Diodes for clipping after a j-fet booster doesn't work. Why?

[Absentidei]

Earlier tonight, I did some experiments with diodes as a passive clipping device straight from the output on the guitar.

I thought that boosting the signal before clipping it would result in a stronger effect, so I went to look for a booster I could build with the parts I had.
I decided to go for this circuit:
http://www.beavisaudio.com/techpages/blocksfragments/images/JFET_Booster_Schematic.gif

I confirmed that the booster worked.
Then I plugged in the little device I built earlier tonight, wich in effect just put two diodes in parralell, pointing each way, from signal to ground. Then I didn't get any sound at all...

I'm trying to learn a bit more about pedals so I can start to build some of my own, but this has me stumped.

Can anyone explain why this happens?

Edit: It doesn't actually go completely silent. The volume is substancially reduced, I get REALLY short sustain and the only sound I get from it is a kind of crackling sound.

[psychedelicfish]

Do you have the diodes to ground in parallel with the 51k resistor? If you had them before the 4u7 capacitor it would short the DC on the drain of the FET to ground. Other than that, one of your diodes might be dead. Try checking the diodes with your multimeter. If you don't have a multimeter that can check diodes, replace the diodes and see if that makes a difference.

[Absentidei]

Yup. Both diodes in parallel with the 51k resistor.
The diodes are in a little passive device I built that has a 12 position rotary switch that let's me choose between any of 7 tone caps or 4 diode configurations in series with a 500k audio taper potmeter wired as a variable resistor.
Two of the settings for diodes are 2 diodes in parallel with different polarity, two of the settings are just one diode.
When I use just one diode, the booster works (I'm not sure about the polarity of that diode.)

I just tried with two different diodes, one 1n4148 silicon, and one bat41 schottky.
When the diode points to ground, in parralell with r6, it makes these sounds:
https://soundcloud.com/a-melodyc-shivering-form/wtf
When they point the other way, it doesn't seem to have any affect at all. No clipping.

[teemuk]

Fit a lowish series resistor after C3. Without it, the FET stage is trying to drive almost a dead short when the diodes begin to clip.

Also, a 500K series resistor for the diodes might be WAY too much. I bet than in circuit such as this such pot will allow clipping only in far extreme of the dial.

[Absentidei]

The 500k pot is turned way down when I'm using the diodes for clipping.
The reason it's wired up like this is that it's going to replace the onboard tone control in a guitar.

I'll try with the resistor.

Edit: I tried with a 10k resistor in series with c3, and put the clipping diodes after that. Still the same result with almost no sound, but occasionally spluttering and crackles.

Edit 2: I tried with a 10k 1:1 transformer on the output, and then it worked with no problems.
One end on the primary on the transformer wired from signal out on the booster and the other end to ground.
One end on the secondary then became my output, and the other end on the secondary I wired to ground.

I'm just trying to figure out the physics behind it.

[Quackzed]

how much boost are you getting without the diodes? this thing should be loud. if its barely boosting you may want to try a different jfet if you have one. they can get fried pretty easily...
maybee try just diodes without the switching device /cap changes, just to simplify...
if it is indeed a LOUD boost without the diodes, try a different c3 cap,maybee the cap is fried or at least double check it that theres no short around the c3 cap...
it should work. i think the resistor after c3 but before the diodes is a good idea, say a 10k or so, but it may/may not be needed...
double check that its boosting good w-out the diodes, check the c3 cap, and try a different jfet, maybee different diodes.
but i'm thinking it may be the device thing, so try to omit that first...

[Absentidei]

I get LOADS of boost with the pedal.

I'll try with a different c3 cap when I get back home.

But why would it work with an output transformer before the clipping diodes, and not else?
If the c3 cap is busted, that would mean that DC would run trough that cap, and then trough the diode that goes from output to ground, and that would mess with the transistor?

