Modding a Big Muff for more gain/compression?

[pott]

Before you think I'm crazy, keep in mind I use lower output buckers to single coils mostly 

I usually use the Muff on 10, with an OD in front and the compressor before everything, all this into the front amp of my amp's crunch channel. It's sonic mayhem, and I love this.

But I'd love for my Muff to have that extra push in itself, so I can just turn it on and control it from the guitar's volume control, see what I mean?

So I'd like more gain from it, maybe a little more compression for added sustain!

Thanks!

PS I have the latest Russian version, with True Bypass, in case that matters.

[alex frias]

 

[Mark Hammer]

Try germanium diodes in the first stage.

[doug deeper]

try replacing the emitter resistors (100 ohm) with jumpers.

[John Lyons]

Sound like good advise.
The germanium diodes with make the Muff compress/squish more and
taking out the 100ohm resistors will run the transistors hotter for more gain.

What transistors are you using now? Do you know the Hfe of them?
You might also want to get some higher gain ones in there if they are lower than 300.

John

[slacker]

Have a look at these threads or do a search for posts by Yun

http://www.diystompboxes.com/smfforum/index.php?topic=55649.0
http://www.diystompboxes.com/smfforum/index.php?topic=51690.0

Some good ideas in there if you think the Big Muff is a bit tame 

[brett]

Hi

RE:

try replacing the emitter resistors (100 ohm) with jumpers.

and

taking out the 100ohm resistors will run the transistors hotter for more gain.

The gain of the gain stages is set by the ratio of the input resistor to the feedback (collector to base) resistor. ie the gain is lower than the ratio of the drain resistor to the emitter resistor, as in common emitter circuits.  Somewhere in the forum RG Keen and I have made these comments before.  Back then, I had mentioned the ratio as being the effective gain, but then RG put it through SPICE modell ing and showed that it was a whisker different (presumably lower by a percent or two) than my ratio.

Therefore, increasing the value of the feedback resistor would work. 
But mine already has hellish gain when cranked (somewhere around 1000?), so it will get fairly difficult to control oscillations and feedback.
cheers

[pott]

I have no clue what transistors are in or their hfe. It's entirely stock. I'd modded it when I got it but didn't like it and got it back to stock.

I'll jumper the first two emitter resistors and see what gives. If that's not enough I'll put in a 1N34 instead of one of the 4148s and see what gives! Thanks guys!


By the way if I understand right, changing the emitter resistor is like changing the cathode resistor on a valve amp, i.e lower value = more gain?

[kurtlives]

I have no clue what transistors are in or their hfe. It's entirely stock. I'd modded it when I got it but didn't like it and got it back to stock.

I'll jumper the first two emitter resistors and see what gives. If that's not enough I'll put in a 1N34 instead of one of the 4148s and see what gives! Thanks guys!


By the way if I understand right, changing the emitter resistor is like changing the cathode resistor on a valve amp, i.e lower value = more gain?

Pretty much

You could also try adding bypass caps to those "cathodes" if they dont already have them.

[Gus]

brett  Gave a good answer.   The series resistor 8.2k and resistance before it and the feedback resistor C to B controls part  of the stages gain.

If one adjusts the 470k C to B resistor you might have to adjust the B to ground resistor(it is the other part of the bias string)

Shorting or bypassing the 100 ohm should not change the gain that much.

note the clipping diodes are AC COUPLED in the feedback path.  Maybe look up inverting opamp circuits the output is the C the input summing node is the B.  The thing is the one stage transistor gain stage makes a very poor inverting opamp because of the limited open loop gain.

