Pyrocumulus fuzz biasing

[sopapo]

Hello to all,
I built the pyrocumulus fuzz, but I have a litle problem here, the vc in q2 ( or q3, the second stage from the input) is low, like 1,2 v, isnt in line with the usual 4,5v. The rest of the stages are properly biased at 4,5v.
This circuit is directly "inspired" by the dice works muff diver, and in this particular design there isnt the usual resistor to ground from the base of the q2...
Anyway, I checked the board, resolder all, checked all the values of resistors in this stage and all is right. The funny thing is that I used the big muff bias simulator of diystompboxes, https://www.diystompboxes.com/biascalc/bmbias.html and with this particular values my vc of 1,2v is right¡¡¡ I am using bc549c with and hfe of 420 aprx.
If I put a transistor with a hfe of 100 the vc up to 4 v (this is a normal behaviour with transistor)
Is this circuit designed so q2 is biased low??...but its too low¡¡ I think with this readings the transistor isn working properly

Thanks in advance

 

[comradehoser]

I have also built the pyrocumulus, but my impression was that it was based on the thorpy FX fallout cloud. Is the earlier genealogy to that the muff diver? In any event, my build exhibits no underbiased behavior. I don't know how much fidelity there is to the exact original sound, but it is a ripping big muff fuzz. Really love it, especially for the huge bass and the tone separation.

 

[sopapo]

I have also built the pyrocumulus, but my impression was that it was based on the thorpy FX fallout cloud. Is the earlier genealogy to that the muff diver? In any event, my build exhibits no underbiased behavior. I don't know how much fidelity there is to the exact original sound, but it is a ripping big muff fuzz. Really love it, especially for the huge bass and the tone separation.

Thanks comradehoser¡¡ yes, the pyrocumulus its based on thorpyfx fallout, but in the fallout, the three first stages are exactly like the dice work muff diver..
Have you checked the voltages of the transistors? I would like to have a reference.
Mine dont sound ok, it have very low volume and very..emm farty?? very chocked and distorted in a not pleasant way, and of course not like a muff

 

[jesuscrisp]

It's a big muff, there's no individual transistor biasing involved here, like in some JFET builds. If you have issues with it sounding low volume and farty, you may have installed some wrong value component(s), have a faulty transistor or fried your transistor when soldering. Or have something else shorting out somewhere.

Please post pictures.

 

[sopapo]

It's a big muff, there's no individual transistor biasing involved here, like in some JFET builds. If you have issues with it sounding low volume and farty, you may have installed some wrong value component(s), have a faulty transistor or fried your transistor when soldering. Or have something else shorting out somewhere.

Please post pictures.

A transistor has to be biased, sometimes high or sometimes low, but It has to have a VC between 1-8v ( assuming 9vcc).
The things is that bipolar are very consistent, nota like jfet, germanium etc so if you built the circuit, the most probably is that the bias is right
All the information i have gathered about muffs says that the vc of the four stages is about vcc/2 aprox....-+2-3 v
The thing is that this particular circuit that doesnt have the usual resistor to ground on the base of q2, so i think that the bias isnt the same..

 

[jesuscrisp]

What I'm getting at is that if it doesn't work, it's not about how it's biased by the values.
Removing the base to ground resistor makes that stage closer to a soft-clipping electra instead of a soft-clipping LPB-1.
That makes it "less stable", thus theoretically dirtier. Can't tell you about what voltage it should be though.

 

[Robert]

The absence of that resistor isn't exclusive to the Muff Diver / Pyroculumus. There were apparently some stock triangle Muffs (which is what the Fallout / Muffroom was alleged to be based on) that didn't have that resistor as well.

The Tonepad version of the triangle Muff, for example, omits this resistor.

 

[sopapo]

The absence of that resistor isn't exclusive to the Muff Diver / Pyroculumus. There were apparently some stock triangle Muffs (which is what the Fallout / Muffroom was alleged to be based on) that didn't have that resistor as well.

The Tonepad version of the triangle Muff, for example, omits this resistor.

Thanks for your answers, Jesúscrisp and Robert!! Ok, this the confirmation that I was looking for, this voltage is normal with this configuración, so I have to look elsewhere for the problem!!!

