big muff gain stages

[gutsofgold]

As I understand it, the first BMP stage is simply setup for a good amount of clean gain to pass along to the rest of the circuit.

Stages 2 and 3 are of course fixed and there simply for clipping and the last stage a buffer.

From a technical stand point, if I have a number of transistor with varying hfe's from 100 - 600 .. what would be the difference between...
1) putting a higher hfe transistor in stage 1 and two equal but lower hfe transistors in stages 2 and 3
2) putting a slightly lower hfe in stage 1 and finding two higher hfe's for stages 2 and 3

scenario one would allow more for clean amplified signal to pass on to the clipping stages of course which would result in more clipping
however scenario two could give more clipping if the hFE is a real deciding factor in those stages..

just trying to work out the technicality in my head here..

[tubelectron]

Hello gutsofgold,

Here is an info about BMP Qs Hfe which would be of some interest to you, I think :

I recently opened a 1980 Big Muff PI for "repairs / strange behaviour" as the owner descibed it to me.

All the Qs are MPSA18, given for an Hfe min of 900 theorically (I often find them at 1100-1200).

The 1st stage showed no gain (the Q is probably dead), the others were working correctly.

The suprising thing is that this "broken - low gain" 1980 BMP worked well : its still have a good sustain and sounds like Carlos Santana's greatests hits... Of course you don't reach that thick-endless-fuzzy well-know BMP sound when maxxed, because of the lack of gain, but instead you have a nice "violin singing sustained" overdrive - which I personally like...

Comparatively, I opened a brand new BMP RI (2009) of similar look (folded sheet metal box) bought for peanuts : all the Qs are BC550C, and the sound is the well-known BMP sound, as would probably give the 1980 model if it was fully restored to its original condition.

As my goal is to try to duplicate that "broken" BMP sound in my RI, I will use a LOW hfe Q in the 1st stage, and left the others with their original Qs (BC550C : Hfe Min = 420, probably 600-700 typical) at the first time, and then see if it's necessary to shift to low hfe also to have the expected tone.

So if you lower the Hfe of Q1, you will have less clipping, less noise, and doing conversely will probably increase the clipping and the hiss. It's a matter of taste, and I will go for the 1st solution, as a lower gain BMP is of interest to me : I will have the tone, but not the outrageous gain...

A+!

[frank_p]

The first transistor biasing setup can provide a lot of voltage gain.  It is clipping slightly with a relatively hot pickup (with a 22KOhm as R_coll and medium gain transistor).

[Earthscum]

All the Qs are MPSA18, given for an Hfe min of 900 theorically (I often find them at 1100-1200).

The 1st stage showed no gain (the Q is probably dead), the others were working correctly.

.....

As my goal is to try to duplicate that "broken" BMP sound in my RI, I will use a LOW hfe Q in the 1st stage, and left the others with their original Qs (BC550C : Hfe Min = 420, probably 600-700 typical) at the first time, and then see if it's necessary to shift to low hfe also to have the expected tone.

So if you lower the Hfe of Q1, you will have less clipping, less noise, and doing conversely will probably increase the clipping and the hiss. It's a matter of taste, and I will go for the 1st solution, as a lower gain BMP is of interest to me : I will have the tone, but not the outrageous gain...

You gave yourself a clue right there... make your first stage a buffer. If you want to mod a stock BMP, you might get away with putting in a PNP in Q1 and biasing it to about 4.5V on the emitter. Swapping the NPN for a PNP will essentially flip the transistor upside down, enabling you to make a buffer stage without having to do some goofy work-around. I think you can actually pull the feedback cap and resistor and bias the base with a resistor to ground, which is already there... just gotta swap the value.

[jmasciswannabe]

A little OT, but I recently was checking out the Royal Beaver...probably could have phrased that better....rimshot please

http://www.bigtonemusic.com/royalbeaver.html

Splits up the different gain stages to achieve different big muff version qualities. Kinda neat. Way too many knobs for a muff though.

[Projectile]

The way the transistors are biased in the big muff, changing the hfe isn't going to effect the gain much. Don't get too attached to the idea that hfe = gain. That's only true in some fuzz circuits. In most circuits transistors are biased in a way as to minimize the effect of hfe on circuit gain. That's why with most pedals, manufacturers can swap out different transistors with little effect on the sound of the pedal.

Here's a general rule of thumb. If you see the emitter of a bjt transistor tied to ground (like in a fuzz face), the hfe is going to make a big difference in the gain of the circuit. However, if you see a resistor on the emitter to ground, then the hfe is  probably only going to only be a small factor in the gain of the circuit.

[tubelectron]

Hi Earthscum, Projectile & frank_p ,

Thanks for your practical informations here - very useful for me (and certainly also to gutsofgold).

A+!

