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This post isn't intended to be a comprehensive list of every known modification for the Large Beaver, but rather a starting point for those wishing to experiment with the circuit.

OTHER BMP VARIATIONS

There have been many variations of the Big Muff Pi and much discussion about how they compare, but one thing that hasn't changed much is the basic circuit. Sure, you'll find variations in the values of various resistors and capacitors, but nearly all BMP versions (we'll ignore the late '70s opamp version) follow the same basic layout - even the original PNP version.



The two shaded components in the above skeleton schematic - a capacitor in the feedback loop of Q1 and a base resistor on Q2 - don't appear on the Large Beaver circuit board due to the particular variation it clones, but these are easy enough to add if you wish to clone a different BMP model. With a schematic for the desired version and the information contained in the Large Beaver walkthrough, you should have no trouble making a Ram's Head or Russian clone, or even a clone of a non-EH BMP derivative such as the Swollen Pickle.

TRANSISTOR SUBSTITUTIONS

It's been said that the key ingredient to the BMP's sound is the transistors. Others have stated just the opposite - that any high-gain transistor will sound about the same in this circuit. There's something to the latter statement, and you only need to refer to the schematic of the effect to see why. Notice the clipping diodes in the feedback loops of Q2 and Q3. The diodes are the components that actually clip (or distort) the signal. The transistor is simply there to provide clean gain.

As for myself, I take a middle-of-the-road approach. Yes, changing the transistor type can affect the character of the distortion, but there are many other parts in the circuit that can have a more profound affect on the pedal's sound. I've built examples with hand-picked 2N5113s, 2N5088s, and 2N5089s that sound very, very similar, and yet others using the same transistor types - with other resistor and capacitor modifications - that sound dramatically different from one another.

Popular transistor types for this circuit include the BC239, 2N5113, 2N5088, and 2N5089, but many other medium to high-gain NPN silcon transistors will work as well. Socket your transistors and experiment away.



Note that I've seen at least one source of BMP information that indicates the 2N5087 is an acceptable lower-gain alternative to the 2N5088. This is incorrect. The 2N5087 is a PNP analog of the NPN 5088 and will not work in a Large Beaver circuit without other changes (specifically, rewiring the pedal to positive ground).

Use a DMM with a transistor socket to measure the gain (hFE) of each transistor before you begin your build. I like to put transistors with similar gain in the Q2 and Q3 clipping stages. If one of the remaining transistors is significantly lower in gain than the other, put the highest one in Q1 and the lowest in Q4. Why? Because the Q4 transistor in the output stage isn't being asked to do much - it's a simple recovery stage for the loss in gain incurred in the tone stage. In contrast, Q1 in the input stage sets the gain for the entire circuit.

CONVERSION TO PNP

Polarity doesn't have a "sound." If you want to convert your pedal to positive ground like the first edition triangle, fine, but don't expect flipping the polarity alone to change the tone of the pedal. If you've got a specific PNP transistor you'd like to try that isn't available in NPN form, then the exercise might be worthwhile. Otherwise, I'd leave the circuit negative ground for a number of reasons and look to other modifications to alter the tone.

BIAS CHANGES

In a stock build, the Q2 transistor in the first clipping stage is mis-biased. This is a result of the circuit conforming precisely to the published schematic for an NPN triangle-era BMP. All other BMP variations I've seen include this resistor. Its omission doesn't create a major problem - there are lots of fine-sounding Beavers out there that don't have it - but we can still make it technically correct. One only needs to top-mount a resistor from the base of Q2 to ground to bring the voltage at Q2's collector up to ~4.5V.

With the circuit board silkscreen-up and the notched edge at the top, the resistor to be added will go in the upper-right corner of the board and will join the topmost leg of the 8.2K resistor nearest Q2 with the topmost leg of the 2.7K resistor at the right-hand edge. An 82K resistor should get Q2 pretty close to 4.5V. Here's a picture of what it looks like...



Virtually every BMP variation has differences in the values of resistors that connect to power or ground - the ones oriented vertically in the schematic. Pick up a schematic for one of these variants and experiment away.

CREAMY DREAMER MOD

One popular mod is to jumper the emitters of transistors Q1, Q2, and Q3 directly to ground. This is often referred to as the "Creamy Dreamer" mod. If you're into Gilmour-ish BMP tones, you might want to give this one a miss, as it yields a massive increase in gain and distortion. But if you're after a more Pumpkins-like tone, give it a try. Socketing these resistor pads will allow you to experiment with resistors and jumpers in these spots.



