Mudbunny Triangle - voltage problem

[Blues Healer]

Hello all,
this is my first mb project, a mudbunny built to triangle specs:


it works, looks great, and sounds OK ... however I checked the voltages and got the following:

Q1:

E - 0.16
B - 0.74
C - 4.42

Q2:

E - 0.1
B - 0.7
C - 1.16

Q3:

E - 0.15
B - 0.72
C - 4.29

Q4:

E - 1.2
B - 1.78
C- 3.78

I reflowed solder on the resistors around Q2 & Q4. Next, I'll double-check those component. In the meantime, I wanted to ask if there's anything else I may be missing?

I appreciate your expert opinions!

[RobA]

Could you have the emitter and collector values you've written down there reversed?

I've never built this version, so I don't know what the values should be, but I do just happen to havea simulation of the two clipping stages for the Big Muff up that I'm working on. So, I altered the parts to match the Triangle specs and took the voltages there. I get emitter = 55mV, base = 700mV, and collector = 900mV. So, not too different from what you've written down (assuming the emitter value and collector values are reversed. I don't have the Q4 stage in the simulation at this point. Hope this helps some.

[Blues Healer]

thanks ... I edited to show correct order

[RobA]

I added in the Q4 stage just to take a look. I left out the tone control section, but that shouldn't matter for the quiescent voltages. For this I get, emitter = 1.14V, base = 1.74V, and collector = ~4.2V. So, not too far off of your voltages again.

The values for Q2 are going to be a bit strange looking because of the weird biasing by dropping R9. I can't say that I understand the point (asymmetric clipping maybe?), but I assume it's doing what was desired.

[Blues Healer]

thanks for your help, Rob ... I wondered about R9 being omitted, but hadn't considered that might alter the bias ... maybe it's OK

[RobA]

I decided to breadboard the Q2 section this morning to directly measure the voltages and what I get is really close to what you measured. So, it seems everything is good with your build.

Yeah, deleting R9 will change the bias point. I'm guessing that it's done to force the signal to swing around a voltage that's close to ground on the output at the collector so that it will clip the bottom of the signal some in a way the top half of the signal won't. I'm guessing that's why the did it on the first clipping segment because it would have been too extreme to do it on the second and the second helps smooth the shape back out a bit.

One thing that might be kinda cool to do would be to put an SPST (or SPDT) switch in there that could toggle an 82k resistor in and out for R9. I haven't tried this though to see what it actually does to the sound. I'd guess someone here probably has though.

[Blues Healer]

Rob,
thanks again for your help, and confirming that my build's OK ...

I took a look at Kit Rae's Triangle info last night:
http://www.kitrae.net/music/big_muff_history.html#TriangleBigMuff
his schematic is about 2/3 of the way down ...



it does show the C2 cap missing, but R9 is there. I don't know what that does either, just what folks have commented about that circuit: that it is more 'articulate'.

anyway, I'm moving on -- I have a few mudbunny boards, so I plan to explore different builds. Next is a Mayo.

thanks again for your help ... I did a search, but didn't really find any Triangle voltages, so I appreciate it.

- Gary

[RobA]

You're welcome on the help. There are so many variations on the Big Muff that it's always interesting to see what's going on with them.

C2 allows some high frequency inverted signal to be feed back to the input signal at the base of Q1 and will work to reduce the high frequency response. So, taking it out lets more of the high frequency through. That could lead to bunches of uncontrolled squealing, but the triangle version uses reduced gain in the first and second stages so that's probably less of a problem. That would make sense to me with the description of more articulate too.

There is one of the circuits on the Big Muff Page that is a variation on the Triangle that deletes R9.

Anyway, have fun with it and the next versions you build too.