Muffin Fuzz (Rams Head)

[psb962]

I'm breadboarding the Violet Rams Head version of the Muffin Fuzz. Output seems low. Looking at LTspice, I see the collector voltages are supposed to be 4V and change, but 6.6V at Q4. All of mine measure as 7V and change. I checked all the resistors, they are correct values. Base voltages are 0.6V on Q1-3 and 1V at Q4. Vcc is 8.8V. Any ideas on what error I could have made that would result in over 7V at all 4 collectors with this circuit?

 

[psb962]

All the caps I used in this build (except the power supply) were not electrolytic - I have film caps in the values that the schematic identified as electrolytic. Does this matter? In the model above, that would be C3, C8, C5, C7, C12, C10, and C11. I'm still wondering why my collector volts are like 3V too high.

 

[cdwillis]

R12 should be 2.7k, not 4.7k. That's the culprit for low volume. Voltages are probably off because of transistor tolerances, maybe.

 

[psb962]

I found a problem. I had a set of 180k resistors in my bag labelled 100k. So all 4 100k bias resistors were in fact 180k. I was wondering who changed the laws of physics, but this is a better explanation.

HOWEVER: this brought all 4 base voltages down to ~0.6V which is what LTSpice says they should be, but I still have all four collector voltages over 7V, and Q4 is at 8V. Last time I took a class on biasing transistors was in 1981. I really have no clue as to how I can resolve this. My 2N5088s are made by Central Semiconductor and came from Mouser. Could these be way off the spec?

 

[cdwillis]

I wouldn't get hung up on what LTspice is saying voltages should be. What does it sound like? Are you certain all your collector resistors are 12k?

 

[psb962]

Output is weak and far less gainy than my EHX Muff. Output of Q3 sounds much better. I think Q4 runs out of headroom.

I swapped out and tested the 12k resistors. All are 12k.

To check LTSpice, I also modelled the BMP circuit shown in electrosmash as that has voltage check points on it - LTSpice matches them within 0.1V.

It looks to me like something isn't right about these transistors. Sadly I have no others to substitute. I'll get back to this tomorrow...

 

[psb962]

Solved. You will find this amusing...

As I hadn't worked with transistors since college 40 years ago, when I built this I looked at the datasheet to determine which leads were C, B, and E, using image below:


...and I misread it. Lead 3 is the collector, and I read the bottom view as top down, when of course its bottom up. So I had all four transistors wrong way round! The miracle is that it worked up through Q3.

Now I have the transistors the correct way round, all the voltages are what they should be, so I will now go away and play the breadboard for a week or so.

The good news is that I learned a heck of a lot about common emitter amplifier DC biasing over last few days. The 'lights came on' when I measured the volts across each resistor, calculated Ic and Ie, and realized that there was no way Ic could be 75ua and Ie 1.7mA when they should be almost exactly the same.

 

[cdwillis]

Think about getting one of these cheap testers if you don't have one yet. They're really handy for checking pinouts, hfe, resistor value, cap value, etc. This is the one I've had for a couple years now and it's well worth the $20 I paid for it:

Amazon.com

 

[owlexifry]

hahah nice one.

incorrect pinout was my first troubleshoot culprit/build error.

also my favourite type of error, very easy to fix.

 

[psb962]

hahah nice one.

incorrect pinout was my first troubleshoot culprit/build error.

also my favourite type of error, very easy to fix.

Yes, I think the reason it took me so long to figure it out is that the circuit actually works up to a point. I was getting decent fuzz like tones off of Q3 but it all went away on the tone stack and Q4. I didn't know that transistors work even if slotted the wrong way around.

 

[psb962]

I'm having trouble evaluating this circuit on the breadboard as it seems very prone to noise. Way worse than other circuits I've tried. I'm using the protoboard. Any advice on how I can quiet this down a bit?

 

[jwin615]

I'm having trouble evaluating this circuit on the breadboard as it seems very prone to noise. Way worse than other circuits I've tried. I'm using the protoboard. Any advice on how I can quiet this down a bit?

View attachment 60551

With that many gainstages, breadboards can be noisy.
Try turning off lamps/lights nearby.
I use an old swiss made florescent drafting lamp on my workbench that was my grandfather's and it screams emf/rfi. actually scared myself a few times when bench testing because I had the bench amp too high.
Some led lamps and bulbs can behave terribly as well. Those crude half wave "switching" supplies are banshees in certain spectrums.

 

[Coda]

I'm having trouble evaluating this circuit on the breadboard as it seems very prone to noise. Way worse than other circuits I've tried. I'm using the protoboard. Any advice on how I can quiet this down a bit?

View attachment 60551

That sounds correct. BMP is noisy in general, with the Ram’s Head version especially noisy…

 

[Feral Feline]

Try to tighten up your layout and use shorter jumpers if you have them (and if you don't, make some).

Lots of long wires sticking up in the air, reminds me of this photo by Dunk:

 

[psb962]

Without the case this design is very noisy. In the case, it's fine, in fact it's exactly the same noise levels as an EHX Big Muff Green Russian reissue that I've got sitting next to it. So don't worry if your breadboard of a Muffin is noisy - they are. Even if you shorten the leads

I played around with the design and settled on the 73 Rams Head 'Gilmour' spec as published by kitrae (below). Very easy for organizing the components as there are a lot of values that are used many times over.

The bonus on this build was that as a result of my snafu on the transistors in the breadboard I learned a heck of a lot about biasing BJT transistors and that helped me better understand the design.



The finished pedal looks like this (below). I'm finding the Tayda predrilled and painted enclosures with matching PedalPCB boards to be a great time saver. For my lettering I used a simple word doc design which I printed on clear waterslide paper with a laserjet. Seems to work pretty well for all my builds. I added a red 'PI' graphic using a car touchup paint stick.

So how does it sound? I can detect a very small difference to my Green Russian reissue Muff (which has a little more bass). Otherwise they are so similar I think I will sell the EHX pedal as that will provide funds for about 4 more PedalPCB builds....