CAPACITOR CHANGES

Another thing unique to the NPN triangle schematic compared to other BMP variations is that it is missing a capacitor in the feedback loop of Q1. This cap acts as a simple low-pass filter which prevents some of the higher frequencies from being passed on to subsequent stages. The addition of this cap won't dramatically change the tone of the pedal, but may help to remove some high-end "gritchiness" from the tone. Values from 470pF to 560pF work well. Adding the cap to the Large Beaver board is a simple process that involves bridging the existing 390K resistor in the input stage.



The 560pF caps in the feedback loops are probably best left stock, but there's nothing magical about the value, so experiment if you want a softer or harder-edged distortion tone.

If you're sourcing your own parts, don't worry about finding precise .05uF caps for the clipping stages. Those are primarily there to block DC and not as tone shapers, and any value between .05uF and .1uF will work just fine.

Leave the remaining .1uF coupling caps at their stock values.

The best place to experiment with other capacitor values is in the tone stack, which we'll cover later.

DIODE CHANGES

Just like any other diode clipping circuit, the diodes in a Large Beaver can be changed to provide different sounds. Because there are two clipping stages, you'll probably want to start by modifying just one of the clipping stages (the second) while leaving the other clipping stage stock.

You can use a different kinds of silicon diodes, germanium diodes, LEDs, or even FETs to clip the signal. The diodes from one or both clipping stages can also be lifted, a la the BYOC Mighty Mouse. While I haven't performed this particular mod myself, I imagine that fuzz would decrease and overall volume would increase pretty significantly.

SIMPLE MIDRANGE ADJUSTMENT

Raising the value of the 39K resistor to ground (the one next to the .0039 cap) towards 100K will reduce the midrange notch and allow more bass frequencies through when the control is turned towards the treble side. Lowering the value will obviously increase the midrange dip.

Alternately, raising the value of the .0039uF cap to around .0056uF will have approximately the same effect. Going further to 0.01uF will completely flatten the dip. Why? Check out the tone stage on the schematic. The tone two filters now have identical component values - a 39K resistor and a 0.01uF cap - just oppositely oriented.

I recommend using socket pins in these component spots if you want to experiment. That way, you can easily try different resistor or capacitor values to tailor the effect's midrange response to your liking.

TONE STACK CHANGES

There are many other things you can do with the tone stack to tailor it to your needs. I highly recommend using the Duncan Tone Stack Calculator at:

http://www.duncanamps.com/tsc/

The TSC is a nifty tool that has the BMP tone stack as a built-in option. Play with the values until the resulting graph looks like something you want to hear. By changing some of the components in this circuit's simple tone stack, you can decrease, increase, or eliminate the midrange notch, or shift it to a lower or higher frequency.

SCOOP/FLAT/BOOST MIDS SWITCH

(This modification comes directly from a byoc post in the old mods section.)

The tone knob in the BMP is a high pass filter and a low pass filter in parallel with the B100k acting as a "mix" between the two.

If you use two equal value caps in the two filters, you will get a flat response at noon on the tone knob. This is why the two "on board" caps have been changed to .0033uf so that when the toggle switch is in the "off" or middle position, the two .0068 caps are lifted out.

By adding a .0068 uf cap in parallel to the .0033uf where the .01uf cap used to be(low pass filter), this would give us approx .01uf again, thus giving use the stock "scooped mids" tone knob.

Flipping the toggle to the opposite "on" position would lift that .0068 and put the other .0068 in parallel to the .0033uf cap where the .0039uf cap used to be(high pass filter) which would produce a mid hump at noon on the tone knob.



IMPROVE CLARITY & DEFINITION

This one's easy - you won't need to make even a single change to your Large Beaver! If you haven't done so already, order yourself a BYOC compressor kit and build a Ross or DynaComp clone. Place it before the Beaver in your signal chain, and engage the compressor when you want your lead lines to have extra definition.

USING JFETs AS CLIPPERS

I mentioned the use of FETs above as clippers in the Large Beaver circuit, so let's expand on that...

If you're not happy with the "wall of fuzzy sustain" offered by the stock circuit, give this mod a try. It will tone down the fuzzy nature of the Beaver while adding a more saturated tube-type drive.

Find yourself a pair of J201 JFETs. Others JFETs may work just fine, but I've only done this using the J201 type. Note the left-to-right Drain/Source/Gate arrangement of the pins if you're looking at the flat side of the transistor. See figure (A).

Short the Source pin to the Drain pin. Some of you are going to say, "I've read elsewhere that you're supposed to connect the Gate and Source pins to make a diode!" Well, you'll just have to trust me that you should ignore that for now and tie the Source and Drain together. See figure (B).

Finally, replace the two clipping diodes in the second clipping stage in the upper-left region of the circuit board with the two JFETs. Like the original diodes, they should face in opposite directions. Putting the flat faces of the transistors together will accomplish this. See figure (C).



That's all there is to it! Your pedal should now have a little more cranked amp sound to it. Sure, some of the fuzzy nature of the original BMP is gone, but you can easily use a switch to select between the original silicon and JFET diodes to have both sounds at your disposal.

