# General Stupid questions



## djmiyta (Jan 1, 2022)

So playing around with my Dr Boogie and Pal800 both utilizing SMD J201’s , the Dr.Boogie sounds fantastic but it seems every 3rd or 4th time I play with it I have to adjust the trimmers slightly to get the certain sound I like. I know that germanium trannys are affected by temp changes but what causes silicone (?) trannys to drift. I thought maybe it was my ears so I did some recordings and I definitely can hear a difference
With the Pal800 using the same J201’s all biased properly I can get NO distortion from it. So with the Pal800 do I need to audition J201’s to get it right and with Dr Boogie what legs of the trimmer get measured for the resistance? Cause if the trannys aren’t drifting then it must somehow be the trimmers shifting. And do you people think trimmers can add to the overall noise of a build meaning if resistors instead of trimmers were used it’d be quieter? 
I’ve read and tried to understand about biasing but haven’t really found answers to these questions especially measuring the trimmers for resistors. I know its a simple thing once I know how
Thanks for any help


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## TravisM (Jan 1, 2022)

The wiper is the center. If you measure the resistance from end to end, it should be the total resistance of the part and not change with turns.

Are you using single or multi turn trimpots? I found that single turn trimpots for biasing can be extremely touch sensitive. If you want to lock it, you can add a dab of glue or wax on top of the trimmer adjuster-to-body transition to keep it from moving due to some sort of mechanical influence.


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## djmiyta (Jan 1, 2022)

If a single turn acts like a potentiometer and the multi turn being the kind that kinda clicks into a detention or spot/place then the single turn ones and yeah they are sometimes really noisy adjusting them and less than hairs width of a turn puts the sound to shit on some 
I had always assumed the multi turn would be limiting in its use for biasing thinking the single turn could be dialed in to the between clicks of a multi is each click like a pre-set value?


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## TravisM (Jan 1, 2022)

Multi turn trimpots that I have used don't have detents, the difference is an internal worm gear arrangement to drive the slider at a finer rate.


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## TravisM (Jan 1, 2022)

Something like this is what I will be using going forward. 


			https://www.mouser.com/ProductDetail/Bourns/3339P-1-103LF?qs=pSi0RBzb0oq3DckZs1IQyg%3D%3D


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## TravisM (Jan 1, 2022)

To answer your other question, a trimmer should not add more noise once it is static, and once you determine an optimal R value, you could always swap in a static resistor if you can find a suitable one.


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## Chuck D. Bones (Jan 2, 2022)

JFETs are very stable with temperature.  Once you dial them in, they should stay dialed-in. If there's a problem with stability, I'd suspect the trimmers.  Trimmers, unless they are locked down with glue, can move when subjected to shock and vibration.  Multi-turn pots are much better because the worm-drive keeps them from moving once they're set.  Pots (including trimmers) can be noisy if there is dirt on the resistance element or if the wiper does not make good contact with the resistance element.  

Make sure your board is clean and dry because moisture or contamination can upset the bias of high-impedance circuits.

Next time you get the Boogie bias where you want it, measure and write down the drain voltages.  Then you can check later to see if they change.  

If the M800 isn't working right, then maybe a post in the troubleshooting section is in order.


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## Caldo71 (Jan 2, 2022)

Wow I just stumbled on this thread and just like @djmiyta I was completely under the impression that trimmers for biasing jfets HAD to be single-turn. And just like him, I find them to be noisy as hell when being adjusted and really really touchy with finding a sweet spot.

So @TravisM and @Chuck D. Bones ’ info about those Bourns multi-turn style jobbies could be a HUGE boon!


