Tube Reamer just passing input signal

[DarkeyesZ33]

I am building my third distortion pedal. The first was my own design, second was the PepperMill (with some modifications I came up with), and the thrid is the Tube Reamer. The latter two are from www.runoffgroove.com

http://runoffgroove.com/tubereamer.html (It seems like most people here have all the same website sources as me =p)

Anyway, I've been trying to get this pedal to work on the breadboard for 3 days. I built it exactly, part for part as in the schematic as well as trying different variations of the TS (such as the son of screamer). It seems like no matter what I do, it just passes the input signal without distorting it at all.

I put the schematic into PSpice (a simulation/design program) and checked all my node voltages. On the opamp the voltages are:
Inverting pin: 4.75v
Noninverting pin: 2.4v [This should be around 4.8v]
Output: 4.75v

I checked my bias network and it's fine. I even biased the input to ground and put the opamp at +/- 9v.

I've checked all my part values and tried several others to make sure there isn't a bad part. I've rebuilt the circuit on a different bread board.. not to mention I've completely rebuilt it at least 10 times testing different theories.

Nothing effects the circuit either. Changing the 470k or messing with the Pot does absolutely nothing to the circuit.

I think it has to do with the Noninverting opamp pin not being biased correctly... but I can't for the life of me figure out why it's at 2.4v

Does anyone have any ideas? or maybe experianced something similar?

Thanks,
Thomas McDonley

[MartyMart]

It's quite a simple circuit, so all I can say is even though you've "re-built" it a number
of times, you're repeating the same mistake somewhere....
Or the breadboard has a "wierd" anomoly, that's causing it to pass clean signal.
Try a piece of perf, copy the ROG layout exactly and make sure that your 470k isnt
a 47k or 4k7 .. ( been their , done that ! )
Lots of people here have built it, with good results
Luck,
Marty.

[DarkeyesZ33]

I've checked the values of the resistors and I tried it on a different breadboard. I don't think something is mistakenly bypassing the opamp out of the circuit. It is definately going through the OpAmp, but for some reason the Non-inverting pin is not the same voltage as the inverting pin... it should be near 4.75V unless driven into distortion (which it's not). For this circuit to work, the non-inverting pin (with no signal) needs to be near 4.5V... calculating the theoretical with the values used in the design, it should be near 4.75V

I don't know. I am sure I am making the same mistake over and over again. I just thought maybe someone would know what would cause the opamp's pins to be at different voltages...

thanks,
Thomas

[MartyMart]

Are you using a single or dual opamp ?  ( that's a pinout Q )
I had something similar with an 808 ( real one ) and it was the opamp
that was at fault.
It caused the 4.5v rail to look "wierd" at various points ....

MM

[DarkeyesZ33]

To above: I've used several different opamps include: 741, jrc4558, 833 and op275... I never assume pinouts are the same. I always download the datasheet.

I think I figured one thing out though... but not why it's not working =P.

The voltage coming off the 1M resistor (same node as the non-inverting pin of the opamp), reads 2.40V... but this is incorrect. It really is 4.5V

The problem is the input resistance of a DMM is not large enough compared to 1Meg, so when you take a reading it's wrong. If you were to hook up a voltage follower to that node and then take the reading from the output of the voltage follower, it would be 4.5v

But I still haven't figured out why the circuit isn't working. =/

[DarkeyesZ33]

It's working!

Basically I got fed up with debugging (this being the 4th day), so I decided to just design my own. I took the response of the TS and started to design a circuit that wasn't a tube screamer but behaved like one. When I started to add filters, it stopped working. I started to suspect a .22uF capacitor (so I switched it out with two .1uF)... and bam it worked. The one I designed sounds like a tube screamer but needed more work to quite it.

I took out the .22uF capacitor in the Tube Reamer, and but in two .1uF in parrallel. Bam, the thing works wonderfully. After inspecting the ten .22uF capacitors I got, most of them are defective. haha what are the chances. That's really annoying.