Big Muff Pi - dry mix in output

[bdevlin]

My GGG Big Muff Pi appears to have a significant amount of the dry signal in the output.  If you can imagine using two amps, one set to clean, and one set to fuzz, that's what I'm hearing.  Is this common, and moreover, is it easily tweakable?

[Meanderthal]

it shouldn't do that. It sounds like you accidentally stumbled into a clean blend mod. Possibly a solder bridge, or maybe something odd with the switch?

[southtown]

sell it to a bass player?

[bdevlin]

Well I tried an audio probe on this and I can hear the distortion only signal at C12 (the tone pot) and at the base and emitter or Q4 but at the Collector I can hear the dry signal.  The collector connects to C13 then to lug 3 of the volume pot.  All of those connections have a dry signal mixed in.  The wiring on the bypass switch appears okay.  I admit I'm not very skilled at audio probe troubleshooting.....where should I go from here?

[bdevlin]

Hmm.  More on this tommorow.  It seems if I put the probe to ground I get a dry signal.  I don't think that's right.  Seeing how the probe is the "tip" connection, this means I was in theory shorting "tip" to ground.  And I should have go no signal right?

[Paul Perry (Frostwave)]

Hmm.  More on this tommorow.  It seems if I put the probe to ground I get a dry signal.  I don't think that's right.  Seeing how the probe is the "tip" connection, this means I was in theory shorting "tip" to ground.  And I should have go no signal right?

Yeah, maybe there is a break in the grounding, or something that should be grounded, isn't.

[Khas Evets]

Does the volume pot fade between dry and distortion?

[bdevlin]

Does the volume pot fade between dry and distortion?

No.  If fades between no signal and the blended wet/dry signal.  I still haven't hd time to probe some more.  I hope to in a few minutes.

[bdevlin]

Damn if I can't figure this out.  Probed it again and I couldn't make heads or tails.  I was wrong about the "signal" in the ground connection.  The only thing I see that might tell someone something are the voltages on Q4.  C=4V, E=1.8, B=1.2.  The GGG build instructions shows 4V,2.5V, and 1.8V respectively.  So my Base and Emitter are different.

Unless anyone has ideas, I may have to roundfile this build.  Too bad, I was on a roll until I hit this stumbling block.

[bdevlin]

Oh boy, I'm doing the shameful "bump".  If anyone has anything they can think of for me to try that would be appreciated.

[R.G.]

Unless anyone has ideas, I may have to roundfile this build.  Too bad, I was on a roll until I hit this stumbling block.

It would be silly to do that now, since you've already found what will probably solve your problem.

If you have the base lower than the emitter of an NPN transistor, it simply cannot be amplifying. The real question is why the collector is down at 4V if the base is lower than the emitter.

You obviously have a problem in the parts around Q4. And it can't be a capacitor problem. There's only four resistors, the transistor, and your build technique that it even possibly can be.

[bdevlin]

If you have the base lower than the emitter of an NPN transistor, it simply cannot be amplifying. The real question is why the collector is down at 4V if the base is lower than the emitter.

Shoot.  I just re-read my voltages that I posted and I messed up.  I have a C=4V, B=1.8, E=1.2.  It is in fact the GGG voltages in the build instruictions that list a base voltage lower than the emitter.  I think JD, or the author just put the readings out of order.  So anyhoo, my voltages seem to be in somewhat of the range.

By they way R.G., this Big Muff is for an adaptation on your Steel Stud GEOFEX rack idea.

[R.G.]

Wow, you did a really nice job on building that rack. It looks great!

Why don't you give the DC voltage test one more pass. Measure all of the DC voltages on transistor pins for all of them and post them here.

It's almost certain to be a short circuit or a wiring bug.

[bdevlin]

My Transistor voltages are as follows:

Q1
C=4
B=.6
E=.03

Q2
C=4
B=.6
E=.03

Q3
C=4
B=.6
E=.03

Q4
C=4
B=1.8
E=1.2

Those are all real close to what is mentioned in the build instructions on GGG except the Base and Emitter on Q4 are about .6 lower than what is cited on GGG. 

[petemoore]

Q3
C=4
B=.6
E=.03
  Strange that all of the first three transistors measure exactly the same, base is almost .6v above emitter, I would guess they are producing gain.
Q4
C=4
B=1.8
E=1.2
  Base is above emitter here, collector has room to switch signal voltage, but I'd audio probe base, then collector, signal should be amplified by a good amount between the input and output [B/C] of this gain recovery stage.
  Could be some wierd anomoly, the BMP is generally very distorted and fuzzy, not known for 'timbre' or clean source sound at output, the clipping stages should each be producing...clipped signal sound, second clipping stage sounding the most clipped, volume drop then after passive loss of tonestack and then the recovery stage re-amplifies the boosted, double clipped, tone controlled signal.

[R.G.]

You know, one thing that would do this is getting the transistors in backwards; swapping collector for emitter. Transistors work that way, but with very low gain.

In most cases, the B-E voltage on a high gain NPN with only microamps flowing in is well under 0.6, often 0.45 to 0.5.

Didja check your transistor pinouts carefully?

[bdevlin]

You know, one thing that would do this is getting the transistors in backwards; swapping collector for emitter. Transistors work that way, but with very low gain.

In most cases, the B-E voltage on a high gain NPN with only microamps flowing in is well under 0.6, often 0.45 to 0.5.

Didja check your transistor pinouts carefully?

I used all 5088 and 5089s and the orientation on the GGG board is the same for all of them.  And, there is a big gain difference the B and C.  Also, if you reference the GGG voltages in the build instructions there is nothing really strange as Pete suggests.  Maybe I'll try a different transistor in Q4?

[bdevlin]

Rolling the bass off helps so a cap change could be in order.  Other than that I should really try and A/B it with a stock EH Big Muff.  I'm convinced I hear a distinct "saturated" signal along with a clear, well defined clean signal.

[sfr]

This is probably stupid and not it, but I did have a problem similar to yours once - turns out I had stray piece of stranded wire shorting the stomp switch, so the input and output where always connected, even if signal was going through pedal as well.  Might be worth checking that the tips of the input jacks are not connected when the pedal is engaged?

[bdevlin]

So I think I'm going to try a cut some lows.  That might mask what I hear as an issue.  I assume the best place to do that is with smaller caps in C1 and C13 (I think those are the input/output caps).  Right now they are .1 uF.