Maybe the easy thing to try is reduce the 8.2K(s) value keeping in mind the stage before its output resistance and coupling cap value for the same RC.

http://www.diystompboxes.com/pedals/triangleMuff.jpg

The 470K has about 4.7 times the 100K voltage drop,  The 100K voltage drop is the transistor VBE plus the 100 ohm voltage drop so the collect should be at 5.7 times(470K +100K) the VBE and 100 ohms drop.  Most of the current goes C to E so the collector current is about equal to the emitter current(the emitter current less than the collector current because of the 470K and 100K from collector to ground).  The voltage across the collector resistor divided by the resistor value is the collector current.  Then you have the transistor hfe. A few  things going on in that stage to set the gain and DC operating points.

R.G. have you worked on a tech of for the BMP?

[pott]

brett  Gave a good answer.   The series resistor 8.2k and resistance before it and the feedback resistor C to B controls part  of the stages gain.

If one adjusts the 470k C to B resistor you might have to adjust the B to ground resistor(it is the other part of the bias string)

Shorting or bypassing the 100 ohm should not change the gain that much.

note the clipping diodes are AC COUPLED in the feedback path.  Maybe look up inverting opamp circuits the output is the C the input summing node is the B.  The thing is the one stage transistor gain stage makes a very poor inverting opamp because of the limited open loop gain.

Maybe the easy thing to try is reduce the 8.2K(s) value keeping in mind the stage before its output resistance and coupling cap value for the same RC.

http://www.diystompboxes.com/pedals/triangleMuff.jpg

The 470K has about 4.7 times the 100K voltage drop,  The 100K voltage drop is the transistor VBE plus the 100 ohm voltage drop so the collect should be at 5.7 times(470K +100K) the VBE and 100 ohms drop.  Most of the current goes C to E so the collector current is about equal to the emitter current(the emitter current less than the collector current because of the 470K and 100K from collector to ground).  The voltage across the collector resistor divided by the resistor value is the collector current.  Then you have the transistor hfe. A few  things going on in that stage to set the gain and DC operating points.

R.G. have you worked on a tech of for the BMP?

Wow that is way too complex for me ... I understood the emitter thing, but that bit's a little too elaborate for me.

[brett]

Hi
Gus is absolutely correct.  If I may highlight the important bits:
1.  The emitter resistance has little to do with the gain.
2.  Lowering the input resistor and/or increasing the feedback resistor will increase gain.
3.  These things also affect the bias and the tone, because of the filters in the system.  But small changes (e.g. 470k feedback resistors changed to 680k will make only modest changes, but don't expect to put 1M in there and not affect things other than gain)

But maybe the first thing you might do is check the value of those resistors (and the gain pot) and make sure that they are the standard values.  This is because most people consider the gain in a standard BMP to be very very high.
(If, for example, you accidentally used a 47k feedback resistor in one of the stages instead of a 470, your gain would be 10x less, and make for a fairly mild BMP.  Similarly, an 82 k input resistor instead of an 8.2k resistor would reduce gain by 10x.)
cheers

[Mark Hammer]

You will note that the input stage of the BMP is pretty much identical to the 2 subsequent stages, with one major exception - it lacks the diode-pair and series capacitor found between base and collector.  It might be worth including these in this first stage.

Note, however, that the diode/cap combo can only HAVE a clipping effect on subsequent stages because the output of stage 1 is unconstrained by diodes and combines with the gain of the next two stages to push the signal level up high enough to produce the saturation that the BMP is known for.  It is also worth noting that clipping at stage 1 is much less likely because you normally feed it a guitar-level signal.

Consequently, it is a bit of a crap shoot whether the net effect of inserting a .1uf-plus-GE-diode-pair network in that first stage will be to introduce a bit more compression, introduce too much of a ceiling on the stage 1 output and detract from stage 2 clipping, or basically have no effect at all.  At a conceptual level, it seems to make sense.  I mean if the quintessence of the BMP is double-clipping, then triple clipping ought to give you more of the same, right?  Ultimately, it is an empirical question, and I encourage interested parties to tack such a network onto the copper side of their board, listen for any noticeable impact, and report back.

