Tonepad Tubescreamer/TS-808 not overdriving

[tommasz]

I built a TS-808 clone with symmetrical clipping using the Tonepad El Griton 2.2 board. It doesn't seem to overdrive. In fact, unless turned all the way up it's more of an attenuator.

Looking at the output on a 'scope with a sine wave being fed to the input jack the Drive pot has no effect, Tone appears to work and Level only gives me the same p-p voltage as the input if turned all the way up.

I've double-checked soldering, wiring and component values multiple times. I can see that the signal from Pin 1 of the 4558 is the same level as the input going into Pin 3. But the signal on Pin 5 is much lower, which is why the output on Pin 7 is so low. What should I be looking at next?

Thanks.

[Kesh]

Pin 1 shouldn't be at the same level as pin 3, unless you are putting in about a volt peak-to-peak, as that is what the diodes limit output to. Even with gain all the way down you should be getting x10 input signal, up to that limit of a volt peak-to-peak.

Pin 5 being down suggests the voltage divider/RC network (consisting of a 1K in series, a 10K to Vb and a 220n tant cap to ground) is somehow wrong. Check values, shorts, etc.

[tommasz]

I checked the 10K to Vb and it was open. Replacing it had no effect, unfortunately. I'm wondering why the 500K Drive pot seems to have no effect on the output. Is there something in that feedback section for the first opamp section I should be looking at now?

[slacker]

If you're getting no distortion and the drive pot does nothing, then probably either there is something shorting pins 1 and 2 of the opamp, or you have a bad connection to the cap and resistor from pin 2. Either of those things reduce the gain of the opamp to 1 and will make the pot do nothing. Wrong values for the cap and resistor off pin 2, or bad components there could also cause it.

[tommasz]

The components in the feedback section seem to be fine, no visible shorts and I reflowed the solder. I checked Pin 1 and 2 with no opamp in the socket and they're not shorted. I have the same signal on Pin 1,2 and 3 which is obviously wrong.

[sco01]

Hi,

I had a very similar problem with a Maxon OD-808 vero build a couple of days ago (see: http://www.diystompboxes.com/smfforum/index.php?topic=100513.0).

I had a cap following the clipping stage with a value that was way off resulting in the same symptoms as you have. Follow the links in my post to see the layout and schematic.

//Stefan

[tommasz]

I have a 47nF (473) cap in parallel with the clipping diodes, which I think is equivalent to what you did. The schematic called for a 51pF, by the way.

[slacker]

You don't want 47n in parallel with the clipping diodes, that should be 51p, 47n will mess with the gain and tone possibly causing your problem. The 47n cap talked about in that thread is the one between pin 2 of the opamp and the 4k7 resistor to vb, Stefan had put 47p in that position by mistake which will give virtually no gain.

What diodes are you using? Germaniums will give very little distortion.

[Kesh]

I have a 47nF (473) cap in parallel with the clipping diodes, which I think is equivalent to what you did. The schematic called for a 51pF, by the way.

47n will bypass the diodes and the gain control at guitar frequencies, resulting in roughly unity gain

it's nearly 1000 times the value it should be.

[tommasz]

I had a 68pF ceramic disk in my box, so I used that. That was definitely the problem. Should I bother replacing it with a value closer to the 51pF or just leave it as is?

[slacker]

Cool, glad you fixed it. If you like how it sounds you can leave the 68p in there, changing it to 51p will give a bit more treble.

[Kesh]

Cool, glad you fixed it. If you like how it sounds you can leave the 68p in there, changing it to 51p will give a bit more treble.

The 51p does very little anyway, i can only hear a difference on full gain, and I use 1 Meg gain pot. 68p might make it a bit warmer.

But please, if you ask for help and have used RADICALLY different components, please say so.

[tommasz]

It wasn't intentional. I either ordered the wrong part or got the wrong value in place of the 51pF and didn't notice.