How to find the right pot to stretch the first quarter of rotation

[eh la bas ma]

Hello,

I built a distorsion some time ago, and the second half of the gain pot's rotation doesn't change anything noticeable  to my ears : the gain remains the same from 12' to fully CW. On the other hand, I am interested by the sounds I can hear in the first quarter of the rotation, from CCW to 9'.

The standard pot value is B100k. I tried A100k, A50k, A1M, but it didn't stretch the first half of rotation. I used log. instead of lin. because I thought logarythmic potentiometers would be more progressive.
The circuit is almost silent when I turn the gain knob all the way down.
It feels like I have only two choices  : silent or crunchy drive in the first quarter, full distorsion in the remaining three quarters of the pot's rotation. I would like to find a way to have more possibilities when I turn the Gain knob, if it's possible.

Here is the circuit :
https://pcbguitarmania.com/wp-content/uploads/2019/08/Mesa-DR-Building-doc.pdf

Thank you very much for your help !

[EBK]

Use a B25k pot in series with a fixed resistor of about 75k.

[eh la bas ma]

Use a B25k pot in series with a fixed resistor of about 75k.

Thanks,

I do have a B25k and a 75k, but how should I place the resistor ? I am not sure what do you mean by "in series".
Is it between pad1 and the pot's terminal ?

Edit : I placed the resistor as described above, now I have a full distorsion and the pot is useless, CCW or CW it's all the same. I guess I did something wrong...

[antonis]

Place resistor in series with pot's lug 3 then..

[idy]

Series means first one and then the other...two resistors one after the other in line. The other big configuration is parallel...

Please clarify: opamp gain? how wired? In things like Distortion + where the gain is a veritable resistor to ground a "C" or reverse log taper is better. Here less resistance makes more gain.

And yes if all you need to use is the first quarter turn you can imitate that, possibly with a smaller pot and a series resistor. But maybe your configuration (like a TS) makes more gain when the resistor is bigger...

The Distortion+ (and many others including tube screamers) have a series resistor in line with the pot, so the value never goes below a point...which you the hacker can select.

When people are talking to you about a R in parallel with the the pot (or two of its terminals) they are altering the taper, something many of us picked up from RG Keene's "The Secret Life of Pots."
http://www.geofex.com/article_folders/potsecrets/potscret.htm

[eh la bas ma]

From the instructions, I can say it's a circuit based on an op-amp (TL74) and J-fet buffers (2x J201).

I understand more clearly what "in series" means now. Thanks !

I should mention that I already did some mods on this Mesa Dr : a ToneTweq (an additional EQ board) post distorsion, and an additional "bass knob" : see replies #11 and #12 on this thread :
https://www.diystompboxes.com/smfforum/index.php?topic=126320.msg1207410#msg1207410

B25k and 75k resistor gave me some encouraging improvements.

There is a bit more range in the gain pot's rotation. The gain does change between CCW and 12'. The second half is still useless. I am guessing I can either modify R8 (68k), or try B5k or B10k with the appropriate resistor (95 or 90k) .

If I try to change R8, would you have any suggestions about wich values I can use to start  my tests ?

Edit: maybe I shouldn't touch R8, looks like it's doing something with C5 and C6... I'll try a C100k, and C5k, because I have one of those around.

[MetalGuy]

A 5% raper log pot would give you more stretched response but it depends on the circuit as well.
You can find Mesa DR and Peavey A1M 5% replacement pots but they could be a bit pricey.

[iainpunk]

Series means first one and then the other...two resistors one after the other in line. The other big configuration is parallel...

Please clarify: opamp gain? how wired? In things like Distortion + where the gain is a veritable resistor to ground a "C" or reverse log taper is better. Here less resistance makes more gain.

And yes if all you need to use is the first quarter turn you can imitate that, possibly with a smaller pot and a series resistor. But maybe your configuration (like a TS) makes more gain when the resistor is bigger...

