DOD FX-55 tone control-please help

[theshade]

Based on a previous post, I have been modifying my old DOD Supra Distortion (first pedal, got it when I was 13).  I have the schem here, but it's not exactly correct for my pedal.  Good for reference though.  http://www.theshadeny.com/fx55.gif

I have a sound I like, but I've been struggling with the tone section.  My intention is to convert the existing tone control to a mid notch filter, then add a BMP style tone control between the last gain stage and the output buffer/gain recovery stage.  I'm a bit of a newbie so figuring some of this stuff out has been trial and error/headache inducing frustration.  I was playing around with it and was able to get a nice notch tone, but it wasn't adjustable via the pot.  Then I kept messing with it and totally lost what I had and can't seem to recreate it.  Keep in mind, I've just been inserting random values and jumpers, trying to come up with something cool.  I totally removed the 2 3.3k resistors and c7 is gone.  I really like the sound when the pot (sans resistors or caps) is almost shorted to ground.  It gets very insane and interesting.  I want to have a switch that makes this happen, but I also want the thing to function as a mid notch that I can use in conjunction with the BMP style tone control. 

My problem with the whole thing is that this tone control is pretty different then most of the others I've looked at from other pedals.  That's why I can't get the notch thing going.  These caps go straight from the pot to ground, and I don't know how to incorporate the T junction or whatever they call the notch filter.  I apologize for the vague-ness of this info, but I feel this is more of a conceptual issue rather than a specific values based issue.  Also, if my terminology is unorthodox, please correct me.

Thanks,
Kaleb

[Mark Hammer]

That's a whole lotta wants.

I've seen this schem before, and I can't judge its accuracy or whether the issue of pedal you have is the same one.  The tone control is essentially a gain control that takes what is fixed in the Rat and makes it variable.

U2B is a noninverting gain stage.  The gain is given by the sum of R8 plus the resistance to ground, divided by the resistance to ground.  In this instance, there are two paths to ground, each with a different cap, and each with a potentially different resistance. Rotate the pot fully one way, and the resistance to ground is 3k3, with a 1uf cap.  That will give you a gain of about 6.5 with a low end rolloff around 48hz.  Meanwhile the path through the .022 cap has a resistance of 100k+3k3, resulting in a gain of (103.3+18)/103.3 = 1.17 for content above 70hz.

If you rotate the tone control completely the other way, you'll have a gain of 6.5 for content above around 2.2khz.  So what happens is that rotating the control shifts where the gain is distributed so that mids and highs either get boosted, or not.

If a midscoop is your pleasure, then I would suggest the following:

a) Convert the Level control into your BMP-style tone pot,
b) Lift the ground side of the .022uf cap in the existing tone control,
c) replace the 220k resistor before the level control with something smaller, like 10k-22k.

What will this do? It will give you a BMP type control, and convert your existing tone control into a gain control (more like a gain recovey control)  You won't have the full range of level settings that a true volume control would give, but since the BMP tone control has some passive loss, and since the modded "tone" control will only be able to change gain from 1.17 to 6.45, you should be able to go from a bit less than unity (when no gain is added to what comes out of the BMP tone control) to somewhat higher than unity. If you want more output level than that, then simply change the 3k3 resistor shown as R17 to 2k2 or even 1k, for gains up to 19.  Nineteen doesn't sound like much but it is multipled by the substantial gain already in the clipping stage.

Another tact to try is to try is this. 

1) First, replace the 220k resistor (R7) with the midscoop filter from the Superfuzz (10k and 22k in series with .001 in parallel, .1 cap to ground from the 10k/22k junction).
2) Second, up the potential gain in U2B by changing R8 (18k) for a larger value (try 27k or 33k).
3) Third, we're going to rewire the existing tone control so it behaves differently.  We're going to stick the .022uf cap in the feedback path of U2B by lifting the ground side of that cap and tying it to pin 7 of the op-amp.  While you're in the neighbourhood, change it for .01uf.  While you're still in the neighbourhood, change the 1uf for .1uf.

So what do we have now?  First, we have a fuzz with a tried and true midscoop, and full-functioning (as before) volume control.  The tone-control, though, now behaves in an interesting way.  Since the wiper of the tone pot is still tied to pin 6 (the inverting input pin), changes to that leg of the pot will increase gain.  When it is at max resistance (assuming we stick a 27k resistor for R8), then the resistance to ground, through C12 will be 103.3k.  With 27k in the feedback path, that will give us a gain of (103.3k+27k)/103.3k = 1.26, witha low end rolloff at about 15hz.  But wait.  We have that pesky .01uf cap sitting in the feedback path.  Even though there is a 3k3 resistance ins eries with it, there will still be noticeable rolloff of the highs.  If there were no 3k3 resistance, the rolloff would begin just under 600hz, so you'd get the thunder of the scoop, but not the lightning.  Adding the 3k3 resistor reduces the impact of the .01 cap so you'll get the rolloff but some mids will come through too.

Rotating the pot the other way, the resistance to ground starts to drop, and the resistance in series with the .01 feedback cap increases, reducing its impact.  At full rotation, there is now 103.3k in series with the feedback cap, and 3k3 in series with the .1 cap to ground.  That will give us a gain of (27k+3k3)/3k3 = 10.1, with a low end rolloff around 480hz.  So, pretty much ALL lightening, and very little thunder.

[theshade]

Jeez.  Didn't think I'd get a reply from the actual dude who gave me the idea in the first place.  Thanks very much for the wealth of info.  If I haven't completely destroyed the pc board by the time I finish putting this shit together, I'll be a happy man.  But seriously, I really appreciate it.  All you guys are great.  It's really strange/great to be interfacing with the dudes from whose web pages I learned how do to this stuff years ago. BTW I just became a member today, which should explain the nostalgia.

Regards,
Kaleb

[Mark Hammer]

Aw shucks.

I've got two schems for the FX55b, one is the one you posted, and the other has some noteworthy changes.  Given your comments in your first post, that may be the one you have.  I guess the most obvious and easy to describe differences would be that in the one you posted, there is a 120pf cap to ground in parallel with the second set of clipping diodes.  In the other, there is a .001uf cap (similar to the DOD250 and MXR Dist+).  In the one you posted/linked, there is a 220k resistor and a .01uf cap between the 2nd set of diodes and the Level control.  In the other, there is simply a .1uf cap between the diodes and the Level pot. The Gain control seems to have a much greater range of values in the one you linked to than in the other one.

The Tone control, however, straddles what is in the original schem you linked to, and what I described in my reply.  In the 2nd 55b, the 100k Tone pot adjusts the gain with the ground leg, at the same time as it places a resistance in series with a .1uf feedback cap to adjust how much high cut there is.  However, there is a 1k resistor and 1uf cap to ground off the junction of the .1uf cap and pot lug.  Not sure how that one works.  In general, though, it works the same way, emphasizing bass at one extreme, and treble boosting at the other.  Whether it does so in a pleasing way is up to you.

If you have this other version, with the .1uf cap linking the Level and diodes, you should be able to park the midscoop where the .1uf cap went.  Solder the two series caps together, solder the .001uf bypass cap in parallel, and thensolder those outside leads to where the .1uf cap used to be.  You can park the .1uf cap to ground wherever you have a ground connection, and run the other lead of the cap to the 22k/10k junction with a wire.  Shouldn't harm anything.