SCOOP MOD

The stock Large Beaver circuit has a midrange dip of about 7dB around 1kHz. This mod allows you to control the depth of the midrange notch from stock to flat response and everywhere in-between.

Click on the image for a larger view.



While this type of modification should be apparent to just about any pedal tweaker, I first saw it documented on the 'net by Jack Orman at the following address. Refer to that link for a more thorough description of the mod, and for a second type of mod that allows even greater tonal variation.

http://www.muzique.com/lab/tone3.htm

I just ordered thirty NOS Fairchild 2n5133 from Small Bear.

I need testing advice in regards to 2n5133 being put in my V.1 triangle beaver?

-For example: Q1 tranny sets the gain stage for the circuit... is there such thing as a too high of an hFE for this stage?

-What about the same question with regards to Q2 & Q3 clipping stages (acceptable hFE values)?

-I know the last stage, Q4 , needs the LOWEST hFE Value, , is there too high or too low of a hFE value for it?

Thanks for reading..... I've got alot of tranny testing coming my way and I just want to be prepared

This thread has helped me ALOT Rog....... you rock!!!!

Just wondering if there'd be any easy way of showing where the "missing" bias resistor and feedback loop cap are on the new Triangle V1 specs? The one with the flat mid/hump/bypass control.

I'm confused looking at the schematic on the walk-through sticky and my circuit board.

EDIT: Okay, nevermind. I just saw the schematic for the Large Beaver v. II (Triangle Muff version 1, if that confuses the issue). It appears that a 560p cap, and an 82k resistor are already a part of the kit.

Here's a modification I like to do with the v.2.0 Large Beaver which adds three switchable clipper configurations to the second clipping stage of the pedal. For this purpose, I solder the clipping components to a small piece of perfboard wired to an on-on-on DPDT toggle switch and connected back to the PCB at the top & bottom connection points of the 2nd clipping stage. For the three sets of clippers, I typically use (1) the stock 1N914 silicon diode pair, (2) a pair of 3mm red LED's, and (3) a pair of J201 JFET's, per CallMeRog's suggestion a few posts up from this one. But you can use any combination of diode clipping arrays that you prefer. There is sufficient room to affix the small piece of perfboard (about one inch square) to the inside of the enclosure top, just above the stomp switch and below the sustain pot. I use double-sided foam tape for this purpose. The toggle switch is mounted through the left side of the pedal as viewed from the top, immediately above the output jack.

I find that this switchable clipper mod increases the tonal flexibility of the pedal very significantly. The red LED's give a volume boost and a fuller, more muscular tone, though with less of a typical "fuzz" character. The JFET clippers are a more subtle change vs. the stock silicon diodes, giving a somewhat smoother, warmer fuzz tone and a slight volume increase. Calling their tone "tube-like" may be a bit of a stretch, since it's still PLENTY fuzzy, but it definitely moves the pedal's tone in that direction. Together with the LB v.2.0's 4-position rotary EQ switch, this mod gives a Beaver with 12 different "voices". Very cool!

Here are a couple of diagrams showing the mod layout and how the enclosure is altered, along with a closeup photo of the clipper perfboard mounted in an actual LB v.2.0 build. You can see a portion of the perfboard and the two black J201's tucked underneath the I/O jacks and the switch wiring.

I plan to attempt duhvoodooman's clipping switch mod on my LB when it arrives but I'm unclear of one thing... Which solder pads do you use on the LB pcb to jump to the new clipping board and then return to the clipping section. I see where the stock 1N914's are highlighted in purple on the LB pcb, I'm just uncertain of where/how it sends and returns. Thanks in advance!

One wire into either of the two upper clipping eyelets, and the other into either of the bottom pair of eyelets. Plus it doesn't matter which wire comes from the switch and which comes from the perfboard.

Hey, could you explain the switching here? How are you making 3 choices with a dpdt switch? It looks to me like you're selecting between
Down position: red LED clipping
Up position: 1n914 + FET clipping

is it necessary to wire the switch as you have shown with terminal 5 tied to 3? (numbered from top left down, and then top right down)
i must be missing something..

edit: i should have researched harder before i said anything. i get it now, although i don't like that a switch with 3 settings is called a double throw. center should count, but i'm not on the committee so who asked me.

What you're missing is that this is a 3-position "on-on-on" DPDT toggle switch. Here's how it works--the one shown in my diagram would be the "on-on-on 1" configuration in this link: http://guitarwiring.blogspot.com/2011/0 ... itars.html

Trace out the connections in the three positions of the switch and you'll see how it works.

Hey DVM,
Am I looking at the perfboard right in that the solder pads of the bottom legs of each clipping option are jumpered? Thanks!