The Distortion+ (and many others including tube screamers) have a series resistor in line with the pot, so the value never goes below a point...which you the hacker can select.

When people are talking to you about a R in parallel with the the pot (or two of its terminals) they are altering the taper, something many of us picked up from RG Keene's "The Secret Life of Pots."
http://www.geofex.com/article_folders/potsecrets/potscret.htm

its a volume control style gain control.

like Antonis said, a resistor in series with pin 3, or alternatively lower R5 to 3k3, this should take off a bunch of gain as well

cheers

[idy]

So if the circuit you are working on is the Mesa DR you linked to...
the gain control is a voltage divider or "volume control" after a buffer and a fixed gain Opamp stage, feeding three additional Opamp stages, a pair of asymmetrical clippers and an output buffer. All the gain stages have tone shaping elements.

The gain control also has treble bleed caps so that when you turn down the highs bypass the pot.

The gain control is 100k pot to ground. The output is the wiper. The first quarter of rotation your signal goes from nada to (approximately, remember these these things are not precise!) 25% of the output from previous stages. To keep the same you could pull terminal 3 out of the board and but a R around 300k(!) and now you will be working that 0 to 1/4. If 75k and 25k still aren't right, maybe  try A and B taper. And/or keep upping that R. 100k and 25k pot....

You could measure the value(s) of the pot at the point(s) where you like it (R from 1 to 2, and from 2 to 3) and work from there. (Power off for R measurements.) You know the positions you like, can you quantify them?

[anotherjim]

IDY beat me to it, but I'll post this anyway!

I don't see a gain control, although it calls it that. It's a pre-volume control and as designed can (pointlessly) be turned down to a silent zero.
So the series resistor will go in the pots lug1 which connects to Vr. See the Sustain pot in the classic Big Muff circuit for the same thing. The pot can be linear or log (the original BMP was a B100k). Note that the series resistor doesn't have to too large compared to the pot. You can keep the 100k pot value and fit a 1k series for instance. I would suggest you temporarily fit a spare pot as the series resistor (lugs 1 & 2) - anything 10k or more and see how much series resistance you need, then take it out and measure the resistance. OTOH, you may have a preset pot that you can set with a screwdriver and leave in there.

Another method, you could have kept just the original B100k and set it to the minimum volume position you liked and then take it out and measure resistance between lug 2 and the other ends. Between lug 2 & 3 is the value of the pot you need and between lug 1 & 2 is the value of the fixed resistor.
 

[eh la bas ma]

Another method, you could have kept just the original B100k and set it to the minimum volume position you liked and then take it out and measure resistance between lug 2 and the other ends. Between lug 2 & 3 is the value of the pot you need and between lug 1 & 2 is the value of the fixed resistor.

I  just tried this method : I read 106.4k between 2 and 3, and 2.3k between 1 and 2.
(106k is strange so I tested the pot and it is going up to 107k)

I guess I need to use a B100k with a 2.2k resistor in series. I'll try that and report

Edit : I tried again with a pot tested and going up to 98k, i read 97k between 2 and 3, 1.9k between 1 and 2.  I think it's still going to be a 2.2k and a B100k.

Edit 2 : I placed the resistor between lug 1 and the pot's lug 1. Now the starting point of the rotation is the sweet spot I chose instead of silence. It's something, it's good, but ...  I'll try Idy's idea : the B25k with 100k or higher just in case I can make use of the whole rotation. There is also Iain's suggestion about lowering R5...

I thank you very much for your help, I will make good use of your advices !

[davent]

Another method, you could have kept just the original B100k and set it to the minimum volume position you liked and then take it out and measure resistance between lug 2 and the other ends. Between lug 2 & 3 is the value of the pot you need and between lug 1 & 2 is the value of the fixed resistor.

