Dist + SWTC problem

[jdub]

Hi-

I'm relatively new (5 builds) to this addicting pastime, and decided to make a first attempt at my own perf layout from a schematic.  I chose a D+ for simplicity, but added a DPDT toggle for diode switching.  The layout works, though I am certain it's not a model of efficient design- any input or tips for improving or changing the layout would be greatly appreciated, 'cause now that I've tried one layout, I'm hooked.  BTW, R2 is the standing resistor next to C2.



Anyway, I'd like to add a tone control to this layout.  Some searching of the forum turned up posts on Mark Hammer's Stupidly Wonderful Tone Control, and I tried it.  According to the post, the procedure involved replacing the 10k resistor (R8) with a 10k pot (lugs 1 & 3 to respective terminals of the resistor) and tying the wiper to a .01 to .018 cap to ground- C7 in the layout (I used a .015).  After hooking it up, it had no effect at all. I couldn't anything in the forum specifically dealing with this.

I'm not sure what I did wrong, a prob with the location of the 10k in my layout, cap size, or what.  Any help would be appreciated.  Thanks to Aron (for the forum) and everyone who takes time to answer these kinds of question for noobs like myself.

Jdub

[Mark Hammer]

Everyone is a noob at one time.....

I'm assuming that you show the tone pot as VR3, and the output f VR3 goes to the volume control.  The greenwire you show, that goes to C8, should go to the outside lug of VR3 only.  The other outside lug goes to the volume pot input.

Replace C8 with a .01-.022uf cap, and connect the non-ground side of C8 to the middle lug (wiper) of VR3.  That'll do it.

Do note that while the SWTC IS effective, sometimes it can seem to be so subtle that one doesn't hear adjustment of less than 1/3 of the rotation producing any audible effect at first.  I assure you that it is there, and effective.

[jdub]

Mark-
Wow, that was a quick reply...

I should have clarified, but VR3 in the diagram is actually the volume control- I mislabeled it in the parts list as VR2.  It has the label VR3 because I had deleted a previous pot and the new version of DIY Layout Creator has some quirks that I haven't come to terms with yet...     The center lug of VR3 would go to the output jack.  C7 was the cap I was using for the tone control (.015)- is its location OK?  Does it matter?

Thanks!

[Mark Hammer]

Quick clarification.  Keep in mind the clipping will add lots more treble, and the tone control is intended to manage that.  So, the tone control should go AFTER the clipping diodes, rather than before.

So....

You can move C7 over to where C8 is now located, or even beside it, given how much space you've left.

When C8 (.001uf) is placed in parallel with the diodes, and after R8, it forms a basic lowpass filter rolling off above 15.9khz.  Clearly, that doesn't cut much treble, so consider increasing it so .0022uf for starters.

From the junction of R8, C8, and the diodes, the wire goes to the SWTC pot and then to the volume pot.  One wire goes to one outside lug of the tone pot, and a second wire from the other outside lug goes to the volume pot.  The tone pot now provides a fixed resistance between the clipping diodes and the volume pot.  What will be varied is how much of the fixed resistance will be applied to the added tone cap.  As the wiper is moved more to the volume-pot side of the tone pot, more resistance is placed before the tone cap and the rolloff produced moves downward.  With the full 10k leading up to that second cap, the rolloff produced starts around 884hz. 

That may sound low, but since the slope of that rolloff is not particularly steep, there is still plenty of treble getting out.  Content at 3.5khz is only 12db down from what it would be without the tone control, and the fizz at 7khz is only around 18db down.

Although moving the tone pot in the opposite direction moves the rolloff upwards, when you get completely to the other end, that .018uf cap is placed directly in parallel with the existing cap that parallels the diodes.  If you leave that cap at .001uf, the rolloff produced starts at just under 840hz.  If you use .0022uf, the rolloff produced there starts around 790hz.

[jdub]

I gather that the problem was likely the fact that I had the tone control before the diodes.

If I understand correctly, I can move C8 right next to the diodes and move C7 after that (i.e. closer to bottom edge of the board).  The non-ground end of C8 goes to outside lug of tone control, other outside lug of tone goes to volume, wiper of tone goes to C7 (other end of C7 to ground).  Wiper of volume would go to output as usual and other lug to ground.  Sound about right? Leave R8 where it's at?

Mark, thanks for your help. Greatly appreciated.

[Mark Hammer]

Yep.  R8 stays put.

[jdub]

Hey Mark, wired it up last night and it works, although it's pretty subtle.  It does roll off some treble, though.  Just out of curiosity, would a higher cap value (say, a .022) produce a more pronounced effect (I'm using a .015)?

Again, many thanks for your help.

jdub

[Mark Hammer]

Possibly.  The intent of the SWTC is not to be any sort of "eq".  Rather it is simply intended to manage the treble such that you can have one treble setting on the amp that remains valid whether you have an overdrive/distortion on or off.  I would imagine that for some amps/guitars/players a higher value cap might be appropriate.