Adding Jack Orman's "body" pot to the BMW tonestack

[mordechai]

I just stumbled across this mod which sounds great.  I would like to add this to a BMP clone I'm making based on the Violet Rams Head version, where the upper and lower tonestack resistors are 33k (I think this is one of the GGG tuned versions, correct?).   

On the AMZ site, the suggestion is to use a 25k pot with a small value resistor (3.3k i think...which i believe sets the smallest possible resistance the pot can offer) wired to lug 3...but i think the standard tonestack circuit this adapts has the "replaced" upper resistor as 22k, which seems close enough to 25k.

What would you suggest I do if I want the pot -- at full clockwise rotation -- to sound like the 33k resistor if i used the 25k pot?Add a resistor to lug 1?  Change the value of the upper capacitor to which it connects?   

And should I also keep the resistor wired to lug 3 at 3.3k?

[digi2t]

First of all, I recommend you download the Duncan Tone stack calculator. It has a BMP tone section tab, and you can modify the values to see the effect that different values will have on the mid range. Get it here; http://www.duncanamps.com/tsc/

The 25K pot, along with the 3.3K, will bring you to 28.3K. Getting closer to 33K would require a larger resistor than 3.3K, but then it would also raise the amount of mid scoop, for which the BMP is known for. It's a balancing act, to which your ears will be the final judge. The values offered by Jack are simply guidelines, which also stay close to the ability to maintain the classic BMP sound. Of course, you're free to experiment with the values. The calculator with offer you a chance to visualize what's going on.

Personally, I found that an audio 25K pot works best. A linear pot tends to bunch up the body to one side. I've recently built a couple of BMP's with the body mod, but my version uses a 0.015uF cap, instead of the AMZ recommended 0.012uF, along with the 3.3K resistor. The bottom side uses a 33K and 0.01uF. Using the calculator, it shows that I get a tad more scoop than the original, and more boost to the mid with the 0.015uF.

All in all, it's a great mod

[mordechai]

Thanks for the reply...your suggested component changes sound good and I'll try them.  I'm a little confused, though...doesn't a larger resistor on the top decrease the mid scoop?  I though that when you change the 22k to a 33k, the mid scoop decreases. 

Also, wouldn't adding the 3.3k resistor to lug 3 **not** increase the pot's total resistance but just provide a minimum of 3.3k when the pot was reduced to lowest resistance?  I was under the impression that when you rotate the pot counter clockwise, the resistance increases -- and with the 3.3k on lug 3, we'd be moving the wiper away fom it.  I thought that adding the 3.3k to lug 1 would increase resistance.  Guess I'm still not grasping how this works...

[GGBB]

doesn't a larger resistor on the top decrease the mid scoop?  I though that when you change the 22k to a 33k, the mid scoop decreases.

Correct.

Also, wouldn't adding the 3.3k resistor to lug 3 **not** increase the pot's total resistance but just provide a minimum of 3.3k when the pot was reduced to lowest resistance?  I was under the impression that when you rotate the pot counter clockwise, the resistance increases -- and with the 3.3k on lug 3, we'd be moving the wiper away fom it.  I thought that adding the 3.3k to lug 1 would increase resistance.  Guess I'm still not grasping how this works...

It's total resistance that matters.  Range of a 25K pot and 3.3K resistor is 3.3K to 28.5K - it doesn't matter which is connected where.  As digi recommended, a linear taper pot doesn't work that well - use audio taper to increase resistance/decrease scoop clockwise or reverse audio to decrease resistance/increase scoop clockwise.  The 3.3K value is the most critical of the two since small changes to minimum resistance make for big changes in scoop.

[mordechai]

This is starting to make more sense to me now.

So when the pot is at minimum resistance but the 3.3k resistor is on lug 3, then the total resistance after the cap and before ground is 3.3k...and when the pot is set at full resistance, it is 28.3k.  If I have that right (Do I have that right?) then the last question is this -- I liked the standard 33k/.004uf combo in the upper part of the BMP tonestack (that's the Rams Head value, I think).  If I'm working with the 25k+3.3k pot as the new resistance, what adjustments should I make to the cap so that the corner frequency and tone remain essentially the same?  I am guessing have to increase the cap value a bit.  I'm having difficulty with the Duncan tonestack calculator (not working on my iPad), otherwise I'd try to calculate the cap value on my own.  On the AMZ site the changed cap is to a .012uf, and digi mentioned a .015uf cap.  But if I want to maintain that Rams Had 33k/.004uf tone when the pot is set to the usual scoped sound, what do you suggest?

[digi2t]

This is starting to make more sense to me now.

So when the pot is at minimum resistance but the 3.3k resistor is on lug 3, then the total resistance after the cap and before ground is 3.3k...and when the pot is set at full resistance, it is 28.3k.  If I have that right (Do I have that right?) then the last question is this -- I liked the standard 33k/.004uf combo in the upper part of the BMP tonestack (that's the Rams Head value, I think).  If I'm working with the 25k+3.3k pot as the new resistance, what adjustments should I make to the cap so that the corner frequency and tone remain essentially the same?  I am guessing have to increase the cap value a bit.  I'm having difficulty with the Duncan tonestack calculator (not working on my iPad), otherwise I'd try to calculate the cap value on my own.  On the AMZ site the changed cap is to a .012uf, and digi mentioned a .015uf cap.  But if I want to maintain that Rams Had 33k/.004uf tone when the pot is set to the usual scoped sound, what do you suggest?

With a .012/3.3k combo, the typical Ram's Head tone should be found near the 3 or 4 position of the Body pot. With the .015/3.3k combo, the mid scoop moves up a bit sending the normal scoop to about 5 on the Body pot, but at 10, the mid gets boosted more. With the .012/3.3k, the mid flat lines at max. I found that the .015 cap not only affords me a deeper scoop than stock, but also covers the original tone, and blasts the mids, if need be.

If you don't have a breadboard to test on, you could always socket the resistor and cap combo, and try different values. Once you're happy, solder in the desired values.

[GGBB]

I liked the standard 33k/.004uf combo in the upper part of the BMP tonestack (that's the Rams Head value, I think).  If I'm working with the 25k+3.3k pot as the new resistance, what adjustments should I make to the cap so that the corner frequency and tone remain essentially the same?  I am guessing have to increase the cap value a bit.

The -3dB knee frequency of a 33k/4.0n combo is equaled by a 28.3k/4.66n combo.  That's very close to a standard 4.7n so you're in luck.  However, that is only part of the equation.  The drop in resistance from 33k to 28.3k causes a slight shift in the bass-treble balance.  You would have to adjust the other side of the control's 33k/10n to 28.3k/11.66n to balance this out if you were really concerned, but this is too slight a difference IMO to worry about.  Alternatively, you could leave the 4n in and change the 3.3k to 8.2k (to get you very close to 33k with no scoop) and live with a milder scoop control (about 60% as deep).  Or another idea would be to use a 50k log pot so that you can go over the original 33k value to make it give you a slight mid "boost" as well as scoop.  With a typical 50k "10% log" pot and 3.3k resistor, your 33k point is going to sit at about 7.75 (higher in the unlikely case that you have a true logarithmic curve pot).

[mordechai]

This is great info...and very helpful.  Thanks so much!