I  just tried this method : I read 106.4k between 2 and 3, and 2.3k between 1 and 2.
(106k is strange so I tested the pot and it is going up to 107k)

I guess I need to use a B100k with a 2.2k resistor in series. I'll try that and report

Edit : I tried again with a pot tested and going up to 98k, i read 97k between 2 and 3, 1.9 between 1 and 2.  I think it's still going to be a 2.2k and a B100k.

Alpha pots typical of pedal builds are 20% tolerance so your measured 107K wouldn't  be a surprise.

[Mark Hammer]

You are probably familiar with the idea of a "compensated volume pot" on a Fender guitar.  In that situation, a capacitor links the input lug of the volume control, and the wiper/output, such that high frequencies  remain intact as you turn the volume down.

Consider R8 and the gain pot in this circuit as being like a 168k volume pot.  Instead of a single capacitor bypassing the pot and linking the input and output/wiper lugs directly, two different caps are used to shape the emphasis of high end in a more complex way.  C5 includes a bit more of the mids than C6 does.  Changing the value of the Gain pot, without also changing the value of C5, C6, and R8, will alter the tone of the circuit, and response of the Gain control.

Ideally, a pot used in the manner shown would be a log/"A"-taper pot, since it is functioning essentially as a volume control for that first gain stage.

Most of the circuit seems to be all about tone-shaping, with 4 fairly modest gain-stages cascaded before we get to anything that clips.  Just for the heck of it, unsolder one end of both C5 and C6 and see what the pot you have in there gets you.

[anotherjim]

I guess I need to use a B100k with a 2.2k resistor in series. I'll try that and report

Which gives a total 102.2k which is well inside the tolerance range of the pot on its own.
As Mark said, there can be a tonal effect by changing the resistance, but as long as the change isn't more than 1/10th from the design values, there should be zero audible change in tone.

Apart from the production tolerance of the resistance of the pot, I think another error is due to its moving wiper contact having some physical width and so shorting across a narrow portion of the resistance track under it. This effect is somewhat erratic depending on contact pressure and wear (and more so with a log pot) - so don't be surprised if the resistances on either half from the wiper don't perfectly add up to the total track resistance between lugs 1 & 3.

[iainpunk]

[off topic]

Apart from the production tolerance of the resistance of the pot, I think another error is due to its moving wiper contact having some physical width and so shorting across a narrow portion of the resistance track under it. This effect is somewhat erratic depending on contact pressure and wear (and more so with a log pot) - so don't be surprised if the resistances on either half from the wiper don't perfectly add up to the total track resistance between lugs 1 & 3.

this is more prominent with Variac's winding and wipers. the wiper can actually SHORT a winding, running quite a high current through that single winding... scary stuff, those cheaper variacs... some more expensive ones have a special notch system to prevent this problem

cheers

[eh la bas ma]

C5 includes a bit more of the mids than C6 does.  Changing the value of the Gain pot, without also changing the value of C5, C6, and R8, will alter the tone of the circuit, and response of the Gain control.

Just for the heck of it, unsolder one end of both C5 and C6 and see what the pot you have in there gets you.

I would if I could, but I've already added a modification on these pads ( replies #11 and #12 on this thread :
https://www.diystompboxes.com/smfforum/index.php?topic=126320.msg1207410#msg1207410 ), and they are already almost overheated from soldering and desoldering. I would be interested to hear what happens without them, but to apply some heat once more on these pads would be, unfortunatly, a perilous endeavour I'm afraid.

I am considering to settle with a 1.8k resistor between lug1 and pad1, and a 100k resistor between pad 3 and lug3 of a B25k. Being able to easily dial the chosen setting (by placing a resistor between lug1 and pad1) is a great improvement. If I have to make another attempt in the future, I'll try to place a 3.3k resistor at R5.

I thought it would be really easy to modify the pot to give it a real gain range through the whole rotation.
I was reaching too far with my current level but I learned some things in the process, so thank you.
For instance, a friend asked me two days ago if I knew about RG Keen's work, and I didn't. I found out he's well-known in this forum, I 'll read carefully his "Secret Life of Pots" and try to understand something if I can.