Resistor value substitution question...MXR Distortion +

[Hiwatt25]

I recently etched my first pcb of a distortion + from tonepad and was about to start populating it when I realized I don't have a 4.7K resistor (R7 in the picture below) and was wondering if I can substitute a resistor of larger or smaller value without any ill effect. 

I would have linked to the schematic at Tonepad but their site was acting strangely.  That's why I posted the picture instead. 

Thanks in advance for the advice.

[Seljer]

the 4k7 resistor defines the max gain, smaller resistor = more gain

[Hiwatt25]

Seljer, you've come through for me again.  Thanks for the input. 

Cheers

[Meanderthal]

 hmmmm... I don't remember R13 from when I built mine(R9 was the gain pot)... or R7 & C7  either, but wouldn't that bit of the circuit be a lowpass filter? Being that it's in the feedback loop wouldn't that then act as a highpass? It would also affect overall gain but it looks to me like it's there also to tighten up the bottom end... Me thinks that as you dial in more distortion it cuts the lows a little to prevent mud. I might be wrong.
Oh, and for best results I'd suggest a 741 or other primitive old fashioned single opamp(I used a salvaged lm307-not 308- for mine. Horrible slew rate... sounds wonderful) The old noisy 'obsolete'  ones will give you more grit and snarl (and of course mojo!).

[GibsonGM]

Why not put two 2.2K resistors in series, legs up like this ^  and the connection wrapped in shrinktube/tape,  and back to the board?  If you want to keep it just 1 resistor there, you can go with a single 2.2K, tho.  You're right, the low end goes away more with increased gain.

[Hiwatt25]

Well, I populated to board and gave it a dry run this afternoon.  The closest thing I had was a 3.9K resistor so I threw it in there.  Now, I've never actually owned a Distortion + but it sure distorted and it sounded good too.  I'm very happy with the way it turned out.  It was my first PCB project and I call this one a winner.  Thanks all for the input. 

[MartyMart]

That's great
DOnt underestimate the good 'ole Dist+ , there's many great sounds to squeeze out
of that little circuit .... DOD250/308/Ross Dist/Liquid drive etc !!
I recently made the "vintage" version with 1N4001 diodes - V nice "gnarly" sound.

MM.

[GibsonGM]

I put a Stupidly Wonderful Tone control in mine, used 1N914's...sounds awesome!  My long-term project is to build a new board, get a bigger enclosure, and run a booster into the front of it with variable gain, for boosting solos into harder territory ;o)   The ckt has tons of gain, so a tone stack doesn't swamp it at its output; there are 100's of ways to mod that baby! 
Someday I'll put in an actual 1M reverse log gain pot, LOL.

[Mark Hammer]

Although values smaller than 4k7 ARE suitable for what you show as R7, note that you can pay a price for it.

The combined resistance of R7 and the gain pot (R13) determine the overall gain of the op-amp.  As the resistance drops, the gain goes up.  With R13 = 0 (set to min) and R7 = 4k7, your maximum gain is [1meg+0k+4k7] divided by 4k7 = 214.  If R7 goes down to 3k9, max gain is 257; a modest i9ncrease but not a day and night difference.

HOWEVER, the combined value  of R13/R7 and C7 determine where that gain is applied.  As it turns out - and in direct contrast to what happens with the TS-9 gain control - bass content gets reduced as gain goes up.  With the stock values, the bass rolloff starts around 720hz at max gain.  This is why a lot of folks ask the perennial question "How can I get more bass out of my Distortion+?".  With R7=3k9, that bass rolloff moves up to just under 870hz.  So, you may get more "drive" but you end up with somewhat less drive for your wound strings than for your unwound ones.

The recommendation is that if you want more drive from the basic Dist+ circuit, by all means go for it.  However, be aware of the price you will pay in low end, and compensate for it.  In this case, upping the value of C7 to .068uf or .1uf would be a good idea and would not take away from the basic Dist+ sound.

[Rafa]

Rafa
Yup thats the gain
2,2 I think is ok,
Rafa

[Mark Hammer]

For people who want more gain but don't want to have to tinker with cap values (or maybe don't have anything else that will fit the space on their PCB), you can also increase gain by increasing the feedback resistor.  Stock, it is 1meg.  Increasing it to 1meg+220k will have the same effect on maximum gain as decreasing R7 to 3k9, though without the change in bass content.  You can simply solder one end of a 1meg into one solder pad, one leg of a 220k into the other pad, and solder their free ends together.

[Hiwatt25]

Very interesting stuff.  Thanks again for all the reply's.  As it turns out, this pedal will be used primarily with my Tele which can get pretty spanky anyhow.  I'll give it a good workout this weekend and see if it sounds too thin.  If so, I'll bounce on down to radio shack and get the appropriate value resistor (or heck, make another one).  The idea of having slightly cleaner low strings seems kind of cool though.  As long as the modification doesn't hurt any of my other pedals or cause itself to blow up and it sounds good...I'm a happy camper. 

Cheers.