BK Butler Tube Driver bass mod

[snk]

Hello,
A couple years ago, I built a BK Butler tube driver pedal (using this veroboard layout, and this schematic). It works fine, I enjoyed it for a while on my electric piano, and then I stopped using it (because for rehearsals, my power supply didn't had a 12V, and in the studio I fell in love with the true tube sound of a Klemt amp).
Today, I took it out again and played with it. I like what it does on medium and high frequencies, but when I use it on bass, I think it lacks bottom end.

My question then is the following : I noticed a 47nF input capacitor : is it, like many other stompbox design, a Hipass filter (tuned for guitar and cutting everything below 100Hz or so), leading to a softer, thinner sound, or is this design working differently (I have seen pedals with a 10nF input cap and very generous in the bass range)?
Would using a 150 or 220nF cap give me back a wider frequency spectrum?

And, if so, should I also change the output cap for a bigger value?

Thank you!

[snk]

schematics (on one of them, the input cap is 33nF).







and : https://patents.google.com/patent/US5022305

[GibsonGM]

Rev. 2 schematic:  You could try doubling the 10n coupling cap after the 2nd half of the opamp and see if that's enough...I think it may well be.  Appears to cut approx. 800Hz right now.

If not, then, I'd lower the slope resistor a bit...the 22k at left of tone stack...maybe to 10k.   And you can raise the value of the 2  47n caps below it.  Funny, I put this in LT Spice and it doesn't seem 'thin'.  So it may well be that raising the coupling cap will do the trick.

The 220k across the 'hi' pot is creating a scoop near 1k, and perhaps that can be raised by lowering the resistance there. 

The input cap, with the 1meg following it, appears to have a cutoff < 10Hz...  ( 1 /  2pi * R * C)  so should be good as-is.   

HTH!

[antonis]

+1 to what Sir Mike said about 10nF cap..
(IMHO, it should be about 5 times bigger, at least, but trial & error is the best way to find out..)

[snk]

Hello, Thank you both.

The input cap, with the 1meg following it, appears to have a cutoff < 10Hz...  ( 1 /  2pi * R * C)  so should be good as-is.   

Yes, 4hz is the figure I got from an online calculator a little while after posting my initial message. So, the bass cut doesn't occurs right at the front of circuit

You could try doubling the 10n coupling cap after the 2nd half of the opamp and see if that's enough...I think it may well be.  Appears to cut approx. 800Hz right now.

Thank you for the tip. I may try that.
Out of curiosity ,and in order to increase my knowledge : could you enlight me? Is the 10nF cap in the sidechain (detection trigger) circuit? That 800Hz reminds me of the Ibanez Tubescreamer 720Hz cutoff in the sidechain... If so, will changing its value to 22nF make it more bass heavy, or bass sensitive?

Funny, I put this in LT Spice and it doesn't seem 'thin'.

Yes, this circuit isn't supposed to be "thin". My version doesn't sound bad at all (I'm glad, I got rid of all the hum, buzz or noise which can occur with such design), and should please any guitarist. But i am a synth player, and with a bass sound, I get some nice harmonics, but i'm losing bass depth. I could keep it as it for other duties, but I'd like to know if a couple tweaks could make it more versatile

[GibsonGM]

Maybe you could check the coupling cap there, and if you like a larger value, put it on a switch...just have the switch jumper it out when thrown. Then the box will have versatility down the road, too!  (For a guitarist perhaps)

I am talking about the 10n cap just after the 2nd half of the opamp, in the 2nd pic you posted, the "Revision 2".  Not a 'sidechain' (did you mean 'feedback loop'?), it's at the opamp output and is used specifically for tone shaping as well as 'dc blocking'.   That coupling cap, on the grid of the 12AX7, is working with the two 10K resistors as a high pass.  That's where I'm using the formula to see what the cutoff is.  Since it's ~800Hz, if you lowered that (using a larger cap, experiment!  Antonis suggest 5x larger, that may well be the case), you may find you like the bass response much more since your synth will have a LOT more to offer than a guitar in that area!      The tone stack seems 'ok as is', in that you won't really get much 'boost' out of it by tweaking it.  The heavy lifting will probably be done by that 10n cap.

If you need MORE, perhaps a smaller cap in the feedback loop....?  Crank up that coupling cap first

[snk]

Thank you!
In the meantime, I checked, and my version is the 903, which is a lower gain version of the original Tube driver, and features a 3 bands eq. Here is the schematic :

In this version, the 10nF is already 47nF.
I'll try with 100nF ?

[antonis]

edit: for 903 version, you have to tweak 100k/10k resistors (not the 10k grid resistor one..)

[snk]

Hi, Antonis, I think our posts crossed
Yes, I figured it out on the schematic.
On the 903, it's already a 47nF.
But when I compared the 903 schematic (with the 47nF cap) and the layout I used, I noticed a couple differences, one being that the 47nF cap goes to a 100k resistor (wich then goes to two 10k resistors). On the layout, the 100K resistor is a 100R resistor 



Could it be that? What does this 100k resistor do in the circuit?

[GibsonGM]

There's your problem    Approx. 10k vs. 110k.  Yes, that might make it slightly bright-sounding        ~340Hz cutoff. Not horrible (for guitar) but for synth, I do think that would be audible!

[snk]

Thank you, GibsonGM 

I noticed also a couple more differences (mainly in the center part of the left side, starting from "footswitch" and ending with the 1.5M resistor... but also a J112 transistor which is not present anymore, etc), I guess there are different schematics floating around.
That 100k becoming 100R looks like a mistake, though

[GibsonGM]

You're welcome, it's funny when you find an error you made years ago! 

I can' t see any reason for a 100R over the 100k!  Not unless you are intentionally making a treble booster    Let us know how it sounds when you change it.

[antonis]

What does this 100k resistor do in the circuit?

Not too many good things..

For a high frequency signal, 47nF cap impedance could be considered negligible (compared to 100k value) so it simply attenuates it by down to 10%..
As signal frequency is getting lower, 47nF impedance is going higher thus adding itself to 100k resulting into further attenuation..

P.S.
A reverse analogy of 100k/10k should be more reasonable..
(always dependent on what frequencies you like to attenuate..)

[snk]

Hello,
I made the mod
From my early test, it seems that some bass is back, but I need to let the tube warm up a bit before being sure.
Thanks a lot for the help!

[GibsonGM]

Glad it worked!