aren't using, but you never open-circuit the collector leg.

let me know if you need a diagram ....

Alright, pretty much how I would have thought to do it. Thanks again! I'm really excited to try this out.

[duck_arse]

to quote stiff little fingers, "you didn't ask for this, but you've only got yourselves to blame".