ITS8 questions

rez667 · February 12, 2010, 06:57:34 AM

good morning ppl.. i have a trouble shooting question.. I just built the Ibenez Tube Screamer 808, schematic is here. http://www.generalguitargadgets.com/pdf/ggg_its8_sc.pdf. I am not getting very much effect when the pedal is activated. I was wondering if anyone could steer me in the right direction of what to troubleshoot first.. I tested the pins of the IC and the voltages are correct. Voltages on both transistors seem to be correct also. Base is .7v off of emitter voltage. It's time for class, but i will answer any questions anyone might have. Also, I did not see a search box, so if this has been posted before i'm sorry for asking the same question :-(

thx

rez

Quackzed · February 12, 2010, 08:09:05 AM

chack the diodes, make sure they are connected and not missing or changed...most 'mods' involve the clipping diodes and someone may have lifted them or added leds etc... if you use clipping diodes with a higher threshold(like leds or si diodes in series), you will get less 'effect' and more clean volume...

BAARON · February 12, 2010, 09:30:47 AM

Does "not very much effect" mean not very much distortion, or not very much volume? Can you be more descriptive about the pedal's symptoms?

Badside · February 12, 2010, 12:24:27 PM

Quote from: Quackzed on February 12, 2010, 08:09:05 AM
chack the diodes, make sure they are connected and not missing or changed...most 'mods' involve the clipping diodes and someone may have lifted them or added leds etc... if you use clipping diodes with a higher threshold(like leds or si diodes in series), you will get less 'effect' and more clean volume...

Actually, on the ITS8 I built, the "no-clipping" option (it comes with a 3-way switch) can get very distorted because of the limited voltage swing available. It's pure opamp overdrive of course, but it sounds surprisingly good with an OPA2064 in there.

However, the germanium diodes sound surprisingly... quiet and undistorted. Rather strange (the silicon diodes sound just perfect).

To the OP: did you install the 3-way switch (the circuit actually won't use any clipping diodes without it) and do the different positions change anything?

rez667 · February 12, 2010, 04:37:26 PM

Yes, i installed the 3 way switch. I brought it to class today and hooked it up to an oscilloscope. In position one it looks square wave..ish. position 2 it looks like a regular Sine wave. and position 3 it looks like a saw tooth wave. The effect that i think is lacking is the distortion and sustain. There is plenty of volume though. I have the led's on the clipping part of the circuit.. Should i try maybe regular diodes instead? if so i have IN4001's available at my house.. If they wont work, whats the best kind to use?

thanks for your help ppl:-) much appreciated

rez

Badside · February 12, 2010, 04:51:26 PM

Sounds like you might be lacking gain
Are you sure you used the right pot in the right position?
Like if you used the level pot for gain, that could be the result

Althought, if you were able to see a square wave, I'd be inclined to say there is enough gain

I must mention though, the Tube Screamer circuit does not work that well in front of a perfectly clean amp. It really shines as a "boost" in a tube amp.
For one thing, you always hear your clean signal mixed in with the distorted signal. It's part of why it's so glorious as a boost, but it's weird when used by itself.
That is imho of course

rez667 · February 12, 2010, 05:13:18 PM

that was one of the first things i checked.. the pots. I got my DMM out, and checked, they are all in the correct spots.. I'm in the process of building a jcm 800 2204.. (once i find a chassis) i think this may be all the effect i can get out of it.. I think im just used to all the digital boxes i have been using in the past.. but any other suggestions, i'd be more than happy to try.

rez

rez667 · February 12, 2010, 07:47:15 PM

would changing the values of the pots make a difference? I know the gain is controlled by the negative feedback resistor.. at least thats what i think from theory class.. I am at a loss here..

rez

Electron Tornado · February 13, 2010, 12:00:52 AM

If you're using the pot values called for in the schematic, then it's unlikely that they are part of the problem unless you have wired them incorrectly.

If you're having trouble that keeps you scratching your head, take a big step back and find out if the pedal is working OK without clipping diodes. Take the diodes out of the circuit - does the pedal function correctly? If not, then you know the diodes are not the problem.

If everything is OK, try using just a pair of 1n914 or 1n4148 diodes for clipping and see if you still have the same problem.

Also, where is the drive pot is set - is it maxed or only turned up part way?

rez667 · February 13, 2010, 10:39:15 AM

The drive pot is maxed, and i will try IN914 tonight.. i have some laying around somewhere.

Electron Tornado · February 13, 2010, 11:37:32 AM

If the drive pot is maxed and you're not getting it to distort, check that the drive pot lugs are not somehow being grounded. I repaired a TS-9 that had that problem.

BAARON · February 13, 2010, 12:02:03 PM

Quote from: rez667 on February 12, 2010, 04:37:26 PM
The effect that i think is lacking is the distortion and sustain.

The Tubescreamer is a) soft overdrive clipping, not distortion, which might explain why it sounds too mild, and b) not a high gain pedal. Stevie Ray Vaughan is probably the world's most well-known user of the TS808/TS9, and listen to what his tone is like.

If you want to increase the maximum gain levels:
Change the gain pot to 1M audio.
Change R6 to 1k.
Change C3 to 0.22µF.
Change R7 to something like 4k7-10k to make sure the "minimum gain" setting doesn't get too dirty.
That'll give you MUCH more gain available at the top end of the knob than the stock circuit.

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ITS8 questions