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## music6000 (Jan 2, 2022)

djmiyta said:


> So playing around with my Dr Boogie and Pal800 both utilizing SMD J201’s , the Dr.Boogie sounds fantastic but it seems every 3rd or 4th time I play with it I have to adjust the trimmers slightly to get the certain sound I like. I know that germanium trannys are affected by temp changes but what causes silicone (?) trannys to drift. I thought maybe it was my ears so I did some recordings and I definitely can hear a difference
> With the Pal800 using the same J201’s all biased properly I can get NO distortion from it. So with the Pal800 do I need to audition J201’s to get it right and with Dr Boogie what legs of the trimmer get measured for the resistance? Cause if the trannys aren’t drifting then it must somehow be the trimmers shifting. And do you people think trimmers can add to the overall noise of a build meaning if resistors instead of trimmers were used it’d be quieter?
> I’ve read and tried to understand about biasing but haven’t really found answers to these questions especially measuring the trimmers for resistors. I know its a simple thing once I know how
> Thanks for any help


How are you Biasing the trimmers on the PAL800, What voltage ???


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## djmiyta (Jan 2, 2022)

music6000 said:


> How are you Biasing the trimmers on the PAL800, What voltage ???


Using a DMM with negative probe to ground positive probe to drain of jfet supply at 9.2volts so biased at 4.6 than adjust by ear


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## djmiyta (Jan 2, 2022)

So those trim pots will make it easier to bias the jfets than the ones I use? And thanks Chuck D Bones I’ve read your posts about jfets so I hoped you’d chime in. I kinda figured it wouldn’t be the jfets I’ve been using the trimmers from Tayda time for a change


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## Caldo71 (Jan 2, 2022)

Hey @djmiyta I hope you don’t mind me semi-hijacking this thread for a moment, but I did have a related question for @Chuck D. Bones : as you have related to us that it is very okay to use multi-turn trimmers to bias jfets…

*Is it also equally okay to use multi-turn trimmers for dialing in EQ settings? *

Like say for example I wanted to make my Dwarven Hammer build more like a Horizon Devices Nano Attack, where the treble EQ is tweaked only via an internal trim pot…?…


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## TravisM (Jan 2, 2022)

Caldo71 said:


> *Is it also equally okay to use multi-turn trimmers for dialing in EQ settings? *
> 
> Like say for example I wanted to make my Dwarven Hammer build more like a Horizon Devices Nano Attack, where the treble EQ is tweaked only via an internal trim pot…?…


It depends on how much of a slope you want. I assume that that the single turn you are used to is linear but has a much steeper response slope than a multi turn. I would think it should be fine theoretically, but I'll defer to experience if someone has tried it.

All my circuits training was 20+ years ago and I moved from hardware to software for my career path pretty soon after college.


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## Popnfreshbass (Jan 2, 2022)

I built a tearjerker wah in a crybaby enclosure. I used the existing pot of the treadle. It currently works, but heel is treble and toe is bass. Is this simpy a matter of switching the wires of 1 and 3 on the pot?


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## djmiyta (Jan 2, 2022)

Still a bit unclear measuring the trimmer to replace with a resistor . It gets measured while still in the circuit ? Measuring the wiper to on of the outside pins? Then pull out trimmer and put resistor into pcb holes of the trimmer legs I took the measurements on?


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## JamieJ (Jan 2, 2022)

@Caldo71 - sometimes those trimmers can be a PITA. They are great for if you want finite control to bias a part but IMO not the best for an internal EQ control which does not need as much precision. The one benefit is that they are often in an online package which works well to split into a 16mm pot spacing.


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## TravisM (Jan 2, 2022)

djmiyta said:


> Still a bit unclear measuring the trimmer to replace with a resistor . It gets measured while still in the circuit ? Measuring the wiper to on of the outside pins? Then pull out trimmer and put resistor into pcb holes of the trimmer legs I took the measurements on?


You want to measure outside of the circuit, not inside it.


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## djmiyta (Jan 2, 2022)

TravisM said:


> You want to measure outside of the circuit, not inside it.


What I thought thank you


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## Caldo71 (Feb 11, 2022)

JamieJ said:


> @Caldo71 - sometimes those trimmers can be a PITA. They are great for if you want finite control to bias a part but IMO not the best for an internal EQ control which does not need as much precision. The one benefit is that they are often in an online package which works well to split into a 16mm pot spacing.


I tried this out with a Tyrian build that I just finished, and I have to say I rather liked the very gradual fine control over eq, and was pleasantly surprised by how clean and flush they looked after I splayed out the tiny wire leads to feed thru the 5mm pitch pads!


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