 

[Chuck D. Bones]

I built a Pyro and at first I thought there was a mistake, but sure enough some Triangle Muffs were missing that base pull-down resistor. I have to wonder if Mike Matthews did that deliberately or if it was mistake that other people blindly copied.

You have some options:
1. Leave it as-is.
2. Use a lower HFE tranny for Q2.
3. Add a 100K resistor from Q2-B to GND.
4. Increase R9 to 1M.

If you change anything, expect the tone to change. It might be very subtle.

I went with Door #1 on mine, but opted to change other stuff. https://forum.pedalpcb.com/threads/pyromania.2653/

 

[sopapo]

Thanks Robert and Chuck¡¡
One question, with the r9 to 1 meg mod, what is achieved? the vc only up to 1,8
Sure its a bassy design, but its very easy to solve this, you can low the 0,1 caps between q1 and q2 to taste.....

 

[Chuck D. Bones]

The idea behind increasing R9 is to get a little more headroom for Q2. The distortion is supposed to come from diode clipping, not transistor saturation.

You are right about changing the coupling caps to reduce the bass content. I added a bass cut up front on mine.

 

[sopapo]

Hello to all¡¡¡. The fuzz is working¡¡ Thanks for the help and responses¡¡¡¡.....the bias of q2 wasnt the problem, I resoldered again all the board and conections and voila¡¡ sound pretty good¡¡ ... two observations, there is noise with the gain up, i would say that there is too much noise, I know its a big muff, but I would like to know how much noise would be tolerable for this design.. The worst thing is that I wired it very carefully to minimize noise, used shielded wire for the input and output ( to board and to 3pdt), I even used shielded cable for the sustain pot wires, tried to lay out the wires to not cross input and outout signals etcetc, all the usual stuff... The noise shutup if I unplug the guitar and if turn down a little the vol pot ( i use high output alnico humbuckers)
The other thing that I have noticed is that the hfe of the transistor matter, I know this design minimize the importance of the hfe, but if you sub a tranny with a hfe of 430 with other with 650 the tone change, its not a huge change but there is, enough to make a few oermutations of transistor to find your sweet spot.. this design use bc549C..but the "C" range of gain is huge, from 420 to 800, enough to make a difference even in this design, even the B range of gain overlap with the C.
I find that in the first spot is sweeter for me a more tamed tranny, I used a 549B (360 HFE), In q2 a 549c (440 hfe) and in Q3 i used a bc108c (660HFE), in q4 used a 549C ( around 400 something hfe).
In general in fuzzes this work very well, the first stage a little tamed in gain and in the next stages something more high.....

 

[sopapo]

to all the people that have made this... its normal that the treble control gets very, very trebly in the upper half??, its unusable in that positions, which cap set the limit freq?? I dont know if i have soemthing wrong... in the videos in yt look like with the control up dont get fizzy at all..
Thanks for all

 

[Chuck D. Bones]

That is what the TREBLE control is supposed to do. The simple solution is don't turn TREBLE up so high.

You can retune the BASS & TREBLE controls.
To reduce the TREBLE cut & boost range, increase R17 to 22K or 33K.
If you only want to limit the boost side of the TREBLE range, then leave R17 alone and insert a resistor in series with pin 3 of the TREBLE pot. Start with 33K and go from there.
To move the TREBLE control freq range down, increase C9 & C10. Try 4.7nF.
If you increase C9 & C10, then you should also increase C13 to 33nF so you don't get too much overlap between BASS & TREBLE.

 

[sopapo]

Thanks Chuck!! Its only that I was getting the impresión that in my build the treble gets a lot more fizzy than in the demos.. sometimes I have Saw that they put he control in the last quarter and the tone is not much trebly, and with single coils!!! I am using humbuckers...
Could be a taper thing but I Guess that if I can get a usable sound in the first quarter all is right...

 

[Chuck D. Bones]

You have to take the demos with a grain of salt. The amp, speakers, mic, recording EQ, playback EQ, speakers at your end all color the sound.

Humbuckers generally have more output than single-coils. In a Big Muff (and every other dirt box), the bigger the guitar signal, the more distortion you get. It's likely that Humbuckers will be more "fizzy" than single coils all other things being equal.