[frank_p]

The way the transistors are biased in the big muff, changing the hfe isn't going to effect the gain much. Don't get too attached to the idea that hfe = gain. That's only true in some fuzz circuits. In most circuits transistors are biased in a way as to minimize the effect of hfe on circuit gain. That's why with most pedals, manufacturers can swap out different transistors with little effect on the sound of the pedal.

Here's a general rule of thumb. If you see the emitter of a bjt transistor tied to ground (like in a fuzz face), the hfe is going to make a big difference in the gain of the circuit. However, if you see a resistor on the emitter to ground, then the hfe is  probably only going to only be a small factor in the gain of the circuit.

Perhaps for small changes.  But a change of Hfe from 70 to 1000 will change the collector voltage from 5.2V to 3.8V DC at idle.  I think the topology of the biasing is a good one for preventing changes in Hfe but the the actual BM resistor values throws that advanage away. Calculate the ratio of Rc/Re: 10K/100=100 : the gain is kind of high: my bet is that Re is too small (or Rc too big) to keep the advanges of that bias topology on Hfe changes.

Tubelectron: Salut et A+.

[PRR]

> Calculate the ratio of Rc/Re:

Rb/Re is more relevant to hFE sensitivity.

> a change of Hfe from 70 to 1000 will change the collector voltage from 5.2V to 3.8V

1:14 change of hFe gives 1.4:1 change of idle voltage? While an OVER-driven stage will change sound somewhat, it still pretty darn stable for such a vast spread of hFE.

[tuckster]

Does anyone hear the difference between transistors with different Hfe values?

[gutsofgold]

thanks for the explanations guys!

my initial wondering stemmed from the claim that you need at least fairly high level hfe transistors in the big muff circuit to get it sounding as one would expect a big muff to sound.

I got my hands on a pile of NOS 2N5133 transistors with hfe's measuring between 70 - 150 with a single one measuring 400. I built an original Triangle model clone and tried the 400 at Q1 with two 140-150's for Q2 and Q3 and while it didn't reach that "so much distortion that it actually sounds smooth" level that most BMP's can reach it still sounded great. I moved the 400 hfe to Q2 and it was nearly identical to the real Triangle model with perhaps not quite as much available distortion but still very close. I've read that the transistor hfe in muffs needs to be high enough from a number of respected forum posters here and I (supposedly) know skreddy called his vintage BMP clone quits after losing his lead on high hfe vintage transistors... so I guess I was trying to technically get down to the real answer..

[frank_p]

> Calculate the ratio of Rc/Re:

Rb/Re is more relevant to hFE sensitivity.

Do you mean all the base biasing resistors or just the one that goes to ground (ususally Rb (to ground) is selected in function of a % of Hfe*Re) ?  For Hfe min of 2N5088 :  R2/(H_FE*Re) = k

But the different BM have so many different R values from version to version...



Thanks Paul.

[Earthscum]

Does anyone hear the difference between transistors with different Hfe values?

Personally, I couldn't really notice much in the clipping stages until I got down to lower than about 175 or so. To my ears, the 2nd and 3rd stage don't sound much different when you swap out trannies. The first stage makes a little difference, more along the lines of swapping the input cap type of difference, though. I fell in love with MPSA13's in first gain stages and buffers because it really lets my bass come through all the way, and that's about what it does in the BMP. The last stage is where I DO notice difference in trannies, but it didn't seem to be as much about the gain as just the characteristic sound of the actual tranny, itself. BCxxx's sound great to me in the last spot, while the darlington didn't sound any different than a 5088 or 3904. Japanese 2SC1815's sounded a little softer to me in the 4th stage.

On a somewhat off-topic, I tried out a fet in the last stage (MPF102, and a J201), and they REALLY get BMP's sounding creamy! If you get 2SK series fets, they are D-G-S, and drop right in where a normal BJT used to lie without twisting legs around. I haven't tried the ones I got yet, but I don't expect them to sound much different than the MPF's.

[tubelectron]

Hi,

I am planning to make a trial with hfe as low as 50 (2N3137, 2N1893) for the 1st stage, and may be for the others... When I will have the time !

A+!

[Projectile]

Perhaps for small changes.  But a change of Hfe from 70 to 1000 will change the collector voltage from 5.2V to 3.8V DC at idle.

Of course, if you make that huge of a change in hfe, it's going to make a difference, but that's a rather extreme example. I'm giving a general rule of thumb.

I'm not sure how you calculated those DC values. Care to share? The values they chose of the BMP makes it a little more complicated than a standard amp. I would have to simulate it to get DC values that I could be confident about, but it does appear that the DC beta of the transistor is going to be a bit unstable, which could attribute people hearing different tones from lower gain transistors.

