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BMP- now I'm scared

6/20/2000 3:44 AM
Gord BMP- now I'm scared
I always wanted to build a BMP, but after that huge post about it, I'm confused. Which one do I build? What mods to do to it? R.G., are you working on ' the technology of....'?  
What's the big secret?  
Anyone? Anyone?  
6/20/2000 9:15 AM
JD Sleep

Here's the easy answer and I don't know the "secret" either. You'll want to build it stock first and I would go with the values listed on the old schematic that R.G. drew up years ago. If you don't build it stock first, you'll _never_ know how good/bad the mods are :)  
I wish we could get the good stuff from that monster thread summed up in a couple paragraphs (including the "secret"). There's some good stuff in there, but there's a lot to wade through to find it.  
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6/20/2000 12:11 PM
The "secret" isn't about the BMP
... and I've told it in public before. The "secret" is about manipulating the size of the clipping knee with respect to the signal, and pretty much applies to all distortion pedals. It's not particular to the BMP.  
Don't let my flippant remark hold you back. It's not going to be like the discovery of nuclear fission when I get it tamed.
6/20/2000 2:15 PM

I really don't know of many mods for the BMP either. Build it stock definitely. Try switching the 1uF coupling caps to 0.1uF for a different fuzz. I believe RG's schematic has these marked.  
A lot of the mods to the sound will have to do with what material capacitors you use and the gains of the transistors you use. Poly film + high gain transistor = super duper fuzz with 'octave decay'. Ceramic + lower gain = Marshall-y crunch.  
Well, I take that back. There is some room for experimentation with the tone stack. I know Aron would love to hear how a BMP sounds with a Sweet Thing tone control. :)  
"The "secret" is about manipulating the size of the clipping knee with respect to the signal, and pretty much applies to all distortion pedals."
Oh yeah, I think I remember this. It has to do with the clipping diodes right?
6/20/2000 4:18 PM
Jay Doyle

"The "secret" is about manipulating the size of the clipping knee with respect to the signal, and pretty much applies to all distortion pedals."
"Oh yeah, I think I remember this. It has to do with the clipping diodes right?"
I think it has to do with the amount of amplification the signal receives before it is clipped by the diodes as well as the "knee" of the diode as it turns on.  
Jack did some tests on diodes just before he closed shop showing how different diodes (or transistor diode junctions) have different turn on slopes as the forward conduction point is reached.  
Now the question I have is what R.G. thinks is better (what is the "secret"?); a super high gain stage that slams into fast turn on diodes (I don't think this is it, as the resultant signal would be a virtual square wave, buzzy and harsh.) or smaller gain stages with slower diodes. The latter is my guess, as you would retain more of the original signals "slope" (if that is the right term) and have "softer" corners.  
My reasoning behind this is that circuits that use diode clipping and are considered "smoother" (for example the Tube Screamer) use a cap in parallel with the diodes to "round off" the corners of the diodes clipped wave.  
Of course this is just a guess, not to mention highly simplified. Of course there are other combinations, such as a high gain stage to slow turn on diodes etc. [If I remember right MOSFETs had a very slow turn on slope when used as a diode, which may explain the great tones that come from a Shaka Bradda III]  
I hope others chime in with their ideas.  
Jay Doyle
6/20/2000 9:00 PM
Ed Rembold

Look I swear to you guys that I'll send Aron  
my modded BMP schem. to post, but first, I really want the Hammer to clip in a 100K at Q3, (if its  
not there) and report his findings.  
this will settle in my mind what he is refering to as "blooming"  
Ed R.
6/20/2000 9:24 PM

OK, Ed, I am waiting! :-) As for the Big Muff: As R.G. said a while ago, check out those 8.2K resistors BEFORE the last 2 clipping stages. This will control how much clipping each stage will see by limiting the signal - at least that's what I remember he said - or something like this. Heck, put sockets there so you can try different values. Replace the 470K resistors from collector to base with trimmers so you can adjust the bias. Replace the tone section with some other Big Muff permutation or the Sweet Thing tone control. Use sockets for the transistors so you can try different gain transistors. Darn it! I got myself worked up about this.... I think I will build it soon! :-)
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