Made a Tube Screamer, no distortion/overdrive :S

[Budisha]

So i made the Tube Screamer (TS-808) from Tonepad:
http://www.tonepad.com/getFileInfo.asp?id=81
The volume and the tone pot work fine, but when i rotate the drive pot, there is apsolutely no change in the sound.
I replaced the 1n914 diodes with 1n4007. I think that might be the problem.
The battery is at 8.5 volts
Here are the voltages:
IC1 (CHN4558C)
P1:4,0V
P2:4.1V
P3:3.9V
P4:0,0V
P5:8.1V
P6:4.0V
P7:4.0V
P8:4.0V
Considering pins are like this:
1 o    5
2       6
3       7
4       8

Q1(2n3904):
E:2.9V
B:2.4V
C:8.0V

Q2(2n3904):
E:2.9V
B:2.3V
C:8.0V

I am almost 100% sure its the diodes. But i want to make sure. If it is the diodes, with which diodes should i replace them since i cant find the 1N914?
Thanks?

[ashcat_lt]

More likely something other than the diodes.  If there is no noticeable change as you turn the drive pot then either there's a short somewhere in the feedback loop (across the pot itself, or maybe across the diodes or the cap) or one of the connections in the resistor>cap>Vb line is open.

[add4]

I once had the same problem, turned out it i accidentally 'subbed' a 100n cap by a 1n one on the signal path ...

[Budisha]

I can check if the pot is bad by replacing it with a 500kOhm resistor, right? If it gets distorted, it means the pot is bad, right?
@add4
You had this problem when making the Tube Screamer?
I have a strange looking yellow-white-ish cap labeled .1K63 in the place where the 100nF cap should be.
I though that mean .1uF. Does it?
Here's the pic of the cap:
http://img542.imageshack.us/img542/8982/dsc06331c.jpg
What do you think?

[add4]

i had this problem making an opamp overdrive/distortion, if i remember correctly it was an OCD.
that's what happen when you drink beer while you build pedals..

[Budisha]

Dunno if this matters. Just realised that i replaced the 51pF with the 56pF cap and the 0.027 cap with the 0.022.
By the way, i checked the pots with a multimeter, they are good

[Seljer]

Those two differences shouldn't matter for the basic sound of the pedal. My bet is something being wrong with either the gain potentiometer (the 500k) and the parts next to it: the 51kiloohm resistor, the 47nF capacitor and the 4.7kiloohm resistor. In once case you'd be stuck on maximum gain and if one of the latter two has a broken connection it'd be stuck on unity gain.

The diodes aren't that crucial, it'll still funciton without them even (you'll just get a huuuuuuge volume boost which will be very apparent)

[add4]

Indeed, the diodes are only there to convert volume to distortion.
if your pedal gives a huge clean boost, it's the diodes, if the volume/gain doesn't work when you turn the gain pot, theres something wrong in the feedback loop as other people said.
you should check for solder bridges, triple check every component value, and be sure of where you soldered them.

[Budisha]

Soooo. If it was the diodes fault, when i would rotate the drive pot, the volume would increase, right? It doesnt.
I dont really know what the feedback loop is. DOnt know that much about electronics and english isnt my native language.
Ill re-check everything tommorow. Its already late here and i've been working the whole day on the pedal. In the meantime, could you explain what the feedback loop is? Thanks. Really appriciate all of your help, guys.
Also, i thought about the diodes. It says if i want symetric clipping, i should put a jumper instead of D3, But isnt D3 (the jumper) making a short circuit and leaving D2 out?

[J0K3RX]

What does it do with the diodes removed?  .1K63 or .1uF is the same as 100nF

[add4]

The feedback loop is the part of the circuit where you have the gain pot. if you look at the schematic you will see a connection between the output of the operational amplifier, and one of it's inputs.
between this connection you will have different components in parallel :
in your case:
the gain pot
the diodes
maybe a small cap (usually in pF)

what this part of the circuit does is controlling the gain of the operational amplifier.
the gain pot controls the gain of the opamp.
the diodes clip the sound waves, generating distortion
the small cap allows the high frequencies to go through the feedback loop without being distorted, which prevents bad sounding distortion.

You could see all of the feedback loop as a box which controls amount of gain of your operational amplifier, and the frequency response of the amplification. if you add diodes into it, it also generates distortion.

In your case, if the sound doesn't change when you turn the gain pot, is sounds like this part of the circuit doesn't work ... OR the output of the effect gets signal from the input  of the effect without going through the amplifier..
check for correct soldering, solder bridges, correct built of the pedal, does the sound change at all if you bypass the effect ?

[twabelljr]

Read this: (a couple times)
http://www.geofex.com/Article_Folders/TStech/tsxtech.htm
Especially the clipping stage section. You will understand a Tube Screamer if you can read through and folow the schematic.

[Budisha]

When i removed the diodes, it remained the same. Volume and tone pots work, but the drive doesnt make a change.
I also tried with a pair of 1n4148 but the result is same
@add4
If with bypass you mean the 3pdt switch, the sound does change. I can control the volume and tone.
And thanks for the explanation

[Seljer]

Here is the drive section with a couple of compoents grayed out.



If you look at it a bit it matches the scheme for the non-inverting amplifier application of an opamp (you can 'forget' about the capacitors and diodes for now), the input is connected to the + input of the opamp, and the output has a resistor going to the - input and then from the - input to ground (or to some point of reference):
http://en.wikipedia.org/wiki/Operational_amplifier_applications#Non-inverting_amplifier

The gain of such a configuration is 1+R1/R2 where R2 in resistance between the output and the - input and R1 is the resistance from the negative input to the reference voltage

So in the tubescreamer:

• R1 is the 4.7k resistor
• R2 is at one end of the drive control just the 51kiloohm resistor, at the other end that resistor in series with the drive potentiometer for a total of 551kiloohms

So the gain can go from 1+51/4.7 = 11.8 ot 1+551/4.7 = 118

I can vouch for my own tubescreamer (where I added a switch to remove the diodes from the circuit) that even at minimum drive it gives a noticeable boost, and at maximum its just insane.

If in your circuit there were to be something wrong with the 4.7k resistor or the capacitor next to it, say there was a bad solder connection, that would mean there would be no connection and R2 would be in theory infinite. By using the formula from befeore: 1+51/infinity = 1, which means the opamp would not be giving you any amplification at all.
I'd check those two parts, try replacing the capacitor to be sure.

[Budisha]

Thanks, Seljer! It seems like there was a bad solder connection with the capacitor. I resoldered it and now the gain pot gives a big boost, now i just have to resolder the diodes and it should work
Thanks for all the help, guys!

[Mike Burgundy]

Considering pins are like this:
1 o    5
2       6
3       7
4       8

For future reference: pin numbering is standardised as
1o 8
2   7
3   6
4   5

That'll prevent some confusion with schem and datasheet numberings.
Good on you for tagging the pin-numbering on your post, though. Never hurts to be very, very clear.
Glad to hear it's working (assuming it IS after putting the diodes in) ;P