Alternate fulldrive type boost option in an overdrive

[aab0mb]

If one were to place a pot in series with the existing gain pot of a feedback clipper like a ts9 or sd-1, if i'm not mistaken it should up the gain as resistance is increased.

Now, if we jump around that pot with a spst toggle or footswitch, would we have a boost option that functions similar to a fulldrive boost yet it doesn't limit gain with pots in parallel and rather gives a "more drive" option with pots in series? 

I'm trying to figure out why anyone would choose the gain "limiting" version instead.

[GibsonGM]

Good observation!   Yes, you can have a 2nd pot in series and jump it to remove it.   There are things that need to be considered, though. (of course, aren't there always?)

What the Fulldrive does is bring DOWN the resistance (and drive) for 'normal playing', and when you jumper the 'boost' pot, you are only bringing the circuit back UP to where it would be if you turned the drive to 10 - maybe for a lead.   There is an upper limit to the drive that will sound good with the pedal, then.   And a limit to how good you can make something sound in only one stage.   

If you took the same TS-9 type circuit and cranked that gain 2x, 4x, or 10x more, and threw the signal thru the diodes - I am willing to bet you'd just get something very fuzzy, bleaty, and nasty sounding.   "More Gain" doesn't always mean what it implies!    The WAY the clipping is done is very important, not just hitting a non-linear element like a diode with as much signal as possible.  To go very high-gain, it usually would take more stages and a more measured approach to clipping, believe it or not...you also shape the tone in between stages to tailor the sound.   Going too fast is harsh and ugly.

But yes, you can easily do what you're thinking, A.  They chose not to because it would sound like poo.    ;o)   This might have an application in a MULTI-STAGE cliipper, though...something more complex.  Something worth thinking about...

[Gus]

Increasing the gain control resistance, can sometimes make the control of the gain worse

It is a diode feedback clipping circuit SO the maximum output is defined by the diodes voltage drop.

If you increase gain control resistance you will have a more rapid increase in less of the controls rotation(keeping other parts at the same values).
You might have all your control in a 1/4 turn instead of the full rotation and be clipping the diodes.

What you might want is a gain control value that gives max gain, clipping at a small distance from full rotation of the gain control with the weakest guitar or bass you will be using the overdrive with.

[aab0mb]

The sd-1 has a 1 meg gain control.  The ts9 has a 500k and a 51pf cap in parallel for the high roll off. 

If one were to add a 500k in series as a switchable boost in a tubescreamer, wouldn't max boost bring you into the higher gain settings of an sd-1?

I feel like this added resistance idea is within reason.  Although, i do agree that slamming a ton of gain though one stage can get iffy sounding.  That's where discretion comes in i guess.

I realize they are interactive the way i'm setting it up, but once dialed in it'll give two distinct

Could we use the same idea of jumping around a pot with a spst to give a regular old volume boost?  Use the switch to jump around an added variable resistance to ground on the master....

[R.G.]

You need to remember that gain is not purely determined by the feedback resistor.

Referring back to "The Technology of the Tube Screamer", the gain of the clipping opamp is 1 + Rf/Ri. The "1" comes from the fact that you're putting the signal into the noninverting input of the opamp, and opamps always follow their noninverting input to the extent they can. The Rf is the total resistance in the feedback path - the gain pot(s) in this case, and the Ri is the resistance to the reference voltage from the inverting input.

So to change gain, you change either - or both! - of Rf and Ri. You can't do much about the "1+" term except put the signal into the inverting input and hold the noninverting input at a constant voltage.

[Gus]

The sd-1 has a 1 meg gain control.  The ts9 has a 500k and a 51pf cap in parallel for the high roll off. 

If one were to add a 500k in series as a switchable boost in a tubescreamer, wouldn't max boost bring you into the higher gain settings of an sd-1?

I feel like this added resistance idea is within reason.  Although, i do agree that slamming a ton of gain though one stage can get iffy sounding.  That's where discretion comes in i guess.

