Big Muff Pi - losing treble notes

[Beo]

I've built a Violet Ram's Head Big Muff Pi. I like it, but it does something weird and I'm not sure whether I have a problem or if I might want to alter some of the resistor/cap values. First, I ready the thread "Technology of the Big Muff - What does what?":
http://www.diystompboxes.com/smfforum/index.php?topic=78111.0

So, what I'm hearing is that if I ring out a high treble note and follow it with a base note, the treble note drops out. If I mute the base note, the treble note jumps back in. This doesn't seem like the base note volume drowning out the treble note... the treble note actually sounds like its being muted.

So, is this related to the caps used in the early gain stages, letting too much base into the transistors? I'll have to double check my circuit, but I followed the Violet Ram's Head schematic on GGG and all these caps are 100n. Maybe I need 220n or even 1u.

Or is this something else? I can't see how the tone control could have this affect.

Thanks!
Travis

[azrael]

Could you post transistor voltages?

[petemoore]

  Sounds like BMP ?
   hard to say if it's 'normal' for bass to swamp the treble isn't outside the possibilities, transistor voltages is always a good idea for active stages.

[Beo]

I'll get the box open this week and measure the voltages and get back to you. Thanks guys.

[jonfoote]

ive noticed that effect when i was messing around with a bmp, try upping R8 & R12

quite an interesting effect if you short circuit them both

[Beo]

So I measured the transistor voltages, all knobs at center. Note that I'm running at 11.88 volts regulated (Small Bear Wart 200).

Q1  C 4.17   B 0.65   E 0.04
Q2  C 4.74   B 0.69   E 0.07
Q3  C 4.14   B 0.65   E 0.04
Q4  C 5.53   B 2.29   E 1.69

[azrael]

Not too weird, I would try and get them closer to 4.5v-5v. Adjust the collector resistor values a bit.

[petemoore]

Q1  C 4.17   B 0.65   E 0.04
Q2  C 4.74   B 0.69   E 0.07
Q3  C 4.14   B 0.65   E 0.04
Q4  C 5.53   B 2.29   E 1.69
   Q4 has 'high emitter voltage compared to the other three, IIRC all the stages are similarly biased, have you measured the E resistor on Q4 [etc]?

[Beo]

   Q4 has 'high emitter voltage compared to the other three, IIRC all the stages are similarly biased, have you measured the E resistor on Q4 [etc]?

I think this is okay. If you look at the schematics for the various BMP versions on GGG, you can see that Q4 has a significantly different E resistor. Also, GGG has voltage readings in their instruction file for one of the versions, and it looks similar to mine, reduced since running on 9v.

http://www.generalguitargadgets.com/pdf/ggg_bmp_instruct.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

9 volt power supply 8.8
Q1
Collector 3.9
Base 0.6
Emitter 0.03
Q2
Collector 3.8
Base 0.6
Emitter 0.03
Q3
Collector 3.8
Base 0.6
Emitter 0.03
Q4
Collector 4.0
Base 2.5
Emitter 1.8

[Astronaurt]

I built a Ram's Head BMP myself and I noticed a similar effect happens on mine, but I always thought that losing note clarity was just par for the course with these kinds of Fuzz boxes. Idk though, because I did do a few parts substitutions so maybe it just isn't as problematic on mine. Even so, I actually kindof like that whole aspect of 60s-70s Fuzz pedals, but If you're looking for a fuzz that keeps clarity, then look at the Quadra-fuzz.

http://www.paia.com/ProdArticles/quadrafz-design.htm

Splits the signal into 4 frequency bands, fuzzes each of them individually, and then mixes them back together. Much too complicated for anything I'd want to build right now, but it's a great case study to read!

In terms of actually lending helpful information though, I'd decrease the coupling caps going into the 2nd and 3rd stages, probably to something like .047uF as that will filter out more of the bass. I'd mess around with those two first and experiment to find what sounds best. I'm also fairly certain that the last stage on the big muff is supposed to be just a recovery amplifier to boost the signal after being mega squashed by distortion, so I don't think that the problem is around that transistor. Actually come to think of it, I would also increase the emitter resistors on Q2 and Q3 from 100Ω to something like 330 or 470. Can't give you an honest reason why that'll work, but I've read somewhere that increasing that resistor on a BJT stage usually lowers the bass content a bit. That's all I got!