[Quackzed]

basically the signal needs room to swing up and down and c3 is there to block dc so the signal will sit around half way between 9v and ground and be able to do that.
if you short around c3 or its busted and theres a short in it, it wont block that dc and the signal can be too close to 9v or to ground to swing positive and negative freely due to the dc pulling it close to 9v or ground...
it will work WITH the transformer because the transformer is 'basically' doing what c3 should be doing, blocking dc from pulling the signal too high or too low, so the transformer is blocking the dc and preventing it from pulling the signal too high or too low and allowing the signal to stay half way between 9v and ground with no dc bias shift. and that lets both diodes clip normally...
so basically the transformer may be doing what the cap 'should' be doing... no reason you cant just leave the transformer in there rather than changing out the cap, but a bad cap 'could' be what was causing the dc bias shift... but some kind of dc bias shift is probably why it wasn't working...

[Absentidei]

Thanks a bundle!
I replaced the output cap, and it works like a charm now.

I understand that adding a potmeter in series with c4 would make me able to controll gain a bit.
But would increasing the value of r5 (source to ground) to something higher, and then adding a 10k potmeter in series with c4(the capacitor from source to ground) give me more gain control?

[Quackzed]

it works like a charm now.

cool, glad that took care of it. i was hoping that would be it, but couldn't be sure...

I understand that adding a potmeter in series with c4 would make me able to controll gain a bit.
But would increasing the value of r5 (source to ground) to something higher, and then adding a 10k potmeter in series with c4(the capacitor from source to ground) give me more gain control?

yeah, the source resistor controlls gain, and also biases the transistor along with r4 - you can think of the transistor as 'sitting' between 9v and ground.
the bias will only start to come into play if the signal is big enough to extend 'beyond' the linear region of the device (jfet) and not super important till you start to really raise gain.
the c4 cap you can think of as a low resistance resistor at whatever frequencies the size of the cap will pass, big(47uf is big) caps will act like a low resistance resistor for all of the guitars frequencies, so the cap allows you to 'really raise the gain' without un balancing the bias of the transistor.
if you used a small cap .001 the gain for only the frequencies the cap will pass (the very highest freqs) will be raised. sort of a treble booster effect...
a common way to get a useable gain range controll is to use a pot 'instead' of the source resistor (one side of the pot to source-opposite side to ground) so that the source always 'sees' the same resistance to ground, and to use the wiper of the pot to go to the cap(c4) and the other end of the cap to ground.
take a look at a fuzz facegain controll to see what i mean here.
i'd try replacing the 2.2k source resistor with a 5k resistor, and use a 5k pot in parallel -wiper to----( c4 )---to ground for @ same gain at max
or a 10k resistor and a 10kpot wired the same for around 1/2 as much gain at max gain... both should give plenty of control for lower gain sounds.

[Quackzed]

if you happen to only have a 10k pot, you can parallel with a 2.7k for around the same max gain but the pot sweep will be a bit bunched up on one end of rotation- but still get full gain range...
leaving the 2.2k with a parallel 10k pot will give slightly more gain and bunch up the range of the pot to one end a 'bit' more.
so in short, basically, to answer your question: yeah you can do that.

[Absentidei]

I did some experimentation (before I read your reply) and found that if I raised the source resistor to about 6k and added a 5k pot (I didn't have a 10k around) in series with the source capacitor, I got a control over the volume I kinda liked, and it breaks up a bit in a kinda crunchy way when I set the gain pot to max.

I then added a dpdt on/off/on from the output to ground, with a single germanium resistor on one side and one germanium and one schottky on the other side, so I can choose between clean, and two different modes of clipping.

The result:
My first ever guitar effect.

I don't have a drill, so I haven't been able to drill the enclosure yet.

I called it the gizmo, since it can be either quite nice or a bit nasty.
I might rebuild the circuit board later to get the bias right, but right now I like how it sounds, so I won't bother quite yet.

Thanks a BUNDLE for all the help.

[Quackzed]

nice! i like it. gizmo was my 'breaking' name  when i was like 12 or 13 (gremlins had just come out and my first name is gil so...)anyway congrats on you first pedal!!!