First Pedal PCB Build: The Muffin

[almondcity]

Not to hijack the thread but I really like my latest build with pots for the tone stack resistors. It's such an easy mod and so useful for different tones. My approach has been to set them at about stock values, set the tone knob where I want it, then use the LP and HP knobs to tweak the amount of mids I want

 

[Big Monk]

A quick update.

I think after I get through building my Paragon Mini, Kliche Mini and Electrovibe, I'm going to redo a few things on my Muffin build.

Talking in another thread with @Coda I realized that while in my mind I targeted the 1973 #3 or "Gilmour" Ram's Head, I actually built the oddball 1973 #5 unit. There ism not a ton of difference but let's look at it together quick:





There are not that many differences. The input resistance goes up in the #5, limiting the gain a touch. The limiting resistors go down in the #5 which bumps up the gain a bit. The Sustain resistor is larger in the #5, so in practice, you can't put the Sustain as low as on the #3. 2 of the collector resistors are higher in the #5 which reduces gain into those stages a touch. The #5 uses 560 pf filter caps in the clipping stages and therefore has a bassier response. Finally, the tone stack is a bit more scooped in the #5.

To me, the input resistance and limiting resistor changes are a bit of a wash. The #3 boosts the input but limits the interstage gain and the #5 does the opposite. What i need to decide is whether the collector resistance makes a big enough difference in light of the fact that I used transistors with much lower gain than typical for a Big Muff.

What I may do is tweak the collector resistors, clipping filter caps and see how I like it.

Not 100% sure yet though.

 

[carlinb17]

if you have a dmm with a diode setting you can touch the red probe to the a side of the led and the black to the k side if you wired it right it will light up, if not try it the other way and you can then tell if it's correct. red to a black to k

 

[Big Monk]

if you have a dmm with a diode setting you can touch the red probe to the a side of the led and the black to the k side if you wired it right it will light up, if not try it the other way and you can then tell if it's correct. red to a black to k

Thanks. I solved the LED issue a while ago.

 

[carlinb17]

Thanks. I solved the LED issue a while ago.

my bad when I clicked on the thread it brought me to top for some reason... good to hear you got it

 

[Chuck D. Bones]

You may have a hard time hearing the difference between the two and even then only at extreme control settings. Given that EHX had a habit of building their pedals with whatever parts were lying around on a particular day, unless someone verified all of the component values in Mr. Gilmour's personal BMP, it's all just conjecture.

 

[Big Monk]

You may have a hard time hearing the difference between the two and even then only at extreme control settings. Given that EHX had a habit of building their pedals with whatever parts were lying around on a particular day, unless someone verified all of the component values in Mr. Gilmour's personal BMP, it's all just conjecture.

I'm inclined to agree here.

EHX made a point of doing something that I advocate: Tweak the circuit based on what you have. You see that many of the changes to the circuits are almost about balancing.

If one version had a 33k input resistor, 10k limiting resistors and 10k collector resistors, another might have a 39k input resistor, 8.2k limiting resistors and a mix of 10k and 15k collector resistors. You can plainly see this is about balance. Maybe they have a ton of 39k resistors one run and they tweaked the circuit to balance everything out.

Obviously the tonestack tweaks make a difference based on what you want. One of the things that seems to come very starkly when you research is that units with lower gain transistors (by accident of course because keeping with EHX's philosophy in those days, you "use what you got") seem to have a special thing going on.

I may simply tweak the tone stack and be done with it. I personally like the tone but would maybe just like a touch less of a scoop in the stock position of my mid switch.

 

[Chuck D. Bones]

If one version had a 33k input resistor, 10k limiting resistors and 10k collector resistors, another might have a 39k input resistor, 8.2k limiting resistors and a mix of 10k and 15k collector resistors. You can plainly see this is about balance. Maybe they have a ton of 39k resistors one run and they tweaked the circuit to balance everything out.

That's not plain to me at all. It might be the case, but it might also be random. 33K vs 39K in the tone stack makes little difference if both sides of the tone control are varied by the same amount in the same direction.

The collector resistor doesn't matter in the first three stages because they have local feedback loops controlling the freq response, clipping and gain. As long as the resistor isn't so large or so small that it messes up the biasing, it doesn't matter whether it's 10K or 22K.

The current-limiting resistors (R12 & R19) matter because they set the max gain of the stage. But a 20% change from 10K to 8.2K makes very little difference. R19 only matters when SUSTAIN is dimed.

The input resistor (R2) has some effect, but the pickup impedance and guitar control setting have a much stronger effect.

The parts in a BMP that have the strongest effect are the coupling capacitors (C1, C3-C7 & C13) and the Rs & Cs in the tone stack.

 

[Chuck D. Bones]

With the lower gain transistors, have you tried replacing the transistors one at a time with lower gain to see which ones effect the tone the most? I think HFE mainly affects the biasing and the 4th stage is the most sensitive to changes in biasing. There could be something else going on, but that's my best guess based on analysis.

