Overdriver - can this be a solution for the "too logarithmic" Gain pot issue?

[yeeshkul]

I was recently sorting out a problem of 5k pot + resistor in parallel for Fuzz Face to get 2k pot. IT works but it works like a LOG pot together.
Then i was looking at some Overdriver issues here and people complain about how the Gain pot works -> it seems to be too logarithmic. And it should be - it is a combination of a pot and a resistor in parallel.

Can i help it this way(picture)? Just by removing the resistor and changing the pot to a smaller value (roughly equal of those two in parallel)?

[yeeshkul]

Also there was a question about 120/100k resistor at the Q1 collector. The scanned schematics say 5V bias on Q2 collector. I guess that can help to answer.

[John Lyons]

Are you sure that a liner pot with a fixed resistor across it makes it log?
I know that if you put the fixed resistor from wiper to ground it will make it log. 25% of the pots value for the fixed resistor will approximate a standard log taper. More resistance will be more log.
Putting the resistor from wiper to the hot lug will approximate a reverse log taper...

It would seen like taking out the 6k8 resistor on the Colorsound Overdriver would help only in that you could use a 5K pot instead of the 10K and the 6k8 fixed  resistor.
Anyone who knows what they are doing want to clarify this?


John

[frankclarke]

That will reduce the gain quite a bit, you don't have 2k in the DC path emitter->ground. The cap blocks the DC, so the 2 arrangements you show are very different.

[yeeshkul]

John, i soldered up my Fuzz Face with 5k pot and 3k resistor in parallel to get 2k pot "like" all together. In the end it was behaving so "logarithmic" with the fuzz only at the very last tiny bit, that i had to take it out again.

actually when we have 10k resistor and 30k pot in parallel (just for easy counting), then the total resistance is
7.5k when the pot is at 30k (fully down)
6k   when the pot is at 15k (pot is half way up)
5k   when the pot is at 10k
3.3k when the pot is at 5k
0.9k when the pot is at 1k (almost up)

that doesn't look linear or .... i am missing something important (as usuall )

[yeeshkul]

That will reduce the gain quite a bit, you don't have 2k in the DC path emitter->ground. The cap blocks the DC, so the 2 arrangements you show are very different.

DC: in the first case i have 6k8 resistor in emitter (the pot is being blocked), in the second case 5k. How does the 1.8k difference reduce gain? I am just curious.
I thought that the emitor resistance influences gain from the point of AC and there yes, but shall it be that much?

[frankclarke]

The Coloursound overdriver doesn't have the bypass cap, so I assume we mean the Fuzz Face. An minimum Fuzz on the FF, increasing the fuzz pot value would decrease the gain for an input signal which isn't clipping. So what circuit are we talking about?

[yeeshkul]

I am talking about Overdriver. I am not sutre what you mean by bypass cap, but Overdriver has 25u cap in the parallel branch to the 6k8k res (like on the picture).
When we talk about signal(not DC), then at minimum fuzz there are 2 resistors in parallel, 6k8 and 10k, which makes 4k emitter resistance.
In the second case it is just one resistor with 5k resistance. Is the difference considerable for the gain?

[John Lyons]

yeeshkul
When you have a fixed resistor in parallel with a pot the total resistance of the "new" pot does not change or become log taper depending on how the pot is set.
What you have is a linear pot with the "new" resistance measured from outer lug to the other outer lug.

Example: If you have a 5K pot and a 5K parallel resistor across the outer lugs then the pot becomes a 2.5K linear pot with 2.5K total resistance from end to end.

The pot example you gave with the 30K pot and 10K resistor looks straight and linear to me. With a log pot the change would happen much more dramatically at the end of the rotation and not much would happen at the first half.

John

[frankclarke]

I understand now . Not considerable, no, excuse me.

[slacker]

The idea of replacing the original Overdriver gain pot with a fuzz face style arrangement  using a linear pot would work from a technical point of view. I doubt it would do very much to improve the "problem" though.
On my Overdriver the distortion/fuzz comes in on the last few hundred ohms of the pot so a more linear taper wouldn't improve things very much.
Here's a quick graph of the combined AC resistance of the original compared with using a Fuzz face arrangement. The vertical scale is the AC resistance to ground and the horizontal scale is the gain setting increasing towards the right. In both cases decreasing the resistance to ground increases the gain. The blue is the original and the red is the fuzz face style.



In my experience to get a better control of the distortion you'd need a very reverse log taper as shown by the green line. I think you could make one of these with a pot and a tapering resistor, but I think you'd have to wire the pot backwards so max gain would be with the pot all the way anticlockwise.

To be honest the pot taper "problem" with the Overdriver doesn't really bother me because I either use mine as a booster with the gain at about 60-75% for a bit of colour or with the gain flat out.

[John Lyons]

Thanks for the confirmation Slacker.

I just hooked up a 10K lin pot and a 10K resistor and the sweep I get is linear.
The total resistance was 4.69K (the pot measured only 8K+
In Quarter turn increments I get 1K, 2K, 3.4K, and 4.68K .
These are rough measurments at approximate rotations.

John

[yeeshkul]

guys thank you for your answers! i guess i'm gonna leave the original setting on since i actually don't want that much fuzz, more kick than fuzzzzz