I think the topology of the biasing is a good one for preventing changes in Hfe but the the actual BM resistor values throws that advanage away. Calculate the ratio of Rc/Re: 10K/100=100 : the gain is kind of high: my bet is that Re is too small (or Rc too big) to keep the advanges of that bias topology on Hfe changes.

Tubelectron: Salut et A+.

After doing some calculations, I see what you mean about them throwing away the advantage of the topology. It does appear to be a bit unstable with the resistor values they chose, but with all of the feedback and swamping effects accounted for, it's hard to say. I'd probably have to simulate it to really know for sure. Maybe I will.

[frank_p]

Perhaps for small changes.  But a change of Hfe from 70 to 1000 will change the collector voltage from 5.2V to 3.8V DC at idle.

Of course, if you make that huge of a change in hfe, it's going to make a difference, but that's a rather extreme example. I'm giving a general rule of thumb.

I'm not sure how you calculated those DC values. Care to share? The values they chose of the BMP makes it a little more complicated than a standard amp. I would have to simulate it to get DC values that I could be confident about, but it does appear that the DC beta of the transistor is going to be a bit unstable, which could attribute people hearing different tones from lower gain transistors.

I think the topology of the biasing is a good one for preventing changes in Hfe but the the actual BM resistor values throws that advanage away. Calculate the ratio of Rc/Re: 10K/100=100 : the gain is kind of high: my bet is that Re is too small (or Rc too big) to keep the advanges of that bias topology on Hfe changes.

Tubelectron: Salut et A+.

After doing some calculations, I see what you mean about them throwing away the advantage of the topology. It does appear to be a bit unstable with the resistor values they chose, but with all of the feedback and swamping effects accounted for, it's hard to say. I'd probably have to simulate it to really know for sure. Maybe I will.

Have a look at this topic you'll have all the info for calculating the bias of that kind of topo.

http://www.diystompboxes.com/smfforum/index.php?topic=88331.msg745760#msg745760

So: I programmed the equations and did a plot of Vc vs Hfe before this topic appeared because I was curious about this particular bias.
The more Hfe is low the more a change in Hfe will have an influence on Vc (negative steep curve).  So if you put hi gain transistors: Vc will be lower and less influenced by the Hfe variations (less steep curve).

Did a quick spice sim beacause I was a bit confused about how to tune the front end resistor for centering the *operation point*. But what I understant is that there is no real quiescent point for that setup : the feedback resistor makes the Q point variable...

[Earthscum]

Did a quick spice sim beacause I was a bit confused about how to tune the front end resistor for centering the *operation point*. But what I understant is that there is no real quiescent point for that setup : the feedback resistor makes the Q point variable...

Hey, Frank... are you talking about R2/8/12? It's rhetorical, but... how would those have anything to do with bias? They are AC coupled with caps.

I tried rebiasing the clipping stages before. I think I dropped the lower gate resistor, which dropped the collector down to around 3V somewhere. My idea with it was trying asymmetrical clipping. New tones, for sure. My next great idea was to bias one high and one low. I don't recall that one sounding very good... kinda farty, maybe.

If you want to really screw with the sound of the BMP, breadboard one with the emitters (npn) tied together and a 100R to ground (or just tap the 2 together on the PCB... then you'll effectively have a 50R to ground). The "in-between" of that is just pulling the resistors out and jumpering the emitters straight to ground. The gain stages will feed off of each other through the ground path. I kinda remember getting some pretty awesome sounds out of it... and, since we're on the subject of biasing, I don't recall it changing the bias much at all.

[frank_p]

- Hey, Frank... are you talking about R2/8/12? It's rhetorical, but... how would those have anything to do with bias? They are AC coupled with caps.

No R4 and R3: I was jumping over R2 by adjusting the level of the sine generator.

- I tried rebiasing the clipping stages before. I think I dropped the lower gate resistor, which dropped the collector down to around 3V somewhere. My idea with it was trying asymmetrical clipping. New tones, for sure. My next great idea was to bias one high and one low. I don't recall that one sounding very good... kinda farty, maybe.

I think (If I recall well the spice sim.) that if you bias near saturation instead of cutoff: the clipping will be softer because the feedback fight against going in full saturation.


- If you want to really screw with the sound of the BMP, breadboard one with the emitters (npn) tied together and a 100R to ground (or just tap the 2 together on the PCB... then you'll effectively have a 50R to ground). The "in-between" of that is just pulling the resistors out and jumpering the emitters straight to ground. The gain stages will feed off of each other through the ground path. I kinda remember getting some pretty awesome sounds out of it... and, since we're on the subject of biasing, I don't recall it changing the bias much at all.

I was more looking after finding how to bias this amp section in a linear (hi-fi) way for using it in other applications.  Ex: suppose you want X amps at the collector, how you adjust the other resistors to have to most of the two feedbacks.
But thanks for the tips Earthscum.