"Mr Drive" OD schematic for you ....

[MartyMart]

http://aronnelson.com/gallery/Martys-layouts-and-photos/MrDrive?full=1

I've been playing with this for a while, it's sort of a cross between a Rat
and a Dist+ circuit.
It will produce nice "clean" tones at lower drive settings and is a medium
gain OD/Fuzz.
Nice and simple, using common components including the much "under-rated" LM741CN opamp.
The pair of Ge diodes with a 1N4001 is a "must" and was the best
combination of many,many variations !!

Quite "full" sounding with a bright but not "brittle" tone, it sounds very
nice through my small tube amps and works well through my small "Park" SS practice amp too.

A good choice for a first "opamp" build or just to try another "OD tone"
circuit.
The small pf cap across pins 1&8 is not required with a compensated
single op amp  !! ( it remains from using a TL070 )

Again, thanks to all here for the help/advice given, making this "small"
contribution possible

Marty.

[Marcos - Munky]

Very interesting. Soundclips?

[Mark Hammer]

Kind of the pedal equivalent of comfort food, eh?  Nothing terribly fancy, but familiar, comforting, and satisfying nonetheless.  Kudos.

One of the differences between this one and the DOD250/Dist+ is that you've elected to vary the feedback resistance to change the gain.  AS the pot value increases, the upper end rolloff is reduced by the interactionof pot and cap.  In the case of the 250/Dist+, it is the equivalent of the 470R resistor shown in your schem that gets used to vary the gain and that changes the low-end rolloff.  Your arrangement will yield more consistent bass throughout the entire gain adjustment range.

The 250/Dist+ includes a small value cap (.001uf) in parallel with the diodes to trim off excess fizz and hiss.  You have not done this, but perhaps the 68pf compensation cap accomplishes the same goal.  For those who use a compensated op-amp (i.e., no need or option for 68pf cap), it may be wiser to increase the 120pf feedback cap to something like 390 or maybe even 470pf.  This will deliver more high-end rolloff at higher gains, taming the fizz and hiss that inevitably arises under those circumstances.

Another alternative is to try the SWTC (stupidly wonderful tone control). Stock a 470R resistor and a 10k pot in series with your output pot.  The fixed resistor comes right after the diodes, and goes to one outside lug.  The other outside lug goes to the input of the volume pot.  The wiper of the added pot goes to ground through a .01uf to .022uf cap.  You'll lose a bit of volume but not that much.

Note that your diode complement is still just a teeny bit asymmetrical.  The threshold for the GE side should probably be somewhere in the 480mv range, where the SI side would likely be somewhere over 600mv.

[MartyMart]

Mark, thanks for the "nod" and the tips too
I know its not a "world-beating" design and we've seen these elements
before, but It does produce a well defined "tone" that is, as you put it, a
bit like "comfort food" !!
There is a mention ( on the schem ) of possibly adding a 1n or even a
2n2 roll off cap right after the diodes  :wink:
Mine doesn't have one and is not suffering with any "high end hash" at
the moment
I did think of the SWTC for this, but left it, as this is your "baby" and I
thought perhaps you would like to incorporate it into a design/mod first.
It would be a useful addition, but i do like my circuit flow - simple !!
I am a fan of asymmetrical clipping, and even thought of just using one
Ge diode ( I used 1N91's black cased military things)

Cheers,
Marty.

[Fret Wire]

Looks like a nice adaptation Marty. Nothing the matter with the 741 in these types of ckts, the 250, D+, and Tube Reamer prove that. Probably most folks won't try a TL070, but pins 5 & 8 tied together with a 22-30pf cap will allow the NE5534 to be used.

I see you didn't use the Dist. + 1M/1M/1uf voltage divider. Was it a case of just using the more standard arrangement of 10k/10k/10uf, or a more specific reason?

There's a lot of nice tones to be had in this type of ckt.

[petemoore]

Interesting and tempting, Look Nice and Simple, which often times is very good, and a nice alternative flavor distortion.
 I trust and thank that you've done some homework on this one, finding a tweek that sounds good !!
 I have numerous 'extra' 741's available, as I've not had so much 'luck with them outside DIST+ or 250 mods.
 Looks good !! Doesn't use a large batch of large value resistors either...which I Don't have right now.

