# Bias trimmer in place of resistor



## DAJE (Jun 15, 2022)

TL;DR - the title sums up what I want to do, I'm just not 100% clear on how to do it. Trimmers have 3 legs, resistors have two. What goes where? 

I've made two versions of Aion's Tone Bender mk2, which has two internal bias trimmers for Q2 and Q3. Made one for me, another for a friend, and another friend now wants one. I don't want anything else from Aion right now but I do have some PedalPCB boards on the agenda so I want to use the local version, the Tone Vendor Mk2. Which has no bias trimmers. (International postage is what I want to avoid paying for twice, in case you're wondering why I can't order from both PCB makers.)

I know the 8.2k resistor is the one to replace. Any other suggestions or pointers welcome.


----------



## Alan W (Jun 15, 2022)

Rather than the single turn, 3  leg trimmer resistors (which will work for this, just use the center (wiper) and only one end), I’ve used 10 turn trimmers, which are more vertical, with a small screw head to adjust. Two leads, and .4 spacing is common. Use an ohm meter to get it at the approximate resistance you want, otherwise, you’re trimming forever. But it’s very easy to get the bias exact this way.


----------



## Alan W (Jun 15, 2022)

I just gave you incorrect info. I went to get a photo, and there are three leads, so I must just use the center and one outer, and bend the leads. It’s been at least 2 years since I‘ve used them.


----------



## DAJE (Jun 15, 2022)

Alan W said:


> I just gave you incorrect info. I went to get a photo, and there are three leads, so I must just use the center and one outer, and bend the leads. It’s been at least 2 years since I‘ve used them.


Good info anyway, thanks for the suggestion.


----------



## benny_profane (Jun 15, 2022)

You need to short pin 2 to 3 (or 1, depending on the sweep you want). This is an easy drop-in board:









						Trimmit! - PedalPCB.com
					

Trim pot adapter




					www.pedalpcb.com


----------



## DAJE (Jun 16, 2022)

Trimmits are out of stock. Looks like the center and right legs are always connected. So it's left leg to resistor hole 1 and center and right legs to resistor hole 2? 

I've seen pics where one leg is removed or left unconnected.


----------



## temol (Jun 16, 2022)

Unused leg is often connected to pin 2. It's a "good practice" not a requirement. 

ps.
Let's assume you rotate the trimmer clockwise. Then:
- if you want to increase voltage, use legs 2 and 3 (resistance goes down),
- if you want to increase resistance of the trimmer, use legs 1 and 2 (voltage goes down).


----------



## DAJE (Jun 16, 2022)

temol said:


> Unused leg is often connected to pin 2. It's a "good practice" not a requirement.
> 
> ps.
> Let's assume you rotate the trimmer clockwise. Then:
> ...


Great, thanks.


----------



## MichaelW (Jun 16, 2022)

DAJE said:


> TL;DR - the title sums up what I want to do, I'm just not 100% clear on how to do it. Trimmers have 3 legs, resistors have two. What goes where?
> 
> I've made two versions of Aion's Tone Bender mk2, which has two internal bias trimmers for Q2 and Q3. Made one for me, another for a friend, and another friend now wants one. I don't want anything else from Aion right now but I do have some PedalPCB boards on the agenda so I want to use the local version, the Tone Vendor Mk2. Which has no bias trimmers. (International postage is what I want to avoid paying for twice, in case you're wondering why I can't order from both PCB makers.)
> 
> I know the 8.2k resistor is the one to replace. Any other suggestions or pointers welcome.


I did it the idiot way. (But it worked) I used my DMM to measure which leg was the variable one and which was fixed. Snipped the fixed one and soldered the other two into the resistor pads. I thought it looked pretty hillbilly but @Big Monk gave me the thumbs up so I feel pretty good about myself


----------



## DAJE (Jun 16, 2022)

MichaelW said:


> I did it the idiot way. (But it worked) I used my DMM to measure which leg was the variable one and which was fixed. Snipped the fixed one and soldered the other two into the resistor pads. I thought it looked pretty hillbilly but @Big Monk gave me the thumbs up so I feel pretty good about myself


If it looks bad but it works properly, then it doesn't look _that_ bad.


----------



## Chuck D. Bones (Jun 21, 2022)

Once the cover is on, who's gonna know?  

Use caution when replacing a resistor with a trimmer because _sometimes _letting the resistance go to zero can be destructive. You're ok replacing the Tone Vendor's R7 with a trimmer because R3 and the ATTACK pot will limit the current in Q3 if you set the trimmer to zero Ohms. You would not be so lucky if you replaced R5 with a trimmer and then set it to zero because you could burn up Q2.


----------



## DAJE (Jun 22, 2022)

Chuck D. Bones said:


> Once the cover is on, who's gonna know?
> 
> Use caution when replacing a resistor with a trimmer because _sometimes _letting the resistance go to zero can be destructive. You're ok replacing the Tone Vendor's R7 with a trimmer because R3 and the ATTACK pot will limit the current in Q3 if you set the trimmer to zero Ohms. You would not be so lucky if you replaced R5 with a trimmer and then set it to zero because you could burn up Q2.


Excellent advice, thanks. I plan to set the trimmer to 8.2K-ish before installing it and then adjust it by ear if needed.


----------



## temol (Jun 22, 2022)

As a side note - trimmers make people lazy... I know they make setting right voltage very easy. But trimmers also allow to take any transistor, even with parameters that are way off from the required.


----------



## djmiyta (Jan 5, 2023)

temol said:


> As a side note - trimmers make people lazy... I know they make setting right voltage very easy. But trimmers also allow to take any transistor, even with parameters that are way off from the required.


For me , trimmers are the only way to set bias it's way too difficult for me to ...1) remember an electrical formula or calculation 2) Even if I could remember one actually putting said formula into action is 100 times harder than number 1 (for me)
I know that once you have the trimmer set where you want it you can measure it than replace with a fixed resistor which is better than a trimmer ... I think, because trimmers can drift over time I remember reading something like that and how some have put loktite on the trimmer to "set it" which seems like a safer move than chancing f'n up your board trying to remove it.
  Anybody hear anything like that? Or did I have 3 too many and am just jacked up?


----------



## Chuck D. Bones (Jan 5, 2023)

You can temporarily lock a trimpot with a tiny drop of silicone sealer.  Then if you need to change the setting later on, you can peel the glue off.  Do not use Locktite, it is too runny and will seep down inside the trimpot and contaminate the resistance element.

I gotta say, I'm in agreement with Temol.  Unless you have a JFET with the right parameters, there may not be a correct trimpot setting.  Sure you might be able to dial the drain voltage in to some "magic" value.  Or you will find a setting that sounds good to you.  But will it sound like an original  production pedal?  Probably not.  Why do you suppose the pros don't use trimmers?  It's because they know that the way to produce consistent results is to select JFETs with the correct parameters.  In a production environment, they can buy them in large quantities and cherry-pick them.  Most of us don't have that luxury.  I've said it before, and I'll say it again: _JFETs and Germanium BJTs are not plug-n-play_.  Having a particular part number is not a guarantee of success.


----------



## temol (Jan 5, 2023)

Chuck D. Bones said:


> I've said it before, and I'll say it again: _JFETs and Germanium BJTs are not plug-n-play_.  Having a particular part number is not a guarantee of success.


Amen. 
We should carve it in stone


----------