 

[Big Monk]

That's not plain to me at all. It might be the case, but it might also be random. 33K vs 39K in the tone stack makes little difference if both sides of the tone control are varied by the same amount in the same direction.

The collector resistor doesn't matter in the first three stages because they have local feedback loops controlling the freq response, clipping and gain. As long as the resistor isn't so large or so small that it messes up the biasing, it doesn't matter whether it's 10K or 22K.

The current-limiting resistors (R12 & R19) matter because they set the max gain of the stage. But a 20% change from 10K to 8.2K makes very little difference. R19 only matters when SUSTAIN is dimed.

The input resistor (R2) has some effect, but the pickup impedance and guitar control setting have a much stronger effect.

The parts in a BMP that have the strongest effect are the coupling capacitors (C1, C3-C7 & C13) and the Rs & Cs in the tone stack.

I’m not saying all that stuff matters from a technical standpoint, just that historically, EHX was known to buy components in bulk from surplus sources. That explains the variation in actual components and could also explain the slight variations in circuit components.

Kind of like, “Hey Mike! We got a metric helluva ton of 39k resistors we need to use. Want us to tweak a few other components to keep the signal the same?”

Whether that actually happened is pure conjecture but it would explain relatively minute differences even within a specific version run of the 70s BMPs.

I agree with you that filter caps in the clipping stage, coupling caps and the tonestack produce the most profound effects. That’s part of why I may just leave mine as is.

I need to play around a bit more and see what I come up with.

 

[Big Monk]

With the lower gain transistors, have you tried replacing the transistors one at a time with lower gain to see which ones effect the tone the most? I think HFE mainly affects the biasing and the 4th stage is the most sensitive to changes in biasing. There could be something else going on, but that's my best guess based on analysis.

I haven’t. My Ram’s Head circuit has 4 Amperex A115 around 171 hFE. I went with those at the outset.

 

[Chuck D. Bones]

171 is not exactly "low gain." It's a subjective term, I know, but in many, maybe most circuits, once you get above a threshold HFE, it just doesn't matter what HFE you have. One of these days, I'll breadboard a BMP and play around with various transistors to see if I can tell the difference. I know from experience that the last stage has enough gain that some TONE control settings will drive the last stage into clipping. Changing the HFE affects the bias and that will move it closer to, or further from saturation.

 

[Coda]

I’m glad I mentioned it, because this has turned into a very interesting and useful thread.

 

[Big Monk]

My Muffin went in for surgery today and I’m glad to report is recovering nicely. Put on the black pointers in preparation for the fancy new enclosure:


Went with the 1973 #3 Ram’s Head and added an On-Off-On Mids switch for Stock-Flat-Boost.

 

[Big Monk]

I just need to troubleshoot the tone control.

At the crossover point between low and high pass filters, namely the treble side, it drops the volume of the pedal and there is not treble side effectively. I need to double check my tone stack changes and make sure my joints are good.

EDIT: I imagine the high pass filter resistor is floating. @Chuck D. Bones and @Coda, what do you guys think?

 

[Chuck D. Bones]

Seems like the high-pass resistor is a like candidate. Check everything. It's easy to disturb something unintentionally when making changes.

 

[Big Monk]

Seems like the high-pass resistor is a like candidate. Check everything. It's easy to disturb something unintentionally when making changes.

We were close! Looks like I disturbed the trace that’s connected from the high pass capacitor to the tone control. I jumped it and it’s working now.

 

[Big Monk]

One thing I noticed is I seem to have continuity between pins 2 and 3 of the tone pot.

Looking at the schematic I don’t see how that’s possible and I can’t seen anything bridged in that area.

Does not seem to be affecting anything but weird nonetheless.

 

[Big Monk]

Gents,

I had some time this morning to play this at moderate volumes.

Short story: I’m floored. I was not unhappy with the 1973 #5 version I had built, necessarily but it was a bit wooly and didn’t cut how I wanted it to with the tone control down. Changing to the 1973 #3, or the so called “Gilmour Muff” has really changed the whole character.

For the sake of posterity, my mid switch is an On/Off/On with 4.7 nF in the middle, 22 nF on the Boost side and 6.8 nF on the Flat side. Tons of versatility.

Im very happy. Many thanks to @Coda for kickstarting my researches into modifying my original.

I have a sweet UV printed enclosure coming for it:

 

[Coda]

Very nice. You know, I was thinking the other day how I always chuckle when I hear someone talking about their pedalboard, like on YouTube, and they say "Big Muff Pi," and then don't specify which version. Then I remember that only crazy people know that there are tons of circuit variations, most that make no difference (although, I CAN hear it), and that if you don't specify the "generation" (Triangle, Ram's Head, etc.), the "class" (year), and the circuit (#), you aren't actually telling anything other than "I have a pedal"...