[MartyMart]

The 10k/10k/10uf was just handy ! no other reason, as a voltage
divider it works fine !
I must have tried 20 different types/combinations of diodes and did a LOT
of "tweaking"  !!
For a simple/straight up medium gain device, its very nice !
You could use a 100k gain pot, but I found that a bit too much, and it
ended up to "Mushy" for this particular circuit.
I have a few of those !!
This is more of a "Blue's/country-rock" device

Marty.

[Mark Hammer]

I've never really understood what the function of the large resistor values in the Vref divider circuit were for.  Was it an attempt to "starve" the 741 in some manner?  Anyone who has built a Dist+ with diode options will recognize that lifting the clipping diodes still failed to eliminate the clipping, even when the gain was set reasonably.  Conceivably, part of the Dist+ "charm" was the further clipping of an already clipped signal, brought on by means of a deliberately problematic bias voltage.  I dunno.  Maybe some of you PSpice wizards out there can try the diodeless circuit with a 10k+10k divider vs a 1M+1M divider and see what happens.  I suppose the other thing Ican ask is that MartyMart momentarily lift his diodes from ground and tell us how clean it sounds when the gain is under 30 or so.

I might point out that on my Dist+, I tried an experiment based on something Dave Barber had posted here a couple years ago.  Dave was extolling the virtues of paralleled semiconductors, stacked op-amps in particular, as a means of pumping more current.  Having inheritted a wad of 5534's from a buddy, and given that it wasn't a particularly critical circuit for me, I stacked one 5534 on top of another, soldered the pins together, and plunked it into a Dist+.  I liked the result very much.  Lots of definition, especially with a pair of 1N914s for clipping, instead of 1N34s.

[petemoore]

Does it matter?
 I don't know but always measure R's for V dividers before installing when they are around 20k or under.
 Maybe I'm dreaming this up because I never verified it and can't conclude anything but... ...it occurred to me that larger value resistors seem to be closer to the code marked values '+/- %age' wise...I've had 10k's that are under 9k...maybe it's a 'meter thing'...can anyone shed more light on this?
 I like to measure the voltage after the divider is built to veryify it's near 1/2 supply.

[aron]

Looks good to me!

[aron]

Mark, check out this post re: voltage divider values

THANKS R.G.!

http://www.diystompboxes.com/sboxforum/viewtopic.php?t=3222&highlight=voltage+divider

[petemoore]

The 'divider' link is taking me to the index.

[Connoisseur of Distortion]

thanks for the link aron! thanks for the article, RG!

.. :!:

thanks for the schemo, marty!  

[MartyMart]

I've never really understood what the function of the large resistor values in the Vref divider circuit were for.  Was it an attempt to "starve" the 741 in some manner?  Anyone who has built a Dist+ with diode options will recognize that lifting the clipping diodes still failed to eliminate the clipping, even when the gain was set reasonably.  Conceivably, part of the Dist+ "charm" was the further clipping of an already clipped signal, brought on by means of a deliberately problematic bias voltage.  I dunno.  Maybe some of you PSpice wizards out there can try the diodeless circuit with a 10k+10k divider vs a 1M+1M divider and see what happens.  I suppose the other thing Ican ask is that MartyMart momentarily lift his diodes from ground and tell us how clean it sounds when the gain is under 30 or so.

OK, here's my findings Re: Diodes and Vref

I have used 1M pairs before, in my Dist+ and have used pairs of 47k's
33k's and lots of 10k's for other builds.
I use 1% metal film as much as possible, so in the Vref position that
means I have a good "equal" pair.

Lifted the diodes from my "Mr Drive" ........ voila ! Instant "Hotcake"
That's how the Hotcake gets its "breakup" - nothing to do with diodes.....
though the hotcake has a "presence' Pot and is a bit "bigger" sounding,
believe me .... IT"S CLOSE  !!

It stays "clean" 'til around half rotation of the 50k pot, then starts to
break up in a quite decent way, I was surprized how nice it sounded !!
The "Gtr" voice comes through very well, kind of "pure ~Gtr" layered with some nice "crunch" - if that make sense ?

So, use a switch to lift the diodes and you have "Mr Drive" and a "Hot Bun"        

Marty.