I realize they are interactive the way i'm setting it up, but once dialed in it'll give two distinct

Could we use the same idea of jumping around a pot with a spst to give a regular old volume boost?  Use the switch to jump around an added variable resistance to ground on the master....

Note the SD-1 has 2 diodes and 1 diode as the clipper
The TS has a symmetrical  2 diode clipper

The gain to clip two diodes is more than one diode.  So I would guess that is part of the design

Reread my post and GibsonGM's post.  If you are DIYing an overdrive you can design for the gain range you want. 

Say you have a 1meg gain and a 500K gain same taper controls
If the 500k set at say 250K clips the diodes you lose the 250K left
what does change is what happens before the diodes clip/clamp the output
and
More gain will have a wider flat top and bottom when clipping

If you use a 1meg the diodes will clip at 1/4 of the rotation (linear control)

It is about balancing what you want with the guitar or bass you will be using

Some friends are pro players and In the past I would have them bring their amps and guitars to my house and we would adjust the gain and eq for that guitar and amp at stage volume.

I think the stock TS and SD-1 values used for setting the gain range are a very good for using different instruments.

If you want dual gain I would use a coarse control and fine control setup

[GibsonGM]

Gus has some good clarification, re. the reaching full gain at 1/4 rotation.   Yes, that's exactly what will happen.   And you can go "a little past that" (my amps go to 11!), where your sound turns from fuzzy & cool to undefined wool + white noise.   So now you have a 3/4 useless pot!     

If you have something on the breadboard, please try it!  It's very educational to see what's going on in the relationship between signal level and clipping diodes, both in the FB loop and on the output of gain stages.   

We hit a wall where the signal has clipped 'nicely'...maybe just a little rounded-ness left on their tops, representing even-order harmonics.   Then we compress it - push it into the clippers...and we get more and more squareness to our sound waves....which produces odd-order harmonics. These tend to sound cruddy (glassy, brittle, harsh, etc).     Push even more, you get a bit more odd-order (increasingly annoying, undefined and painful sound).  Then, almost no change no matter how much gain you throw into the clippers.

What is really cool is to use STAGES of clipping, each one shaping the signal in a certain way, which is then fed to another clipping stage.   2 or 3 stages can give you EVH type brown tones and great definition, where the single stage was just putting out odd-order noise.      In short - yes, the TS-9 is already engineered to do what was intended.  That's why it sounds really good a little less than wide open with the volume cranked...it has 'room to breathe'... You can tweak it, but I wouldn't put the effort into setting up massive gain - I'd build something new and adjust all that on the breadboard!  )

[aab0mb]

R.G., the Tech of TS article is a regular stop on the internet for me.  What can i say? I love how this type of circuit sounds and Feels.  I like the squishy-ness.

What i'm gathering is enough info to see the logic behind the fulldrive setup. That's for sure.

For the record, i have added diodes for clipping so my forward voltages aren't typical ranges. I think it's somthing like two 1n4001s on one side and a stock diode on the other. Haven't peaked in there in awhile.

So i need to basically tune the circuit so at max gain and clipping i'm happy with it. Then add a subtractive element for rhythm playing.  I guess it's the fulldrive idea....  I was rethinking it because someone mentioned about it being subtractive and they thought it should be the other way around. Seemed like a fair enough idea to explore for added control.

The subtractive idea could be accomplished Either way i guess.  It's just tuning the max parts first and subtracting from there if I'm putting this together right.

I'm not really trying to milk a lot more clipping out of these boxes.  My goal was to increase my ranges of available control on the floor. Two overdrive pedals with a boost one each gives a lot of control right at the feet.  If it can be squeezed into a boss or ibanez style pedal, even better!

Looking for added controls that are implemented in interesting and creative ways. 

As it sits right now i have the pots and switches mounted on the pedals in the configuration in question. Might change it up considering all the comments from the experts.

[aab0mb]

Forgot to mention since you guys mentioned designed a circuit...

I had a thread started about a discrete op amp clipper similar to the blues driver.  Might be something to look at of you guys like talking dirt boxes... 

http://www.diystompboxes.com/smfforum/index.php?topic=104437.msg936323#msg936323