Breadboarding Germanium Big Muff Pi

[lmorse]

I have just breadboarded a PNP Big Muff Pi using 2N2907's (PNP). All seems OK.

I have just started experimenting with GE Transistors... so I swapped out the 100k emitter resistors for 15k.

I have tried some GT308v's (hfe 60-80), which sound OK-ish. I have also tried some Russian ge's that have an hfe of between 250 - 380 - these distort a HUGE amount with the sustain turned right down, pretty awesome. When I turn the sustain up around halfway they start squealing, like feedback.

On Q1-3 I get bV of around .120v and cV of around 4v, on Q4 I get bV .400v and cV of about 6.5v.

I have also experimented with using just 1 of the high gain ge's as the last stage, and Q1-3 as the GT308v's, which sounds ok. I had imagined using the high gain GE's throughout, but at the moment the squealing is making them unusable. Can anyone give me some guidance on the best way forward?

[MaxPower]

I thought if one was going for a germanium BMP one would use germanium diodes...

[vigilante397]

I thought if one was going for a germanium BMP one would use germanium diodes...

Fuzzes don't generally use germanium clipping diodes, just germanium transistors.

I had imagined using the high gain GE's throughout, but at the moment the squealing is making them unusable. Can anyone give me some guidance on the best way forward?

Hands down the best sounding fuzz I have ever played is a ZVex Fuzz Factory (I've built 7 of them), which uses AC128 germanium transistors. There are definitely more settings that squeal than settings that don't, but the Fuzz Factory has a noise gate that allows you to tune in exactly where you want the squeals to cut off. It's pretty neat.

[lmorse]

I have just replaced R7 with a 500k pot so that I can tweak the Q4 cV to around 4.5 v, which has cut some squeal, but still far too much at around 12 o'clock on the sustain.

[mac]

I have just started experimenting with GE Transistors... so I swapped out the 100k emitter resistors for 15k.

Do you mean base resistor?

A 15k from base to gnd set the collector near 3v - 4v or so for many germs.
But you might need to adjust the 0.1uf decoupling cap and 10k series resistor. Try 1u.

You can replace the 100 ohm emiter resistor with a silicon diode so as not to change the original bias network.

mac

[lmorse]

Sorry, I made a mistake there.....

Yes, I meant the base!!!!

But you might need to adjust the 0.1uf decoupling cap and 10k series resistor. Try 1u.

You can replace the 100 ohm emiter resistor with a silicon diode so as not to change the original bias network.

I'll try this, thank you.

[lmorse]

But you might need to adjust the 0.1uf decoupling cap and 10k series resistor. Try 1u.

You can replace the 100 ohm emiter resistor with a silicon diode so as not to change the original bias network.

I am guessing you are referring to R6 and C3? I have played around with these and it hasn't changed the squeal much?

[mac]

I am guessing you are referring to R6 and C3? I have played around with these and it hasn't changed the squeal much?

If you are using GGG schematic I mean C1, {C3, C4 and R8}, {C7, R13} and C12

Try without or with bigger C to B caps.

Are using positive or negative ground? PNP and NG can be tricky.

BTW, breadboard can be noisy with this super high gain pedal

mac

[lmorse]

Cool, thanks for clearing that up.

I have a positive ground PNP.

I have noticed that the breadboard is noisy, and I was hoping that it would clean up when sent to PCB. 

[digi2t]

Maybe this might help too...

http://www.diystompboxes.com/smfforum/index.php?topic=56874.msg440127#msg440127

[mac]

Insert Quote
Maybe this might help too...

http://www.diystompboxes.com/smfforum/index.php?topic=56874.msg440127#msg440127

The schem is in my gallery now

mac

[lmorse]

Hey Mac, that's useful, thank you.

Using diodes on the emitter has made a BIG difference.

Still very noisy on the breadboard though.

[digi2t]

Try this just for shits and grins;

If your breadboard has a metal backplane, pick a spot where you can chip some of the paint off. Get it right down to the metal, and clip an alligator clip jumper to it. Connect the other end to the ground of your breadboard circuit. Does it get quieter? Also, kill any fluorescent lighting that you may have in the room.

If you find that the noise level is substantially reduced, chances are that it might be much better when you box it up.

[mac]

My desk is made of dense wood. It catches EM fields 
Sometimes I have to suspend the breadboard in the air, or use my amp as a desk

It's also important to have a good earth connection.

mac

[lmorse]

I have noticed that if I touch the outside of the output jack plug that the whistling intensifies - I thought that might have something to do with grounding? Also if I touch the collector of the final stage transistor?


EDIT:
This got me thinking, and I may have found the issue. I reversed the leads on the output jack so that the grounding was correct and the whistling stopped there. Still get whistling if I touch last stage transistor collector. I'm unsure if that is usual?

EDIT #2:
I checked the grounding on the input. I had it reversed too 

She has cleaned right up and the squeal on the collector of the last stage has now gone.

SHE SOUNDS AWESOME!!!!!!!!!!! A real face melter! 

[pinkjimiphoton]

well done, lee!!

[lmorse]

Thanks BLUSHING!

[mac]

SHE SOUNDS AWESOME!!!!!!!!!!! A real face melter!  icon_twisted

mac

[digi2t]

Holy shnorkers... got this on the breadboard now too. 2SB176's, 1N4004 diodes for Q1, 2, 3 emitters, and 1N695 diodes for the clipping. I figured if I'm going germanium, I would go all the way. I used a trimmer for Q4E, 5K ball park seems to give me decent voltages. I'm using a mix of Triangle, Ram's Head, and Russian values for resistors and caps, sort of my personal favs.

This thing has some real kick-in-the-pant roar and tone. Compared to my other BMP builds, this one is throatier flat out, and seems to clean up better when I roll off the guitar volume. Not clean, but more of a gentle over-drive, and not as harsh as my silicon versions. I feel that the guitar's volume is more usable with this circuit, better interaction and drive ramp than with the silicon BMP. Breaking the connection for Q2 clipping cap gives a nice solo boost too. I feel a "Boost" footswitch here would be choice.

Thanks for the info mac, and lmorse for starting this thread, really opened my eyes (and ears) to a great twist on the venerable BMP. I'm working on the vero now (using a modified Muffarray vero layout), and I'm boxing this sucker.

I'll post more on this project soon.

[lmorse]

I agree!!!! It is an awesome sounding pedal!!!!

Like you, I added a switch to knock out the second clipping stage (I assume you are using reversed numbering for the transistors? Q1 being the last stage?). I also added a switch to change the capacitor in the tone stack to reduce the scoop. A very different sounding pedal when both these switches are flicked - some real BOOOOM.

A great project, one I am glad I tried, and yes, definitely being boxed up.

I am now looking for a simple-ish metal type distortion to try - suggestions welcomed.