[tcobretti]

Try putting a clipping diode pair (which is two diodes pointed in the opposite direction) to ground somewhere in the signal path.  Maybe after the emitter of the first stage or after the emitter of the third stage before the tone circuit.  I can't decide how much affect it'll have, but it's a classic way of dirtying up a pedal.

You might also try the pair in the path, i.e oriegted so the signal flows thru them to get to the next component of the circuit.

Ultimately, what I think is probly the absolute best course of action is to take a very high gain design, probably an IC based Boss pedal, then put a BMP tone stack after it.  It'll still be Big Muff like but will have crazier gain.  I think you are gonna get into serious oscillation/noise problems if you start boosting the gain of the existing transistors.

You also might try building a IC based Big Muff, but do something crazy like adding another whole clipping stage.

[R.G.]

You do know that more gain = more hiss and noise, right?

[Gus]

Ok something more complex but easy to do and easy to undo if you don't like it.
Note I have not built this

Look at the first stage.  Stock the input R is set more by the 39K

See the feedback resistor the 470K from collector to base? Remove that and the 500pf cap

Next get two resistors a 220K and 270K connect them in series for a total of  490K(close enough to 470K)

Install them in place of the 470K C to B (270K to base)and add say a 10uf electro + terminal at the center of the resistors and the other end of the 10uf  to ground.  This will remove the AC feedback gain control of the stock 470K but it  will have about the same DC bias and let the stage run at the gain set by the collector and emitter resistor.  If you like it try reducing the 39K value.

Maybe this will add the gain you want,  I don't know how much gain the OD you use adds.  It changes the input and output R a bit but it might work good enough.

You could also just build the OD circuit and install it in the BMP.

[Mark Hammer]

I guess the other thing to do is to take note of Mike Matthews own admission that BMPs vary (or least varied) considerably, even within the same issue and batch.  I've made 3 of them, all using the exact same schematic, and same nominal transistors.  They all sound different.  One of them is nice and polite ("Um, is it alright if I distort a wee bit?").  One is pretty "normal sounding" for a BMP.  The third just screams at you if you look at it the wrong way.

Maybe you/Pott don't need to change the nature of the basic circuit.  Maybe you just need to get lucky with your trannies, diodes, and component tolerances.

[CASTOR]

I recently got my hands on a russian BM that I used to have but got tired of it and sold it to a friend. Well he gave it to me and I wondered as you did, how can I make it more powerful?
Id recommend the so-called "creamy dreamer mods" basically a jumper across the first three trannies emitter resistors. I turned my BM into a MONSTER with this. It sets your amp on fire evry time you kick it in. It has tons of sustain and gain and its louder too. You will indeed get along better with single coils but beware of the noise (the modded pedal will be MUCH noisier...). I use mine with a strat and step on an overdrive pedal for soloing (total madness!) if you wanna hear it, check out my band's myspace: www.myspace.com/castortrio its all over the first song. Have fun!

[fpaul]

I was also disappointed in my BMP(GGG, earlier version than the current).  It distorts okay but little sustain or gain.  The tranny voltages match the posted values and it isn't gated at all so I think the circuit is okay.  I'll try jumping the emitter resistors when I have time.  Thanks to everyone for the posts.  Funny thing is my dr. boogey is really low gain too; I was expecting a monster, and yes I've tweaked the bias from 2.5 volts up to 5.5 volts and tried two sets of J201s.  Metal distortion, low gain/sustain.  But I'll start another topic on that when I have time to start tweaking.   I got lucky with my BSIABII;  I just stuck in the parts an it has PLENTY of gain with no changes at all.  Funny how same parts stuck in a circuit can give such different results.  I guess I'm learning what MOJO really is.

[brett]

Hi
Instead of a jumper on R5 (the emitter resistor on the input stage), try a bypass cap (it's a "jumper" for AC, but won't shift the DC bias).  A value of 10uF should work very well.  It'll roll off a little bit of bass "flab" that BMPs sometimes have.  For keeping 100% of the standard bass, use a 22uF or 47uF bypass cap.
cheers