how can i add a mids control to a tube screamer circuit?

[snow123]

To a close approximation, you could: 

hook up a 1k resistor in series with one of the exact same LEDs you used and a 9V battery like this     GND --- LED K-A----1 K resistor ---- battery +

Measure the voltage from the LED anode to ground, that's its forward voltage.  If you were doing this in a lab, you'd have a specified test current but for grins and stompbox use, this will be close enough!   

and im assuming i use the diode test mode on my DMM?

[snow123]

im not getting anything on the diode test mode, but im getting 1.9v when I put the DMM on DC voltage test.

[GibsonGM]

Your DMM MAY measure Vf directly...better models do.  Mine won't go much over .7V, tho, so I can't use it for LEDs, only small signal diodes.

So no - you have to breadboard and use the 1k resistor + battery, measure across the LED, DC volts. Sounds like you did....1.9V is a reasonable LED Vf.

Let's see what Tom thinks when he comes back   I think you'll be decreasing your gain at that point.

[snow123]

@ElectricDruid

[GibsonGM]

rn its more of a scoop control 'cause it starts to distort HEAVILY (almost like a boss ds1) if i let the mids go above 3db.

Once we've worked out how much you need to reduce the level to stop it distorting in the tone section, I think the best option might be to tweak the values of R1/R2 since they act as a divider. Increasing R1 will reduce the level, and then you just need to reduce C1 by the same factor to compensate and not change the frequency response.

He was probably gonna use that pesky math to calculate how much to reduce the level based on what the LEDs are allowing thru, but you COULD do something with R1 as suggested - I'd replace it with a pot wired as variable resistor and dial that sucker in, personally.  Maybe use a 10k pot, and be aware C1 needs to change after.

[ElectricDruid]

So if the Vf is 1.9V, are these LEDs we're talking about? If they're standard silicon, then I think there's still something wrong with our testing. But maybe it's not that important.

+1 agree with Gibson's suggestion - stick a pot in for R1 and increase it until you stop the distortion. I suspect a 10K pot might not be enough (since R2 is also 10K) so you might need to go up to 47K or whatever.

Once you've got a good value for the pot, take it out and measure it and then we can work out what the cap needs to be.

[snow123]

rn its more of a scoop control 'cause it starts to distort HEAVILY (almost like a boss ds1) if i let the mids go above 3db.

Once we've worked out how much you need to reduce the level to stop it distorting in the tone section, I think the best option might be to tweak the values of R1/R2 since they act as a divider. Increasing R1 will reduce the level, and then you just need to reduce C1 by the same factor to compensate and not change the frequency response.

He was probably gonna use that pesky math to calculate how much to reduce the level based on what the LEDs are allowing thru, but you COULD do something with R1 as suggested - I'd replace it with a pot wired as variable resistor and dial that sucker in, personally.  Maybe use a 10k pot, and be aware C1 needs to change after.

do you mean adjusting the resistor after the tone control, that goes into the output buffer? 'cause the input cap and resistor only affect the low end rather than the whole signal.

[snow123]

and i just tried swapping that 1k resistor for a 25k pot and nothing changed.

[GibsonGM]

The signal didn't decrease at all? Cuz with 1k and the following 10k to Vr, you get about 90% of the signal...if you make the 1k 25k, you should get about 1/4 of the signal, which should then not overdrive the following stage.  You can try a higher value pot if you have one - if not, use the 25k and a 22k or 33k resistor in series, etc, to gain more total R...   

Are you 'listening' to the input of the next stage with an audio probe? Maybe the gain of 4.5 it has is too much (?)   Kind of have to isolate where the 'too much drive' is happening.

[snow123]

The signal didn't decrease at all? Cuz with 1k and the following 10k to Vr, you get about 90% of the signal...if you make the 1k 25k, you should get about 1/4 of the signal, which should then not overdrive the following stage.  You can try a higher value pot if you have one - if not, use the 25k and a 22k or 33k resistor in series, etc, to gain more total R...   

Are you 'listening' to the input of the next stage with an audio probe? Maybe the gain of 4.5 it has is too much (?)   Kind of have to isolate where the 'too much drive' is happening.

i just did some probing, and it starts to distort at the output of the clipping amp (which i think is normal) but i get the distortion (that im getting when the pedal is plugged in normally) at the output of the mids control.

[snow123]

and this didnt happen on the breadboard

[snow123]

and this only happens when the mids control and level control is maxed.

[GibsonGM]

Looks like an 'adjust to taste' situation. I'd try maxing the mids and level (output) with amp turned low, and adjusting the level of gain on U6, as long as U3's level sounds good using the controls associated with it.   See if you can get it to have boost, AND sound good...mids are like that, they're the heart of the guitar range and can really over power.    I'd say you have to decrease the amount of gain you are giving the mids.

[snow123]

Looks like an 'adjust to taste' situation. I'd try maxing the mids and level (output) with amp turned low, and adjusting the level of gain on U6, as long as U3's level sounds good using the controls associated with it.

wdym by that

[m4268588]

If you change 1k (R1), it will affect C1(220n) and the tone control, so you will have to readjust them.
By setting R1=2.2k, R2=1.5k, or R1=3.3k, R2=1.2k, etc., can be performed only by volume drop without affecting the others.

[snow123]

ive been swapping the value of the 27k resistor, and it seems like the distortion wont go away until the mids go back to where they are stock.

[snow123]

If you change 1k (R1), it will affect C1(220n) and the tone control, so you will have to readjust them.
By setting R1=2.2k, R2=1.5k, or R1=3.3k, R2=1.2k, etc., can be performed only by volume drop without affecting the others.

OHHHHHH. i thought you guys were talking about the output 1k and 1uf.

[m4268588]

... i don't know too

[snow123]

and i just replaced r1 with a 2k2 and it still distorts

[m4268588]

And "R2".