I finally finished this project I began late last year. I was in need of some sort of overdrive/distortion pedal because I have a couple of single-channel tube amps that don't begin distorting nicely unless turned up very loud, which isn't always practical. I had some old germanium transistors in my parts bin that I pulled out of a radio or something over 30 years ago when I was a kid, so I figured I'd put them to use.

The pedal's design employs two stages, the first being an emitter-follower acting as a buffer and the second being a Class-A amp with a couple of germanium diodes (new) that can be switched on and off for clipping the signal. The "gain" knob isn't working quite as I hoped. Yes, when turned all the way up, the gain does increase, but turning it all the way down doesn't even come close to killing the signal. I guess there's still plenty of gain provided by R11.

Attached are a couple photos and a 5 minute clip demonstrating the sound. Thanks for the help previously received. I'll be back for more soon.

 

Nice job.

The "gain" knob isn't working quite as I hoped.

Perhaps replace Re with a pot as the gain control, instead of using R12/C5?

 

Not a bad idea. That would certainly allow me to reduce the gain to practically zero, but when turned up all the way, I would lose some stability I believe. Re = 0?

I'll experiment some more on my breadboard and see if I can stumble upon a mod..

 

Re = 0?

No. You'd use the pot as a potentimeter, not as a rheostat. That way Re =1k for all pot settings.

 

Ahhhhh I see. Yeah, let me play with that. Thanks.

 

Very nice. Have you built any other guitar effects pedals. I am just starting out.
Just a beautiful pedal and a great sound.

How could you change the circuit to increase the volume even more?

 

Very nice. Have you built any other guitar effects pedals. I am just starting out.
Just a beautiful pedal and a great sound.

How could you change the circuit to increase the volume even more?

Thanks finom. This is my first guitar effect pedal, but I have another one in the planning stages. I'm going to build a phaser modeled roughly after the MXR 90. I already have the parts, I just need the time to breadboard and experiment. Right now, I've been busy building some speaker cabinets to go with a few tube heads I have.

As far as making the Coppertone louder, there's only so much I can do with such a simple circuit. When the germanium diodes are switched off, I get nearly +/- 4 volts (limited basically by the 9 volt DC supply), but with those diodes clipping the signal, the peaks are roughly +/- 0.3 volts. Driving it harder only makes the rise and fall of the signal steeper (more like a square wave), but never greater in amplitude. In order to make that fuzz tone louder, I would have to amplify that clipped signal without rolling off any highs and losing the fuzz tone. It's ok though. My amps have plenty of power to get louder.

 

You could add a variety of diode combos to clip the output with just a couple more toggle switches. Two silicon diodes in series would let you get 1.4 volt peaks. Three to four times the output voltage.

 

You could add a variety of diode combos to clip the output with just a couple more toggle switches. Two silicon diodes in series would let you get 1.4 volt peaks. Three to four times the output voltage.

I thought about that. There is no shortage of ideas for something like this, but at the time, I just wanted to get something basic built.

If I ever build another distortion pedal, I will add more features but probably also make it opamp-based. Playing with discrete transistors (especially old vintage ones) was fun and a good exercise, but kind of a pain in the @$$. No wonder the rest of the world went to integrated circuits.

 

I'm coming up with 3.9k for R8 in order to get R10 to have 4 volts across it with no signal.

 

I'm coming up with 3.9k for R8 in order to get R10 to have 4 volts across it with no signal.

It might very well be. I think I used a 5k or 10k trimmer pot for R8. That 1.4k value might be what I actually had to adjust to get the Q-point to sit at 4 volts. Remember: very old germanium transistor and probably leaky as hell. I'll check it out the next time I open her up.

 

I didn't realize the transistors were also germanium.

 

Hello, your stuff sounds definitely great. Did you design the schematic yourself ? What are the PNP transistors you use ?

 

Hello, your stuff sounds definitely great. Did you design the schematic yourself ? What are the PNP transistors you use ?

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